<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2025 Workiva-->
<!--r:53e78aec-2649-4ab2-90ec-c7c6a28c71f0,g:4235458f-970f-4287-908f-8ec45290af38-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://www.magn.com/role/Cover" xlink:type="simple" xlink:href="magn-20241228.xsd#Cover"/>
  <link:definitionLink xlink:role="http://www.magn.com/role/Cover" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationLineItems_2c314f7c-1921-41f0-9c9f-0f7cea34fbde" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_DocumentInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationTable_3f942af6-5288-4194-8904-68e0d20a7581" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_DocumentInformationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_dei_DocumentInformationLineItems_2c314f7c-1921-41f0-9c9f-0f7cea34fbde" xlink:to="loc_dei_DocumentInformationTable_3f942af6-5288-4194-8904-68e0d20a7581" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressesAddressTypeAxis_936c0717-d1bc-4252-be9d-7fd2635c7da4" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityAddressesAddressTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dei_DocumentInformationTable_3f942af6-5288-4194-8904-68e0d20a7581" xlink:to="loc_dei_EntityAddressesAddressTypeAxis_936c0717-d1bc-4252-be9d-7fd2635c7da4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_AddressTypeDomain_936c0717-d1bc-4252-be9d-7fd2635c7da4_default" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_AddressTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_EntityAddressesAddressTypeAxis_936c0717-d1bc-4252-be9d-7fd2635c7da4" xlink:to="loc_dei_AddressTypeDomain_936c0717-d1bc-4252-be9d-7fd2635c7da4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_AddressTypeDomain_e678b064-2112-4717-bb7a-86f95ad2fe8e" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_AddressTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_EntityAddressesAddressTypeAxis_936c0717-d1bc-4252-be9d-7fd2635c7da4" xlink:to="loc_dei_AddressTypeDomain_e678b064-2112-4717-bb7a-86f95ad2fe8e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_FormerAddressMember_8e251637-7e8e-4ef3-b9e4-5b615ab658a7" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_FormerAddressMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_AddressTypeDomain_e678b064-2112-4717-bb7a-86f95ad2fe8e" xlink:to="loc_dei_FormerAddressMember_8e251637-7e8e-4ef3-b9e4-5b615ab658a7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentType_ef689352-1eb5-4968-95bd-440f6550140e" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_DocumentType"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_2c314f7c-1921-41f0-9c9f-0f7cea34fbde" xlink:to="loc_dei_DocumentType_ef689352-1eb5-4968-95bd-440f6550140e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentQuarterlyReport_fe55b440-938c-421b-a033-0d6792bd2c9e" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_DocumentQuarterlyReport"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_2c314f7c-1921-41f0-9c9f-0f7cea34fbde" xlink:to="loc_dei_DocumentQuarterlyReport_fe55b440-938c-421b-a033-0d6792bd2c9e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentPeriodEndDate_19017e4a-4d3e-4e3e-b9ca-0902cb32d98d" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_DocumentPeriodEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_2c314f7c-1921-41f0-9c9f-0f7cea34fbde" xlink:to="loc_dei_DocumentPeriodEndDate_19017e4a-4d3e-4e3e-b9ca-0902cb32d98d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentTransitionReport_1c497d86-5f5d-4ae4-bf87-b80144a822df" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_DocumentTransitionReport"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_2c314f7c-1921-41f0-9c9f-0f7cea34fbde" xlink:to="loc_dei_DocumentTransitionReport_1c497d86-5f5d-4ae4-bf87-b80144a822df" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressAddressLine1_1811460a-210a-4ad3-af82-5bb6b2e06051" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityAddressAddressLine1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_2c314f7c-1921-41f0-9c9f-0f7cea34fbde" xlink:to="loc_dei_EntityAddressAddressLine1_1811460a-210a-4ad3-af82-5bb6b2e06051" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressAddressLine2_9a9eb9bc-3a05-4c8e-8ea9-aded94dda9cc" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityAddressAddressLine2"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_2c314f7c-1921-41f0-9c9f-0f7cea34fbde" xlink:to="loc_dei_EntityAddressAddressLine2_9a9eb9bc-3a05-4c8e-8ea9-aded94dda9cc" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressCityOrTown_fc4db3d3-4f68-4069-9595-dcd462571d09" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityAddressCityOrTown"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_2c314f7c-1921-41f0-9c9f-0f7cea34fbde" xlink:to="loc_dei_EntityAddressCityOrTown_fc4db3d3-4f68-4069-9595-dcd462571d09" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressStateOrProvince_0896f9e2-5979-4e09-a4ef-85e923cf2c1e" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityAddressStateOrProvince"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_2c314f7c-1921-41f0-9c9f-0f7cea34fbde" xlink:to="loc_dei_EntityAddressStateOrProvince_0896f9e2-5979-4e09-a4ef-85e923cf2c1e" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressPostalZipCode_7c9ecb49-da98-47bc-8aa1-d1d9cb0d4833" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityAddressPostalZipCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_2c314f7c-1921-41f0-9c9f-0f7cea34fbde" xlink:to="loc_dei_EntityAddressPostalZipCode_7c9ecb49-da98-47bc-8aa1-d1d9cb0d4833" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CityAreaCode_004db422-6de6-425b-9dbd-0579573d9634" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_CityAreaCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_2c314f7c-1921-41f0-9c9f-0f7cea34fbde" xlink:to="loc_dei_CityAreaCode_004db422-6de6-425b-9dbd-0579573d9634" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LocalPhoneNumber_57d884de-3025-4cd1-a0e6-6a3a7448ad3f" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_LocalPhoneNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_2c314f7c-1921-41f0-9c9f-0f7cea34fbde" xlink:to="loc_dei_LocalPhoneNumber_57d884de-3025-4cd1-a0e6-6a3a7448ad3f" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFileNumber_96f4df9e-3b11-45b8-a5af-dbc471aaf378" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityFileNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_2c314f7c-1921-41f0-9c9f-0f7cea34fbde" xlink:to="loc_dei_EntityFileNumber_96f4df9e-3b11-45b8-a5af-dbc471aaf378" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityRegistrantName_2dfb1335-962b-4a00-b646-c8ffa0ee8d1d" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityRegistrantName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_2c314f7c-1921-41f0-9c9f-0f7cea34fbde" xlink:to="loc_dei_EntityRegistrantName_2dfb1335-962b-4a00-b646-c8ffa0ee8d1d" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityTaxIdentificationNumber_1462f071-79d4-4e6d-acc8-940f388c430e" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityTaxIdentificationNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_2c314f7c-1921-41f0-9c9f-0f7cea34fbde" xlink:to="loc_dei_EntityTaxIdentificationNumber_1462f071-79d4-4e6d-acc8-940f388c430e" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityIncorporationStateCountryCode_29b82008-4d8d-4dc8-a187-4f694e737e0a" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityIncorporationStateCountryCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_2c314f7c-1921-41f0-9c9f-0f7cea34fbde" xlink:to="loc_dei_EntityIncorporationStateCountryCode_29b82008-4d8d-4dc8-a187-4f694e737e0a" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_Security12bTitle_176b61cf-1023-4b8d-abc5-b357f262dbc3" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_Security12bTitle"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_2c314f7c-1921-41f0-9c9f-0f7cea34fbde" xlink:to="loc_dei_Security12bTitle_176b61cf-1023-4b8d-abc5-b357f262dbc3" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_TradingSymbol_0d8fd1b9-475f-4ac7-b1b7-2b7ae786740f" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_TradingSymbol"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_2c314f7c-1921-41f0-9c9f-0f7cea34fbde" xlink:to="loc_dei_TradingSymbol_0d8fd1b9-475f-4ac7-b1b7-2b7ae786740f" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_SecurityExchangeName_31b79f99-00ca-4fd5-8575-7f6fcb913a22" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_SecurityExchangeName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_2c314f7c-1921-41f0-9c9f-0f7cea34fbde" xlink:to="loc_dei_SecurityExchangeName_31b79f99-00ca-4fd5-8575-7f6fcb913a22" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCurrentReportingStatus_b4f9661a-8339-4825-8558-9e515657ccb3" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityCurrentReportingStatus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_2c314f7c-1921-41f0-9c9f-0f7cea34fbde" xlink:to="loc_dei_EntityCurrentReportingStatus_b4f9661a-8339-4825-8558-9e515657ccb3" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityInteractiveDataCurrent_9ee1c94c-d04c-4725-8ab1-6ca3c51fd16d" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityInteractiveDataCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_2c314f7c-1921-41f0-9c9f-0f7cea34fbde" xlink:to="loc_dei_EntityInteractiveDataCurrent_9ee1c94c-d04c-4725-8ab1-6ca3c51fd16d" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFilerCategory_04cf55dd-abdf-44d6-8ac0-341c1f28845f" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityFilerCategory"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_2c314f7c-1921-41f0-9c9f-0f7cea34fbde" xlink:to="loc_dei_EntityFilerCategory_04cf55dd-abdf-44d6-8ac0-341c1f28845f" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntitySmallBusiness_286d6280-0bfc-43b1-8756-c015a006888a" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntitySmallBusiness"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_2c314f7c-1921-41f0-9c9f-0f7cea34fbde" xlink:to="loc_dei_EntitySmallBusiness_286d6280-0bfc-43b1-8756-c015a006888a" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityEmergingGrowthCompany_2e133e9f-75c5-4099-ac8d-97bee5f4f723" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityEmergingGrowthCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_2c314f7c-1921-41f0-9c9f-0f7cea34fbde" xlink:to="loc_dei_EntityEmergingGrowthCompany_2e133e9f-75c5-4099-ac8d-97bee5f4f723" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityShellCompany_c628cc8a-4747-4b4c-827f-a4cf2dc14589" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityShellCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_2c314f7c-1921-41f0-9c9f-0f7cea34fbde" xlink:to="loc_dei_EntityShellCompany_c628cc8a-4747-4b4c-827f-a4cf2dc14589" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCommonStockSharesOutstanding_13255a6d-23b4-4024-a155-a21d52f5b4c9" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityCommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_2c314f7c-1921-41f0-9c9f-0f7cea34fbde" xlink:to="loc_dei_EntityCommonStockSharesOutstanding_13255a6d-23b4-4024-a155-a21d52f5b4c9" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_AmendmentFlag_d30fc6ca-6970-4b97-9480-53e1b381adfb" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_AmendmentFlag"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_2c314f7c-1921-41f0-9c9f-0f7cea34fbde" xlink:to="loc_dei_AmendmentFlag_d30fc6ca-6970-4b97-9480-53e1b381adfb" xlink:type="arc" order="25"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCentralIndexKey_638ff7ab-40f9-4bbd-816e-2b71313793c6" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityCentralIndexKey"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_2c314f7c-1921-41f0-9c9f-0f7cea34fbde" xlink:to="loc_dei_EntityCentralIndexKey_638ff7ab-40f9-4bbd-816e-2b71313793c6" xlink:type="arc" order="26"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CurrentFiscalYearEndDate_4720b53d-374b-4049-91c1-92468988f6c4" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_CurrentFiscalYearEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_2c314f7c-1921-41f0-9c9f-0f7cea34fbde" xlink:to="loc_dei_CurrentFiscalYearEndDate_4720b53d-374b-4049-91c1-92468988f6c4" xlink:type="arc" order="27"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalPeriodFocus_96736f73-e4b2-4d71-a20a-ab877663cb6a" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_DocumentFiscalPeriodFocus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_2c314f7c-1921-41f0-9c9f-0f7cea34fbde" xlink:to="loc_dei_DocumentFiscalPeriodFocus_96736f73-e4b2-4d71-a20a-ab877663cb6a" xlink:type="arc" order="28"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalYearFocus_c5714c36-738d-4496-8356-67f4f8f11ea0" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_DocumentFiscalYearFocus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_2c314f7c-1921-41f0-9c9f-0f7cea34fbde" xlink:to="loc_dei_DocumentFiscalYearFocus_c5714c36-738d-4496-8356-67f4f8f11ea0" xlink:type="arc" order="29"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magn.com/role/ConsolidatedandCombinedStatementsofChangesinStockholdersEquity" xlink:type="simple" xlink:href="magn-20241228.xsd#ConsolidatedandCombinedStatementsofChangesinStockholdersEquity"/>
  <link:definitionLink xlink:role="http://www.magn.com/role/ConsolidatedandCombinedStatementsofChangesinStockholdersEquity" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_a4d8183e-c0f5-4795-b283-3bdafc1e0051" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_5a2d15b1-e6b7-4656-89d5-c5f003fca46f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_a4d8183e-c0f5-4795-b283-3bdafc1e0051" xlink:to="loc_us-gaap_StatementTable_5a2d15b1-e6b7-4656-89d5-c5f003fca46f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_3692e1ce-c017-4e3d-be87-cc41c2119706" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_5a2d15b1-e6b7-4656-89d5-c5f003fca46f" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_3692e1ce-c017-4e3d-be87-cc41c2119706" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_3692e1ce-c017-4e3d-be87-cc41c2119706_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_3692e1ce-c017-4e3d-be87-cc41c2119706" xlink:to="loc_us-gaap_EquityComponentDomain_3692e1ce-c017-4e3d-be87-cc41c2119706_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_4a71fa51-17ae-462d-a726-63473c364880" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_3692e1ce-c017-4e3d-be87-cc41c2119706" xlink:to="loc_us-gaap_EquityComponentDomain_4a71fa51-17ae-462d-a726-63473c364880" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MemberUnitsMember_361e1198-4ef9-405b-84c6-d663a07203f3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_MemberUnitsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_4a71fa51-17ae-462d-a726-63473c364880" xlink:to="loc_us-gaap_MemberUnitsMember_361e1198-4ef9-405b-84c6-d663a07203f3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_c4f2925c-7848-4067-ba06-906f82534244" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_4a71fa51-17ae-462d-a726-63473c364880" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_c4f2925c-7848-4067-ba06-906f82534244" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_ed3fa011-d733-4420-a9a1-b2c3bb800cc0" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_4a71fa51-17ae-462d-a726-63473c364880" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_ed3fa011-d733-4420-a9a1-b2c3bb800cc0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_2c2ee897-2fc5-466d-b376-f378e833bc56" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_4a71fa51-17ae-462d-a726-63473c364880" xlink:to="loc_us-gaap_RetainedEarningsMember_2c2ee897-2fc5-466d-b376-f378e833bc56" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_beba10e7-26ac-4d87-89fe-d35e411e72b1" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_a4d8183e-c0f5-4795-b283-3bdafc1e0051" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_beba10e7-26ac-4d87-89fe-d35e411e72b1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_ace85840-496e-4dea-9f83-6d9d7e8637b1" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_beba10e7-26ac-4d87-89fe-d35e411e72b1" xlink:to="loc_us-gaap_StockholdersEquity_ace85840-496e-4dea-9f83-6d9d7e8637b1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_d2046c14-4145-4b40-9d3b-04f03c09ec79" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_beba10e7-26ac-4d87-89fe-d35e411e72b1" xlink:to="loc_us-gaap_NetIncomeLoss_d2046c14-4145-4b40-9d3b-04f03c09ec79" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_ba7329fd-401a-454d-995c-41e46d242ebd" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_beba10e7-26ac-4d87-89fe-d35e411e72b1" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_ba7329fd-401a-454d-995c-41e46d242ebd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_TransfersToParentNet_dd190644-64df-43a6-abbc-e6f404acfcca" xlink:href="magn-20241228.xsd#magn_TransfersToParentNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_beba10e7-26ac-4d87-89fe-d35e411e72b1" xlink:to="loc_magn_TransfersToParentNet_dd190644-64df-43a6-abbc-e6f404acfcca" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_TransfersFromParent_e1b1f836-638f-4246-81d3-568ec6c4ea7e" xlink:href="magn-20241228.xsd#magn_TransfersFromParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_beba10e7-26ac-4d87-89fe-d35e411e72b1" xlink:to="loc_magn_TransfersFromParent_e1b1f836-638f-4246-81d3-568ec6c4ea7e" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_DistributionOfParentsNetInvestment_08a142c8-fe19-4816-8c32-c399b9e4f6f7" xlink:href="magn-20241228.xsd#magn_DistributionOfParentsNetInvestment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_beba10e7-26ac-4d87-89fe-d35e411e72b1" xlink:to="loc_magn_DistributionOfParentsNetInvestment_08a142c8-fe19-4816-8c32-c399b9e4f6f7" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_AdjustmentsToAdditionalPaidInCapitalAcquisition_43b23a86-637e-4686-b2e1-b740c049612a" xlink:href="magn-20241228.xsd#magn_AdjustmentsToAdditionalPaidInCapitalAcquisition"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_beba10e7-26ac-4d87-89fe-d35e411e72b1" xlink:to="loc_magn_AdjustmentsToAdditionalPaidInCapitalAcquisition_43b23a86-637e-4686-b2e1-b740c049612a" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_3de972e3-71c6-462a-a480-c60219864949" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_beba10e7-26ac-4d87-89fe-d35e411e72b1" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_3de972e3-71c6-462a-a480-c60219864949" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_3dbe963f-2120-4427-a4af-90ec1a4e85e8" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StockholdersEquity"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magn.com/role/BasisofPresentationDetails" xlink:type="simple" xlink:href="magn-20241228.xsd#BasisofPresentationDetails"/>
  <link:definitionLink xlink:role="http://www.magn.com/role/BasisofPresentationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestEffectsOfChangesNetLineItems_fe118edd-fd58-44aa-960e-aa8d8f794129" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestEffectsOfChangesNetLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestEffectsOfChangesNetTable_4cc14922-82aa-4560-9068-7407fb5e01c4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestEffectsOfChangesNetTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestEffectsOfChangesNetLineItems_fe118edd-fd58-44aa-960e-aa8d8f794129" xlink:to="loc_us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestEffectsOfChangesNetTable_4cc14922-82aa-4560-9068-7407fb5e01c4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_OwnershipAxis_2c784195-ebbe-4d12-8612-3b16e0354cde" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_OwnershipAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestEffectsOfChangesNetTable_4cc14922-82aa-4560-9068-7407fb5e01c4" xlink:to="loc_srt_OwnershipAxis_2c784195-ebbe-4d12-8612-3b16e0354cde" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_OwnershipDomain_2c784195-ebbe-4d12-8612-3b16e0354cde_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_OwnershipDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_OwnershipAxis_2c784195-ebbe-4d12-8612-3b16e0354cde" xlink:to="loc_srt_OwnershipDomain_2c784195-ebbe-4d12-8612-3b16e0354cde_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_OwnershipDomain_14bae544-01f4-4a24-bfe8-f0e299879e6b" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_OwnershipDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_OwnershipAxis_2c784195-ebbe-4d12-8612-3b16e0354cde" xlink:to="loc_srt_OwnershipDomain_14bae544-01f4-4a24-bfe8-f0e299879e6b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_MagneraCorporationMember_65f90af3-0ee3-4a72-bdee-f1400db471d2" xlink:href="magn-20241228.xsd#magn_MagneraCorporationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_OwnershipDomain_14bae544-01f4-4a24-bfe8-f0e299879e6b" xlink:to="loc_magn_MagneraCorporationMember_65f90af3-0ee3-4a72-bdee-f1400db471d2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_TreasureHoldcoInc.Member_1315c2fe-ee44-44fd-8ddd-097348b8b8cd" xlink:href="magn-20241228.xsd#magn_TreasureHoldcoInc.Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_OwnershipDomain_14bae544-01f4-4a24-bfe8-f0e299879e6b" xlink:to="loc_magn_TreasureHoldcoInc.Member_1315c2fe-ee44-44fd-8ddd-097348b8b8cd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_9d3c9d07-4c80-4d86-abf9-b1802e5e837e" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestEffectsOfChangesNetTable_4cc14922-82aa-4560-9068-7407fb5e01c4" xlink:to="loc_dei_LegalEntityAxis_9d3c9d07-4c80-4d86-abf9-b1802e5e837e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_9d3c9d07-4c80-4d86-abf9-b1802e5e837e_default" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_9d3c9d07-4c80-4d86-abf9-b1802e5e837e" xlink:to="loc_dei_EntityDomain_9d3c9d07-4c80-4d86-abf9-b1802e5e837e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_b4b94468-4c5d-4d0d-bcac-8351440d0d76" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_9d3c9d07-4c80-4d86-abf9-b1802e5e837e" xlink:to="loc_dei_EntityDomain_b4b94468-4c5d-4d0d-bcac-8351440d0d76" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_TreasureShareholdersMember_8c1f1543-87f7-4528-b640-7298ec1c3ab0" xlink:href="magn-20241228.xsd#magn_TreasureShareholdersMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_b4b94468-4c5d-4d0d-bcac-8351440d0d76" xlink:to="loc_magn_TreasureShareholdersMember_8c1f1543-87f7-4528-b640-7298ec1c3ab0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_GlatfelterShareholdersMember_bc281a53-a437-4dc1-b65b-6905f7adc221" xlink:href="magn-20241228.xsd#magn_GlatfelterShareholdersMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_b4b94468-4c5d-4d0d-bcac-8351440d0d76" xlink:to="loc_magn_GlatfelterShareholdersMember_bc281a53-a437-4dc1-b65b-6905f7adc221" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_BerryGlobalGroupInc.Member_df672c46-22f7-4195-8eeb-7c51ff61985a" xlink:href="magn-20241228.xsd#magn_BerryGlobalGroupInc.Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_b4b94468-4c5d-4d0d-bcac-8351440d0d76" xlink:to="loc_magn_BerryGlobalGroupInc.Member_df672c46-22f7-4195-8eeb-7c51ff61985a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LimitedLiabilityCompanyLLCOrLimitedPartnershipLPMembersOrLimitedPartnersOwnershipInterest_27591856-5736-47f8-a5f0-2d4e6b1e9735" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LimitedLiabilityCompanyLLCOrLimitedPartnershipLPMembersOrLimitedPartnersOwnershipInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestEffectsOfChangesNetLineItems_fe118edd-fd58-44aa-960e-aa8d8f794129" xlink:to="loc_us-gaap_LimitedLiabilityCompanyLLCOrLimitedPartnershipLPMembersOrLimitedPartnersOwnershipInterest_27591856-5736-47f8-a5f0-2d4e6b1e9735" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magn.com/role/AcquisitionScheduleofRecognizedIdentifiedAssetsAcquiredandLiabilitiesAssumedDetails" xlink:type="simple" xlink:href="magn-20241228.xsd#AcquisitionScheduleofRecognizedIdentifiedAssetsAcquiredandLiabilitiesAssumedDetails"/>
  <link:definitionLink xlink:role="http://www.magn.com/role/AcquisitionScheduleofRecognizedIdentifiedAssetsAcquiredandLiabilitiesAssumedDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetAcquisitionLineItems_1fa6274d-0c2a-454c-ac21-c754200c78e5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AssetAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetAcquisitionTable_03e23fe2-91f9-43f6-928e-fa4be7942ab1" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AssetAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AssetAcquisitionLineItems_1fa6274d-0c2a-454c-ac21-c754200c78e5" xlink:to="loc_us-gaap_AssetAcquisitionTable_03e23fe2-91f9-43f6-928e-fa4be7942ab1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_b35a6346-de42-4c09-afcf-c00aff281559" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_AssetAcquisitionTable_03e23fe2-91f9-43f6-928e-fa4be7942ab1" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_b35a6346-de42-4c09-afcf-c00aff281559" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_b35a6346-de42-4c09-afcf-c00aff281559_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_b35a6346-de42-4c09-afcf-c00aff281559" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_b35a6346-de42-4c09-afcf-c00aff281559_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_7c6f56c3-4508-4db5-97c7-64290a629d1e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_b35a6346-de42-4c09-afcf-c00aff281559" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_7c6f56c3-4508-4db5-97c7-64290a629d1e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_GLTAcquisitionMember_4a88f859-204b-40eb-a650-cd841a7fd306" xlink:href="magn-20241228.xsd#magn_GLTAcquisitionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_7c6f56c3-4508-4db5-97c7-64290a629d1e" xlink:to="loc_magn_GLTAcquisitionMember_4a88f859-204b-40eb-a650-cd841a7fd306" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_3e85f22b-e3d7-4a26-8dd2-45fd199bf3e1" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_AssetAcquisitionTable_03e23fe2-91f9-43f6-928e-fa4be7942ab1" xlink:to="loc_dei_LegalEntityAxis_3e85f22b-e3d7-4a26-8dd2-45fd199bf3e1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_3e85f22b-e3d7-4a26-8dd2-45fd199bf3e1_default" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_3e85f22b-e3d7-4a26-8dd2-45fd199bf3e1" xlink:to="loc_dei_EntityDomain_3e85f22b-e3d7-4a26-8dd2-45fd199bf3e1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_bd65b683-6e99-4f3e-9ca1-88725367e0f8" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_3e85f22b-e3d7-4a26-8dd2-45fd199bf3e1" xlink:to="loc_dei_EntityDomain_bd65b683-6e99-4f3e-9ca1-88725367e0f8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_BerryGlobalGroupInc.Member_eac44c27-7c27-41db-a3fc-264123352a3f" xlink:href="magn-20241228.xsd#magn_BerryGlobalGroupInc.Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_bd65b683-6e99-4f3e-9ca1-88725367e0f8" xlink:to="loc_magn_BerryGlobalGroupInc.Member_eac44c27-7c27-41db-a3fc-264123352a3f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable_e88cb578-af47-44f5-b358-f4efd95a94c5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetAcquisitionLineItems_1fa6274d-0c2a-454c-ac21-c754200c78e5" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable_e88cb578-af47-44f5-b358-f4efd95a94c5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_e20281a3-2050-419f-8e34-c550b9b77a44" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetAcquisitionLineItems_1fa6274d-0c2a-454c-ac21-c754200c78e5" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_e20281a3-2050-419f-8e34-c550b9b77a44" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents_86872dd5-3d00-4a23-b077-65fb9da36442" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_e20281a3-2050-419f-8e34-c550b9b77a44" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents_86872dd5-3d00-4a23-b077-65fb9da36442" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedWorkingCapital_4ff434ce-fda1-4e23-b63b-42651047fb35" xlink:href="magn-20241228.xsd#magn_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedWorkingCapital"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_e20281a3-2050-419f-8e34-c550b9b77a44" xlink:to="loc_magn_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedWorkingCapital_4ff434ce-fda1-4e23-b63b-42651047fb35" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment_4a37afbd-4738-4758-9b84-a2d47332f87f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_e20281a3-2050-419f-8e34-c550b9b77a44" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment_4a37afbd-4738-4758-9b84-a2d47332f87f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_f444773b-1175-4dfa-9c9e-e75eebf50ddf" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_e20281a3-2050-419f-8e34-c550b9b77a44" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_f444773b-1175-4dfa-9c9e-e75eebf50ddf" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherNoncurrentAssets_24acb61a-ccee-4aad-b9f7-4ac396e2b0de" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherNoncurrentAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_e20281a3-2050-419f-8e34-c550b9b77a44" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherNoncurrentAssets_24acb61a-ccee-4aad-b9f7-4ac396e2b0de" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther_85c28ef4-72d1-45b7-9cb4-712d3be0a001" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_e20281a3-2050-419f-8e34-c550b9b77a44" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther_85c28ef4-72d1-45b7-9cb4-712d3be0a001" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesLongTermDebt_515c5374-deb4-4fdb-93b9-5a1a7fe242e6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesLongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_e20281a3-2050-419f-8e34-c550b9b77a44" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesLongTermDebt_515c5374-deb4-4fdb-93b9-5a1a7fe242e6" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_e7fc2a71-0319-482b-8b5e-8cd16f9bdb29" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_e20281a3-2050-419f-8e34-c550b9b77a44" xlink:to="loc_us-gaap_Goodwill_e7fc2a71-0319-482b-8b5e-8cd16f9bdb29" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_79059a59-1bc7-4e0b-91fe-e596ea3324eb" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_e20281a3-2050-419f-8e34-c550b9b77a44" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_79059a59-1bc7-4e0b-91fe-e596ea3324eb" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentInventory_e01db69c-378c-4203-801e-a65fcf99f76d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentInventory"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetAcquisitionLineItems_1fa6274d-0c2a-454c-ac21-c754200c78e5" xlink:to="loc_us-gaap_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentInventory_e01db69c-378c-4203-801e-a65fcf99f76d" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magn.com/role/AcquisitionNarrativeDetails" xlink:type="simple" xlink:href="magn-20241228.xsd#AcquisitionNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.magn.com/role/AcquisitionNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetAcquisitionLineItems_b9dd139f-6c25-4a5d-8c60-ca43ce1e0368" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AssetAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetAcquisitionTable_e2c3c2e2-225d-4afa-9f16-d0c724bde1ea" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AssetAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AssetAcquisitionLineItems_b9dd139f-6c25-4a5d-8c60-ca43ce1e0368" xlink:to="loc_us-gaap_AssetAcquisitionTable_e2c3c2e2-225d-4afa-9f16-d0c724bde1ea" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_5fe1344c-fe66-4bc0-a650-d8ff6c54e4b1" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_AssetAcquisitionTable_e2c3c2e2-225d-4afa-9f16-d0c724bde1ea" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_5fe1344c-fe66-4bc0-a650-d8ff6c54e4b1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_5fe1344c-fe66-4bc0-a650-d8ff6c54e4b1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_5fe1344c-fe66-4bc0-a650-d8ff6c54e4b1" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_5fe1344c-fe66-4bc0-a650-d8ff6c54e4b1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_5b30c0f0-953c-4841-8bb1-9d6e1db867ae" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_5fe1344c-fe66-4bc0-a650-d8ff6c54e4b1" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_5b30c0f0-953c-4841-8bb1-9d6e1db867ae" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_GLTAcquisitionMember_e6c967c3-c15b-4e41-ac37-89cff57b42aa" xlink:href="magn-20241228.xsd#magn_GLTAcquisitionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_5b30c0f0-953c-4841-8bb1-9d6e1db867ae" xlink:to="loc_magn_GLTAcquisitionMember_e6c967c3-c15b-4e41-ac37-89cff57b42aa" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_aae46d79-7c25-4388-b0d0-9f53a6c306ee" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_AssetAcquisitionTable_e2c3c2e2-225d-4afa-9f16-d0c724bde1ea" xlink:to="loc_dei_LegalEntityAxis_aae46d79-7c25-4388-b0d0-9f53a6c306ee" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_aae46d79-7c25-4388-b0d0-9f53a6c306ee_default" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_aae46d79-7c25-4388-b0d0-9f53a6c306ee" xlink:to="loc_dei_EntityDomain_aae46d79-7c25-4388-b0d0-9f53a6c306ee_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_2325fd5a-4c64-4b2b-804b-a197424edc27" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_aae46d79-7c25-4388-b0d0-9f53a6c306ee" xlink:to="loc_dei_EntityDomain_2325fd5a-4c64-4b2b-804b-a197424edc27" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_BerryGlobalGroupInc.Member_0967918b-c50d-460f-9583-b14594efefec" xlink:href="magn-20241228.xsd#magn_BerryGlobalGroupInc.Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_2325fd5a-4c64-4b2b-804b-a197424edc27" xlink:to="loc_magn_BerryGlobalGroupInc.Member_0967918b-c50d-460f-9583-b14594efefec" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionsProFormaRevenue_2b00d88d-78b2-4d22-9d84-c5999cd18052" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BusinessAcquisitionsProFormaRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetAcquisitionLineItems_b9dd139f-6c25-4a5d-8c60-ca43ce1e0368" xlink:to="loc_us-gaap_BusinessAcquisitionsProFormaRevenue_2b00d88d-78b2-4d22-9d84-c5999cd18052" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss_700dfb39-6fd9-4278-9b09-7d8b29722f94" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetAcquisitionLineItems_b9dd139f-6c25-4a5d-8c60-ca43ce1e0368" xlink:to="loc_us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss_700dfb39-6fd9-4278-9b09-7d8b29722f94" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magn.com/role/TransactionandOtherActivitiesScheduleofRestructuringandRelatedCostsDetails" xlink:type="simple" xlink:href="magn-20241228.xsd#TransactionandOtherActivitiesScheduleofRestructuringandRelatedCostsDetails"/>
  <link:definitionLink xlink:role="http://www.magn.com/role/TransactionandOtherActivitiesScheduleofRestructuringandRelatedCostsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveLineItems_0aee0f51-2933-4584-b47a-c95f8d68927a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestructuringCostAndReserveLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_8e795a75-6aa8-466e-898d-e9579db17e0b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfRestructuringAndRelatedCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_0aee0f51-2933-4584-b47a-c95f8d68927a" xlink:to="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_8e795a75-6aa8-466e-898d-e9579db17e0b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_44b89e82-be08-4fa1-ba2c-b1039b22ecf5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_8e795a75-6aa8-466e-898d-e9579db17e0b" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_44b89e82-be08-4fa1-ba2c-b1039b22ecf5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_44b89e82-be08-4fa1-ba2c-b1039b22ecf5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_44b89e82-be08-4fa1-ba2c-b1039b22ecf5" xlink:to="loc_us-gaap_SegmentDomain_44b89e82-be08-4fa1-ba2c-b1039b22ecf5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_01c9d3ed-c7ff-4c3b-adef-9c9dd4a2fa12" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_44b89e82-be08-4fa1-ba2c-b1039b22ecf5" xlink:to="loc_us-gaap_SegmentDomain_01c9d3ed-c7ff-4c3b-adef-9c9dd4a2fa12" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_AmericasSegmentMember_7e1bb2c5-4ba1-49d4-9c4b-a6ee2cf80c5e" xlink:href="magn-20241228.xsd#magn_AmericasSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_01c9d3ed-c7ff-4c3b-adef-9c9dd4a2fa12" xlink:to="loc_magn_AmericasSegmentMember_7e1bb2c5-4ba1-49d4-9c4b-a6ee2cf80c5e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_RestOfWorldSegmentMember_20dfba78-45d8-47a1-abdf-ef3684a9eae3" xlink:href="magn-20241228.xsd#magn_RestOfWorldSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_01c9d3ed-c7ff-4c3b-adef-9c9dd4a2fa12" xlink:to="loc_magn_RestOfWorldSegmentMember_20dfba78-45d8-47a1-abdf-ef3684a9eae3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCharges_aa74f806-fab6-49f1-b699-0d08962dc00b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestructuringCharges"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_0aee0f51-2933-4584-b47a-c95f8d68927a" xlink:to="loc_us-gaap_RestructuringCharges_aa74f806-fab6-49f1-b699-0d08962dc00b" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magn.com/role/TransactionandOtherActivitiesScheduleofRestructuringReservebyTypeofCostDetails" xlink:type="simple" xlink:href="magn-20241228.xsd#TransactionandOtherActivitiesScheduleofRestructuringReservebyTypeofCostDetails"/>
  <link:definitionLink xlink:role="http://www.magn.com/role/TransactionandOtherActivitiesScheduleofRestructuringReservebyTypeofCostDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveLineItems_32f3a40f-158b-4c16-bde9-99b8e0bc9934" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestructuringCostAndReserveLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_559d51eb-67c8-4f91-9209-d38e67dbb90e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfRestructuringAndRelatedCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_32f3a40f-158b-4c16-bde9-99b8e0bc9934" xlink:to="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_559d51eb-67c8-4f91-9209-d38e67dbb90e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveAxis_7925db17-5050-4381-9623-8eb53a15857a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestructuringCostAndReserveAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_559d51eb-67c8-4f91-9209-d38e67dbb90e" xlink:to="loc_us-gaap_RestructuringCostAndReserveAxis_7925db17-5050-4381-9623-8eb53a15857a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_7925db17-5050-4381-9623-8eb53a15857a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_TypeOfRestructuringDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_7925db17-5050-4381-9623-8eb53a15857a" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_7925db17-5050-4381-9623-8eb53a15857a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_33134616-662b-4fc6-96d4-03c5de93e35d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_TypeOfRestructuringDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_7925db17-5050-4381-9623-8eb53a15857a" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_33134616-662b-4fc6-96d4-03c5de93e35d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeSeveranceMember_dba7b170-8517-4b4b-9797-fdc79f35b383" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EmployeeSeveranceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_33134616-662b-4fc6-96d4-03c5de93e35d" xlink:to="loc_us-gaap_EmployeeSeveranceMember_dba7b170-8517-4b4b-9797-fdc79f35b383" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_TransactionActivitiesMember_976d50ae-fff6-4c0a-8878-c5472ef14cf4" xlink:href="magn-20241228.xsd#magn_TransactionActivitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_33134616-662b-4fc6-96d4-03c5de93e35d" xlink:to="loc_magn_TransactionActivitiesMember_976d50ae-fff6-4c0a-8878-c5472ef14cf4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherRestructuringMember_3f7e606d-bc3d-48c4-a6a6-a639391328df" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherRestructuringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_33134616-662b-4fc6-96d4-03c5de93e35d" xlink:to="loc_us-gaap_OtherRestructuringMember_3f7e606d-bc3d-48c4-a6a6-a639391328df" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringReserveRollForward_894b204c-559d-4b83-9bec-594cca579191" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestructuringReserveRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_32f3a40f-158b-4c16-bde9-99b8e0bc9934" xlink:to="loc_us-gaap_RestructuringReserveRollForward_894b204c-559d-4b83-9bec-594cca579191" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringReserve_a16d5fd3-2078-492f-9816-e5c94e2dc0cb" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestructuringReserve"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringReserveRollForward_894b204c-559d-4b83-9bec-594cca579191" xlink:to="loc_us-gaap_RestructuringReserve_a16d5fd3-2078-492f-9816-e5c94e2dc0cb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCharges_ca671d57-a27c-4ac0-a585-4dad2e84511f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestructuringCharges"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringReserveRollForward_894b204c-559d-4b83-9bec-594cca579191" xlink:to="loc_us-gaap_RestructuringCharges_ca671d57-a27c-4ac0-a585-4dad2e84511f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsForRestructuring_036af2db-30ee-438b-b10d-72745db849b0" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PaymentsForRestructuring"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringReserveRollForward_894b204c-559d-4b83-9bec-594cca579191" xlink:to="loc_us-gaap_PaymentsForRestructuring_036af2db-30ee-438b-b10d-72745db849b0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringReserve_44db8d5a-f0c7-40ed-8aa2-7068c76b9a76" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestructuringReserve"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_TransactionBonuses_0b03f286-a100-4228-8713-c84660b19a21" xlink:href="magn-20241228.xsd#magn_TransactionBonuses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_32f3a40f-158b-4c16-bde9-99b8e0bc9934" xlink:to="loc_magn_TransactionBonuses_0b03f286-a100-4228-8713-c84660b19a21" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magn.com/role/LongTermDebtScheduleofLongtermDebtDetails" xlink:type="simple" xlink:href="magn-20241228.xsd#LongTermDebtScheduleofLongtermDebtDetails"/>
  <link:definitionLink xlink:role="http://www.magn.com/role/LongTermDebtScheduleofLongtermDebtDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_1a88440f-b105-4020-bfd0-528e4a764651" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_b1611cbc-45ad-43c8-ae6f-a2e45af40a15" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_1a88440f-b105-4020-bfd0-528e4a764651" xlink:to="loc_us-gaap_DebtInstrumentTable_b1611cbc-45ad-43c8-ae6f-a2e45af40a15" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_47635725-813c-4622-84ab-04c9f15726a7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_b1611cbc-45ad-43c8-ae6f-a2e45af40a15" xlink:to="loc_us-gaap_DebtInstrumentAxis_47635725-813c-4622-84ab-04c9f15726a7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_47635725-813c-4622-84ab-04c9f15726a7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_47635725-813c-4622-84ab-04c9f15726a7" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_47635725-813c-4622-84ab-04c9f15726a7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_c8e80beb-830d-4d7c-b611-d74ff418d7cd" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_47635725-813c-4622-84ab-04c9f15726a7" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_c8e80beb-830d-4d7c-b611-d74ff418d7cd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_RevolvingCreditFacilityDueNovember2029Member_f17d4e3a-58e0-4937-98b7-52d52feaa170" xlink:href="magn-20241228.xsd#magn_RevolvingCreditFacilityDueNovember2029Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_c8e80beb-830d-4d7c-b611-d74ff418d7cd" xlink:to="loc_magn_RevolvingCreditFacilityDueNovember2029Member_f17d4e3a-58e0-4937-98b7-52d52feaa170" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_TermLoanDueNovember2031Member_bd8125b1-fdaf-4bcc-8ebf-5c71306448ae" xlink:href="magn-20241228.xsd#magn_TermLoanDueNovember2031Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_c8e80beb-830d-4d7c-b611-d74ff418d7cd" xlink:to="loc_magn_TermLoanDueNovember2031Member_bd8125b1-fdaf-4bcc-8ebf-5c71306448ae" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_A7.250FirstPrioritySeniorSecuredNotesMember_0ba8d171-2b5f-4622-825e-6b6a61bdd73f" xlink:href="magn-20241228.xsd#magn_A7.250FirstPrioritySeniorSecuredNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_c8e80beb-830d-4d7c-b611-d74ff418d7cd" xlink:to="loc_magn_A7.250FirstPrioritySeniorSecuredNotesMember_0ba8d171-2b5f-4622-825e-6b6a61bdd73f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_A4.75FirstPrioritySeniorSecuredNotesMember_af5e5289-5b04-4715-89d9-631595aa8c4f" xlink:href="magn-20241228.xsd#magn_A4.75FirstPrioritySeniorSecuredNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_c8e80beb-830d-4d7c-b611-d74ff418d7cd" xlink:to="loc_magn_A4.75FirstPrioritySeniorSecuredNotesMember_af5e5289-5b04-4715-89d9-631595aa8c4f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_c703671a-fa6d-43c2-a134-0dacc9acd3c6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_b1611cbc-45ad-43c8-ae6f-a2e45af40a15" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_c703671a-fa6d-43c2-a134-0dacc9acd3c6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_c703671a-fa6d-43c2-a134-0dacc9acd3c6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_c703671a-fa6d-43c2-a134-0dacc9acd3c6" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_c703671a-fa6d-43c2-a134-0dacc9acd3c6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_7cad9a65-0519-4baf-b4fb-de92ac96fddf" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_c703671a-fa6d-43c2-a134-0dacc9acd3c6" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_7cad9a65-0519-4baf-b4fb-de92ac96fddf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_78273d7c-c642-4d7f-b948-f2b489a4d105" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_7cad9a65-0519-4baf-b4fb-de92ac96fddf" xlink:to="loc_us-gaap_LineOfCreditMember_78273d7c-c642-4d7f-b948-f2b489a4d105" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeniorNotesMember_b4eb6e6e-b4a2-450a-9407-877a81e5286d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SeniorNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_7cad9a65-0519-4baf-b4fb-de92ac96fddf" xlink:to="loc_us-gaap_SeniorNotesMember_b4eb6e6e-b4a2-450a-9407-877a81e5286d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_d84b508b-a53e-42fc-b3f2-76eede5a3f38" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_1a88440f-b105-4020-bfd0-528e4a764651" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_d84b508b-a53e-42fc-b3f2-76eede5a3f38" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_11f6cf48-e592-4d46-adb2-54013de8f007" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_1a88440f-b105-4020-bfd0-528e4a764651" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_11f6cf48-e592-4d46-adb2-54013de8f007" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet_80d85cfe-61c4-434e-bf16-8aadf0a2a4a2" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_1a88440f-b105-4020-bfd0-528e4a764651" xlink:to="loc_us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet_80d85cfe-61c4-434e-bf16-8aadf0a2a4a2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebt_981e18ad-8a55-4a01-abb6-9d358d0b0068" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_1a88440f-b105-4020-bfd0-528e4a764651" xlink:to="loc_us-gaap_LongTermDebt_981e18ad-8a55-4a01-abb6-9d358d0b0068" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtCurrent_ad2831b6-08ec-4088-a646-dda8bc2af0b4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LongTermDebtCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_1a88440f-b105-4020-bfd0-528e4a764651" xlink:to="loc_us-gaap_LongTermDebtCurrent_ad2831b6-08ec-4088-a646-dda8bc2af0b4" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtNoncurrent_3586cd79-6625-451c-82f4-6e514ed95895" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LongTermDebtNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_1a88440f-b105-4020-bfd0-528e4a764651" xlink:to="loc_us-gaap_LongTermDebtNoncurrent_3586cd79-6625-451c-82f4-6e514ed95895" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magn.com/role/LongTermDebtAdditionalInformationDetails" xlink:type="simple" xlink:href="magn-20241228.xsd#LongTermDebtAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://www.magn.com/role/LongTermDebtAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_3e31e327-38e2-4a42-b7a1-0cb7acf65e18" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_c54a86e6-729e-44bc-9bae-7aca9d2c218c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_3e31e327-38e2-4a42-b7a1-0cb7acf65e18" xlink:to="loc_us-gaap_DebtInstrumentTable_c54a86e6-729e-44bc-9bae-7aca9d2c218c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_ef8b64b1-d5b0-46fb-a6ec-21cf25753816" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_c54a86e6-729e-44bc-9bae-7aca9d2c218c" xlink:to="loc_us-gaap_DebtInstrumentAxis_ef8b64b1-d5b0-46fb-a6ec-21cf25753816" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_ef8b64b1-d5b0-46fb-a6ec-21cf25753816_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_ef8b64b1-d5b0-46fb-a6ec-21cf25753816" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_ef8b64b1-d5b0-46fb-a6ec-21cf25753816_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_a0e52c49-2f85-4c2d-8685-45fa491e9e9a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_ef8b64b1-d5b0-46fb-a6ec-21cf25753816" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_a0e52c49-2f85-4c2d-8685-45fa491e9e9a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_TermLoanDueNovember2031Member_9c519414-bf66-4f3e-a2a3-fe4a6310aa2e" xlink:href="magn-20241228.xsd#magn_TermLoanDueNovember2031Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_a0e52c49-2f85-4c2d-8685-45fa491e9e9a" xlink:to="loc_magn_TermLoanDueNovember2031Member_9c519414-bf66-4f3e-a2a3-fe4a6310aa2e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_A7.250FirstPrioritySeniorSecuredNotesMember_e277d8e0-1449-4653-b3aa-fed7e3c16eee" xlink:href="magn-20241228.xsd#magn_A7.250FirstPrioritySeniorSecuredNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_a0e52c49-2f85-4c2d-8685-45fa491e9e9a" xlink:to="loc_magn_A7.250FirstPrioritySeniorSecuredNotesMember_e277d8e0-1449-4653-b3aa-fed7e3c16eee" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_RevolvingCreditFacilityDueNovember2029Member_839542dd-e1fd-4619-b056-0a5c5e4de522" xlink:href="magn-20241228.xsd#magn_RevolvingCreditFacilityDueNovember2029Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_a0e52c49-2f85-4c2d-8685-45fa491e9e9a" xlink:to="loc_magn_RevolvingCreditFacilityDueNovember2029Member_839542dd-e1fd-4619-b056-0a5c5e4de522" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_A4.75FirstPrioritySeniorSecuredNotesMember_fbed3ec1-530f-43c9-b6c7-7972e3e6bb26" xlink:href="magn-20241228.xsd#magn_A4.75FirstPrioritySeniorSecuredNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_a0e52c49-2f85-4c2d-8685-45fa491e9e9a" xlink:to="loc_magn_A4.75FirstPrioritySeniorSecuredNotesMember_fbed3ec1-530f-43c9-b6c7-7972e3e6bb26" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_227e1d1c-64e0-4f83-ad66-a1c154e3ecea" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_c54a86e6-729e-44bc-9bae-7aca9d2c218c" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_227e1d1c-64e0-4f83-ad66-a1c154e3ecea" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_227e1d1c-64e0-4f83-ad66-a1c154e3ecea_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_227e1d1c-64e0-4f83-ad66-a1c154e3ecea" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_227e1d1c-64e0-4f83-ad66-a1c154e3ecea_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_de1079ed-9356-4310-94b0-3c03e4737a3f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_227e1d1c-64e0-4f83-ad66-a1c154e3ecea" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_de1079ed-9356-4310-94b0-3c03e4737a3f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_50713eeb-3bc6-4e80-a32e-0407bcc669ff" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_de1079ed-9356-4310-94b0-3c03e4737a3f" xlink:to="loc_us-gaap_LineOfCreditMember_50713eeb-3bc6-4e80-a32e-0407bcc669ff" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeniorNotesMember_ea7a63b7-2224-4c21-81e0-d399328d9f62" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SeniorNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_de1079ed-9356-4310-94b0-3c03e4737a3f" xlink:to="loc_us-gaap_SeniorNotesMember_ea7a63b7-2224-4c21-81e0-d399328d9f62" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_0a6989f4-805a-4bc6-9a62-d4c4c97e12c3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_3e31e327-38e2-4a42-b7a1-0cb7acf65e18" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_0a6989f4-805a-4bc6-9a62-d4c4c97e12c3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_7c80a5e2-5857-4ee8-9c90-443497d413da" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_3e31e327-38e2-4a42-b7a1-0cb7acf65e18" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_7c80a5e2-5857-4ee8-9c90-443497d413da" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magn.com/role/FinancialInstrumentsandFairValueMeasurementsNarrativeDetails" xlink:type="simple" xlink:href="magn-20241228.xsd#FinancialInstrumentsandFairValueMeasurementsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.magn.com/role/FinancialInstrumentsandFairValueMeasurementsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_fb0ebc47-9a37-4092-8d58-651e70778a0d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByBalanceSheetGroupingTable_ef654f7e-2bba-4e70-b059-a14f5605e7b7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueByBalanceSheetGroupingTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_fb0ebc47-9a37-4092-8d58-651e70778a0d" xlink:to="loc_us-gaap_FairValueByBalanceSheetGroupingTable_ef654f7e-2bba-4e70-b059-a14f5605e7b7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_f8fb43ba-31a9-4102-a4f0-da14f5570c94" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueByBalanceSheetGroupingTable_ef654f7e-2bba-4e70-b059-a14f5605e7b7" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_f8fb43ba-31a9-4102-a4f0-da14f5570c94" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_f8fb43ba-31a9-4102-a4f0-da14f5570c94_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_f8fb43ba-31a9-4102-a4f0-da14f5570c94" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_f8fb43ba-31a9-4102-a4f0-da14f5570c94_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_c73ad2f6-20ca-4cca-a20e-b0198f4eed82" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_f8fb43ba-31a9-4102-a4f0-da14f5570c94" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_c73ad2f6-20ca-4cca-a20e-b0198f4eed82" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_CrossCurrencySwapMaturingNovember2027Member_774253ae-22a7-42fe-bf12-39145a56f35e" xlink:href="magn-20241228.xsd#magn_CrossCurrencySwapMaturingNovember2027Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_c73ad2f6-20ca-4cca-a20e-b0198f4eed82" xlink:to="loc_magn_CrossCurrencySwapMaturingNovember2027Member_774253ae-22a7-42fe-bf12-39145a56f35e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_CrossCurrencySwapMaturingNovember2029Member_28ceca0a-4dcc-4df9-82ad-bd58565818fe" xlink:href="magn-20241228.xsd#magn_CrossCurrencySwapMaturingNovember2029Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_c73ad2f6-20ca-4cca-a20e-b0198f4eed82" xlink:to="loc_magn_CrossCurrencySwapMaturingNovember2029Member_28ceca0a-4dcc-4df9-82ad-bd58565818fe" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeNotionalAmount_70ce7221-2638-425f-a303-f55ffbe0b9fb" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeNotionalAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_fb0ebc47-9a37-4092-8d58-651e70778a0d" xlink:to="loc_us-gaap_DerivativeNotionalAmount_70ce7221-2638-425f-a303-f55ffbe0b9fb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetImpairmentCharges_6aa733de-3c8b-4ff0-a8f8-67d38fa62825" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AssetImpairmentCharges"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_fb0ebc47-9a37-4092-8d58-651e70778a0d" xlink:to="loc_us-gaap_AssetImpairmentCharges_6aa733de-3c8b-4ff0-a8f8-67d38fa62825" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueOptionAggregateDifferencesLongTermDebtInstruments_af960fac-5c8e-4d33-a68f-7f06bac47c7a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueOptionAggregateDifferencesLongTermDebtInstruments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_fb0ebc47-9a37-4092-8d58-651e70778a0d" xlink:to="loc_us-gaap_FairValueOptionAggregateDifferencesLongTermDebtInstruments_af960fac-5c8e-4d33-a68f-7f06bac47c7a" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magn.com/role/FinancialInstrumentsandFairValueMeasurementsScheduleofFairValueofDerivativesandLocationonConsolidatedBalanceSheetsDetails" xlink:type="simple" xlink:href="magn-20241228.xsd#FinancialInstrumentsandFairValueMeasurementsScheduleofFairValueofDerivativesandLocationonConsolidatedBalanceSheetsDetails"/>
  <link:definitionLink xlink:role="http://www.magn.com/role/FinancialInstrumentsandFairValueMeasurementsScheduleofFairValueofDerivativesandLocationonConsolidatedBalanceSheetsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_b3a67e83-48d9-47c4-b7f7-6bedf10ea31a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByBalanceSheetGroupingTable_3763f12a-1d91-44cf-a4f4-3cd233b79d48" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueByBalanceSheetGroupingTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_b3a67e83-48d9-47c4-b7f7-6bedf10ea31a" xlink:to="loc_us-gaap_FairValueByBalanceSheetGroupingTable_3763f12a-1d91-44cf-a4f4-3cd233b79d48" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_4c876175-3757-4e25-b706-4b2d75f6b46e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueByBalanceSheetGroupingTable_3763f12a-1d91-44cf-a4f4-3cd233b79d48" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_4c876175-3757-4e25-b706-4b2d75f6b46e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_4c876175-3757-4e25-b706-4b2d75f6b46e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_4c876175-3757-4e25-b706-4b2d75f6b46e" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_4c876175-3757-4e25-b706-4b2d75f6b46e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_ec701999-67fa-4e27-9860-7cfd9f8a1063" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_4c876175-3757-4e25-b706-4b2d75f6b46e" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_ec701999-67fa-4e27-9860-7cfd9f8a1063" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CurrencySwapMember_a5037ccb-8414-4351-89fc-cc0c34371004" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CurrencySwapMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_ec701999-67fa-4e27-9860-7cfd9f8a1063" xlink:to="loc_us-gaap_CurrencySwapMember_a5037ccb-8414-4351-89fc-cc0c34371004" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationAxis_1c2a5eaf-1209-4df7-9210-5104c4b41e48" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_HedgingDesignationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueByBalanceSheetGroupingTable_3763f12a-1d91-44cf-a4f4-3cd233b79d48" xlink:to="loc_us-gaap_HedgingDesignationAxis_1c2a5eaf-1209-4df7-9210-5104c4b41e48" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_1c2a5eaf-1209-4df7-9210-5104c4b41e48_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_HedgingDesignationAxis_1c2a5eaf-1209-4df7-9210-5104c4b41e48" xlink:to="loc_us-gaap_HedgingDesignationDomain_1c2a5eaf-1209-4df7-9210-5104c4b41e48_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_47516795-fad2-403e-97a8-4b43087856de" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_HedgingDesignationAxis_1c2a5eaf-1209-4df7-9210-5104c4b41e48" xlink:to="loc_us-gaap_HedgingDesignationDomain_47516795-fad2-403e-97a8-4b43087856de" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DesignatedAsHedgingInstrumentMember_3b5777d8-c601-4ab3-8e33-012989f81432" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DesignatedAsHedgingInstrumentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingDesignationDomain_47516795-fad2-403e-97a8-4b43087856de" xlink:to="loc_us-gaap_DesignatedAsHedgingInstrumentMember_3b5777d8-c601-4ab3-8e33-012989f81432" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeFairValueOfDerivativeAsset_49b50f61-a5c5-43a4-9dc1-774cc4a58de4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeFairValueOfDerivativeAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_b3a67e83-48d9-47c4-b7f7-6bedf10ea31a" xlink:to="loc_us-gaap_DerivativeFairValueOfDerivativeAsset_49b50f61-a5c5-43a4-9dc1-774cc4a58de4" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magn.com/role/FinancialInstrumentsandFairValueMeasurementsScheduleofIncomeorLossfromDerivativeInstrumentsDetails" xlink:type="simple" xlink:href="magn-20241228.xsd#FinancialInstrumentsandFairValueMeasurementsScheduleofIncomeorLossfromDerivativeInstrumentsDetails"/>
  <link:definitionLink xlink:role="http://www.magn.com/role/FinancialInstrumentsandFairValueMeasurementsScheduleofIncomeorLossfromDerivativeInstrumentsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_93b31bc0-c950-4033-af38-5f955163efa1" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByBalanceSheetGroupingTable_d50c6c2e-9906-4599-aacc-c3601e4354ed" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueByBalanceSheetGroupingTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_93b31bc0-c950-4033-af38-5f955163efa1" xlink:to="loc_us-gaap_FairValueByBalanceSheetGroupingTable_d50c6c2e-9906-4599-aacc-c3601e4354ed" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_d09a6676-00ee-49c5-9ce6-530e2c6b9f0c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueByBalanceSheetGroupingTable_d50c6c2e-9906-4599-aacc-c3601e4354ed" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_d09a6676-00ee-49c5-9ce6-530e2c6b9f0c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_d09a6676-00ee-49c5-9ce6-530e2c6b9f0c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_d09a6676-00ee-49c5-9ce6-530e2c6b9f0c" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_d09a6676-00ee-49c5-9ce6-530e2c6b9f0c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_2ecc82f7-7428-43ff-b4e1-6e77063a4408" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_d09a6676-00ee-49c5-9ce6-530e2c6b9f0c" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_2ecc82f7-7428-43ff-b4e1-6e77063a4408" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CurrencySwapMember_9d2af49f-731c-4fd2-93e9-aa0005c3e378" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CurrencySwapMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_2ecc82f7-7428-43ff-b4e1-6e77063a4408" xlink:to="loc_us-gaap_CurrencySwapMember_9d2af49f-731c-4fd2-93e9-aa0005c3e378" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainLossOnFairValueHedgesRecognizedInEarnings_ceb15c7d-c6dd-437a-a925-aa70c919738e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_GainLossOnFairValueHedgesRecognizedInEarnings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_93b31bc0-c950-4033-af38-5f955163efa1" xlink:to="loc_us-gaap_GainLossOnFairValueHedgesRecognizedInEarnings_ceb15c7d-c6dd-437a-a925-aa70c919738e" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magn.com/role/FinancialInstrumentsandFairValueMeasurementsScheduleofAssetsMeasuredatFairValueonNonrecurringBasisDetails" xlink:type="simple" xlink:href="magn-20241228.xsd#FinancialInstrumentsandFairValueMeasurementsScheduleofAssetsMeasuredatFairValueonNonrecurringBasisDetails"/>
  <link:definitionLink xlink:role="http://www.magn.com/role/FinancialInstrumentsandFairValueMeasurementsScheduleofAssetsMeasuredatFairValueonNonrecurringBasisDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_cbe9264f-b7ee-44b8-a411-8d3d7f3623c9" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByBalanceSheetGroupingTable_432954af-8fec-454d-900c-de7f69ee49c0" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueByBalanceSheetGroupingTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_cbe9264f-b7ee-44b8-a411-8d3d7f3623c9" xlink:to="loc_us-gaap_FairValueByBalanceSheetGroupingTable_432954af-8fec-454d-900c-de7f69ee49c0" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_1c68a7d4-34c8-4e01-b03e-a7adedc956a8" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueByBalanceSheetGroupingTable_432954af-8fec-454d-900c-de7f69ee49c0" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_1c68a7d4-34c8-4e01-b03e-a7adedc956a8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_1c68a7d4-34c8-4e01-b03e-a7adedc956a8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_1c68a7d4-34c8-4e01-b03e-a7adedc956a8" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_1c68a7d4-34c8-4e01-b03e-a7adedc956a8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_523be08b-d973-4018-b2ba-a41627adc517" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_1c68a7d4-34c8-4e01-b03e-a7adedc956a8" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_523be08b-d973-4018-b2ba-a41627adc517" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_cd566048-9c0a-4a3e-a33b-282bfcd21534" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_523be08b-d973-4018-b2ba-a41627adc517" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_cd566048-9c0a-4a3e-a33b-282bfcd21534" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_96a7c852-7a28-4957-a2a1-753703375210" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_523be08b-d973-4018-b2ba-a41627adc517" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_96a7c852-7a28-4957-a2a1-753703375210" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_e9417a91-02f8-4f23-8796-1e2502a3b88f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_523be08b-d973-4018-b2ba-a41627adc517" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_e9417a91-02f8-4f23-8796-1e2502a3b88f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_e5d7f9fc-ab72-4a10-895b-6e62513f0e49" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueByMeasurementFrequencyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueByBalanceSheetGroupingTable_432954af-8fec-454d-900c-de7f69ee49c0" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_e5d7f9fc-ab72-4a10-895b-6e62513f0e49" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_e5d7f9fc-ab72-4a10-895b-6e62513f0e49_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_e5d7f9fc-ab72-4a10-895b-6e62513f0e49" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_e5d7f9fc-ab72-4a10-895b-6e62513f0e49_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_b57153dc-bb3c-406a-be8b-c1c02b56ce74" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_e5d7f9fc-ab72-4a10-895b-6e62513f0e49" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_b57153dc-bb3c-406a-be8b-c1c02b56ce74" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsNonrecurringMember_708a1bd5-cc0b-41d1-902f-9d3cab8857c3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueMeasurementsNonrecurringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_b57153dc-bb3c-406a-be8b-c1c02b56ce74" xlink:to="loc_us-gaap_FairValueMeasurementsNonrecurringMember_708a1bd5-cc0b-41d1-902f-9d3cab8857c3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosureAbstract_9093079e-4bc0-4efe-b5fb-1c93348a23bc" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AssetsFairValueDisclosureAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_cbe9264f-b7ee-44b8-a411-8d3d7f3623c9" xlink:to="loc_us-gaap_AssetsFairValueDisclosureAbstract_9093079e-4bc0-4efe-b5fb-1c93348a23bc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwillFairValueDisclosure_6f7b7eb9-532f-48c3-a400-feff4745bb08" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwillFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_9093079e-4bc0-4efe-b5fb-1c93348a23bc" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwillFairValueDisclosure_6f7b7eb9-532f-48c3-a400-feff4745bb08" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillFairValueDisclosure_de9693b7-6e71-42be-a486-0db48aca2888" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_GoodwillFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_9093079e-4bc0-4efe-b5fb-1c93348a23bc" xlink:to="loc_us-gaap_GoodwillFairValueDisclosure_de9693b7-6e71-42be-a486-0db48aca2888" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure_236c0bfc-452f-40be-b872-eb1b717b0cec" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_9093079e-4bc0-4efe-b5fb-1c93348a23bc" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure_236c0bfc-452f-40be-b872-eb1b717b0cec" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentFairValueDisclosure_5e128881-0145-4490-911c-a736c7b12102" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PropertyPlantAndEquipmentFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_9093079e-4bc0-4efe-b5fb-1c93348a23bc" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentFairValueDisclosure_5e128881-0145-4490-911c-a736c7b12102" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosure_08ef8a12-2a7c-46c6-9ed3-2fc514e13b8b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AssetsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_9093079e-4bc0-4efe-b5fb-1c93348a23bc" xlink:to="loc_us-gaap_AssetsFairValueDisclosure_08ef8a12-2a7c-46c6-9ed3-2fc514e13b8b" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetImpairmentChargesAbstract_d1104e11-cbc5-4628-9f85-c68c8ae44653" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AssetImpairmentChargesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_cbe9264f-b7ee-44b8-a411-8d3d7f3623c9" xlink:to="loc_us-gaap_AssetImpairmentChargesAbstract_d1104e11-cbc5-4628-9f85-c68c8ae44653" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ImpairmentOfIntangibleAssetsIndefinitelivedExcludingGoodwill_eb2f310c-50f1-461c-a6b5-0ada18c11825" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ImpairmentOfIntangibleAssetsIndefinitelivedExcludingGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetImpairmentChargesAbstract_d1104e11-cbc5-4628-9f85-c68c8ae44653" xlink:to="loc_us-gaap_ImpairmentOfIntangibleAssetsIndefinitelivedExcludingGoodwill_eb2f310c-50f1-461c-a6b5-0ada18c11825" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillImpairmentLoss_6aa71f41-8947-49ed-9702-f37abecc26c5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_GoodwillImpairmentLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetImpairmentChargesAbstract_d1104e11-cbc5-4628-9f85-c68c8ae44653" xlink:to="loc_us-gaap_GoodwillImpairmentLoss_6aa71f41-8947-49ed-9702-f37abecc26c5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ImpairmentOfIntangibleAssetsFinitelived_654c2ddb-1f9e-44e2-a110-48ba3a5d0a2a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ImpairmentOfIntangibleAssetsFinitelived"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetImpairmentChargesAbstract_d1104e11-cbc5-4628-9f85-c68c8ae44653" xlink:to="loc_us-gaap_ImpairmentOfIntangibleAssetsFinitelived_654c2ddb-1f9e-44e2-a110-48ba3a5d0a2a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ImpairmentOfLongLivedAssetsHeldForUse_a7000471-52de-4a99-801f-6e6346e014dc" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ImpairmentOfLongLivedAssetsHeldForUse"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetImpairmentChargesAbstract_d1104e11-cbc5-4628-9f85-c68c8ae44653" xlink:to="loc_us-gaap_ImpairmentOfLongLivedAssetsHeldForUse_a7000471-52de-4a99-801f-6e6346e014dc" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetImpairmentCharges_26099e50-074d-4595-acea-1f95290f7459" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AssetImpairmentCharges"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetImpairmentChargesAbstract_d1104e11-cbc5-4628-9f85-c68c8ae44653" xlink:to="loc_us-gaap_AssetImpairmentCharges_26099e50-074d-4595-acea-1f95290f7459" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magn.com/role/SegmentandGeographicDataScheduleofFinancialandOtherInformationbySegmentDetails" xlink:type="simple" xlink:href="magn-20241228.xsd#SegmentandGeographicDataScheduleofFinancialandOtherInformationbySegmentDetails"/>
  <link:definitionLink xlink:role="http://www.magn.com/role/SegmentandGeographicDataScheduleofFinancialandOtherInformationbySegmentDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_5f4dc5b4-607c-4fd2-a9f9-457b2d59c219" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_e4cdcda2-f633-420c-82d9-dbb91ba75542" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_5f4dc5b4-607c-4fd2-a9f9-457b2d59c219" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_e4cdcda2-f633-420c-82d9-dbb91ba75542" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_101adb92-e0b6-426c-916e-4e96651f1d4c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_e4cdcda2-f633-420c-82d9-dbb91ba75542" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_101adb92-e0b6-426c-916e-4e96651f1d4c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_101adb92-e0b6-426c-916e-4e96651f1d4c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_101adb92-e0b6-426c-916e-4e96651f1d4c" xlink:to="loc_us-gaap_SegmentDomain_101adb92-e0b6-426c-916e-4e96651f1d4c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_7c425e29-4b34-4c6d-aa69-d9022b4581c5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_101adb92-e0b6-426c-916e-4e96651f1d4c" xlink:to="loc_us-gaap_SegmentDomain_7c425e29-4b34-4c6d-aa69-d9022b4581c5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_AmericasSegmentMember_a7e0ceb3-90f7-4567-9c28-e94cb312f4bb" xlink:href="magn-20241228.xsd#magn_AmericasSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_7c425e29-4b34-4c6d-aa69-d9022b4581c5" xlink:to="loc_magn_AmericasSegmentMember_a7e0ceb3-90f7-4567-9c28-e94cb312f4bb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_RestOfWorldSegmentMember_41412eff-fd77-4700-9d6c-e5de9de0d3d2" xlink:href="magn-20241228.xsd#magn_RestOfWorldSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_7c425e29-4b34-4c6d-aa69-d9022b4581c5" xlink:to="loc_magn_RestOfWorldSegmentMember_41412eff-fd77-4700-9d6c-e5de9de0d3d2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_4e1b9426-1743-420f-a235-86c9e2c088ea" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_5f4dc5b4-607c-4fd2-a9f9-457b2d59c219" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_4e1b9426-1743-420f-a235-86c9e2c088ea" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingIncomeLoss_989075fc-b8d1-4f28-a97b-67f437f3c920" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OperatingIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_5f4dc5b4-607c-4fd2-a9f9-457b2d59c219" xlink:to="loc_us-gaap_OperatingIncomeLoss_989075fc-b8d1-4f28-a97b-67f437f3c920" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepreciationDepletionAndAmortization_5687e481-61c0-4201-9b34-f5d341885b6d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DepreciationDepletionAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_5f4dc5b4-607c-4fd2-a9f9-457b2d59c219" xlink:to="loc_us-gaap_DepreciationDepletionAndAmortization_5687e481-61c0-4201-9b34-f5d341885b6d" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magn.com/role/SegmentandGeographicDataScheduleofSelectedInformationbyGeographicalRegionDetails" xlink:type="simple" xlink:href="magn-20241228.xsd#SegmentandGeographicDataScheduleofSelectedInformationbyGeographicalRegionDetails"/>
  <link:definitionLink xlink:role="http://www.magn.com/role/SegmentandGeographicDataScheduleofSelectedInformationbyGeographicalRegionDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_c19026a5-d3f9-4459-bec5-7076c0b9b2ed" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_0dfe0922-60fc-4bdc-9e25-d0db6c9af92c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_c19026a5-d3f9-4459-bec5-7076c0b9b2ed" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_0dfe0922-60fc-4bdc-9e25-d0db6c9af92c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_49f0b682-01c6-4b67-960f-d0268a2723d8" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_0dfe0922-60fc-4bdc-9e25-d0db6c9af92c" xlink:to="loc_srt_StatementGeographicalAxis_49f0b682-01c6-4b67-960f-d0268a2723d8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_49f0b682-01c6-4b67-960f-d0268a2723d8_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_49f0b682-01c6-4b67-960f-d0268a2723d8" xlink:to="loc_srt_SegmentGeographicalDomain_49f0b682-01c6-4b67-960f-d0268a2723d8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_29d95954-3044-431f-ae8d-42fb365a302c" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_49f0b682-01c6-4b67-960f-d0268a2723d8" xlink:to="loc_srt_SegmentGeographicalDomain_29d95954-3044-431f-ae8d-42fb365a302c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_UnitedStatesAndCanadaMember_19ac913f-c76d-4e2e-89b4-6dba44013b51" xlink:href="magn-20241228.xsd#magn_UnitedStatesAndCanadaMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_29d95954-3044-431f-ae8d-42fb365a302c" xlink:to="loc_magn_UnitedStatesAndCanadaMember_19ac913f-c76d-4e2e-89b4-6dba44013b51" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LatinAmericaMember_a5970c41-6b10-4f67-80b0-f93c5edae923" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_LatinAmericaMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_29d95954-3044-431f-ae8d-42fb365a302c" xlink:to="loc_srt_LatinAmericaMember_a5970c41-6b10-4f67-80b0-f93c5edae923" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_RestOfWorldMember_1246caf1-b8bf-47be-8b39-e89d7613d44b" xlink:href="magn-20241228.xsd#magn_RestOfWorldMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_29d95954-3044-431f-ae8d-42fb365a302c" xlink:to="loc_magn_RestOfWorldMember_1246caf1-b8bf-47be-8b39-e89d7613d44b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_977ed485-6cef-4e6b-b59d-65e073450df6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_c19026a5-d3f9-4459-bec5-7076c0b9b2ed" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_977ed485-6cef-4e6b-b59d-65e073450df6" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magn.com/role/SegmentandGeographicDataScheduleofSelectedInformationbyProductLineDetails" xlink:type="simple" xlink:href="magn-20241228.xsd#SegmentandGeographicDataScheduleofSelectedInformationbyProductLineDetails"/>
  <link:definitionLink xlink:role="http://www.magn.com/role/SegmentandGeographicDataScheduleofSelectedInformationbyProductLineDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_0c128209-0ffb-4984-ac0a-61bbddcd5ffd" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_30d37241-99fa-46f5-b935-5fb934cfcbdc" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_0c128209-0ffb-4984-ac0a-61bbddcd5ffd" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_30d37241-99fa-46f5-b935-5fb934cfcbdc" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_86ca92ab-0741-4e00-b79a-843aeba7f87b" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_30d37241-99fa-46f5-b935-5fb934cfcbdc" xlink:to="loc_srt_ProductOrServiceAxis_86ca92ab-0741-4e00-b79a-843aeba7f87b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_86ca92ab-0741-4e00-b79a-843aeba7f87b_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_86ca92ab-0741-4e00-b79a-843aeba7f87b" xlink:to="loc_srt_ProductsAndServicesDomain_86ca92ab-0741-4e00-b79a-843aeba7f87b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_326ce067-a0af-409e-be62-94c328f785ba" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_86ca92ab-0741-4e00-b79a-843aeba7f87b" xlink:to="loc_srt_ProductsAndServicesDomain_326ce067-a0af-409e-be62-94c328f785ba" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_PersonalCareMember_b9ad079b-4829-46ec-919a-7b93997719fe" xlink:href="magn-20241228.xsd#magn_PersonalCareMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_326ce067-a0af-409e-be62-94c328f785ba" xlink:to="loc_magn_PersonalCareMember_b9ad079b-4829-46ec-919a-7b93997719fe" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_ConsumerSolutionsMember_dcc1e1bd-e830-47ac-b607-640f3c710d9a" xlink:href="magn-20241228.xsd#magn_ConsumerSolutionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_326ce067-a0af-409e-be62-94c328f785ba" xlink:to="loc_magn_ConsumerSolutionsMember_dcc1e1bd-e830-47ac-b607-640f3c710d9a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_34c99453-c40d-4274-a6d8-aac2a263c1d5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ConcentrationRiskByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_30d37241-99fa-46f5-b935-5fb934cfcbdc" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_34c99453-c40d-4274-a6d8-aac2a263c1d5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_34c99453-c40d-4274-a6d8-aac2a263c1d5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_34c99453-c40d-4274-a6d8-aac2a263c1d5" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_34c99453-c40d-4274-a6d8-aac2a263c1d5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_95eb056c-e148-47ba-83f3-2c3f459bfcc9" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_34c99453-c40d-4274-a6d8-aac2a263c1d5" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_95eb056c-e148-47ba-83f3-2c3f459bfcc9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProductConcentrationRiskMember_8c263558-b924-4e5d-b578-30c229af7435" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ProductConcentrationRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_95eb056c-e148-47ba-83f3-2c3f459bfcc9" xlink:to="loc_us-gaap_ProductConcentrationRiskMember_8c263558-b924-4e5d-b578-30c229af7435" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_3c535b1f-9848-4b99-9f98-ce43559ecb82" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_30d37241-99fa-46f5-b935-5fb934cfcbdc" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_3c535b1f-9848-4b99-9f98-ce43559ecb82" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_3c535b1f-9848-4b99-9f98-ce43559ecb82_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_3c535b1f-9848-4b99-9f98-ce43559ecb82" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_3c535b1f-9848-4b99-9f98-ce43559ecb82_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_3828e4b3-e957-4568-8490-0936415ce63c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_3c535b1f-9848-4b99-9f98-ce43559ecb82" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_3828e4b3-e957-4568-8490-0936415ce63c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SalesRevenueNetMember_ef962ff1-0024-4d47-908e-ebb42da3fa59" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SalesRevenueNetMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_3828e4b3-e957-4568-8490-0936415ce63c" xlink:to="loc_us-gaap_SalesRevenueNetMember_ef962ff1-0024-4d47-908e-ebb42da3fa59" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskPercentage1_3b5e4d37-4a0f-410d-a610-394f340092f4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ConcentrationRiskPercentage1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_0c128209-0ffb-4984-ac0a-61bbddcd5ffd" xlink:to="loc_us-gaap_ConcentrationRiskPercentage1_3b5e4d37-4a0f-410d-a610-394f340092f4" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magn.com/role/ContingenciesandCommitmentsDetails" xlink:type="simple" xlink:href="magn-20241228.xsd#ContingenciesandCommitmentsDetails"/>
  <link:definitionLink xlink:role="http://www.magn.com/role/ContingenciesandCommitmentsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesLineItems_c985dc06-debc-41ed-a301-e52f40a322d7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LossContingenciesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesTable_abe41ab8-7abd-4071-8f4c-6a1abeaf4b41" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LossContingenciesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LossContingenciesLineItems_c985dc06-debc-41ed-a301-e52f40a322d7" xlink:to="loc_us-gaap_LossContingenciesTable_abe41ab8-7abd-4071-8f4c-6a1abeaf4b41" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_0bc06f7f-0d50-4598-8e7c-c335dc7f41ba" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_abe41ab8-7abd-4071-8f4c-6a1abeaf4b41" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_0bc06f7f-0d50-4598-8e7c-c335dc7f41ba" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_0bc06f7f-0d50-4598-8e7c-c335dc7f41ba_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_0bc06f7f-0d50-4598-8e7c-c335dc7f41ba" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_0bc06f7f-0d50-4598-8e7c-c335dc7f41ba_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_254bbc7c-3f3c-4a8b-9b14-b7e06281b4d8" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_0bc06f7f-0d50-4598-8e7c-c335dc7f41ba" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_254bbc7c-3f3c-4a8b-9b14-b7e06281b4d8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_PreviousAcquisitionMember_cc2e38c2-c521-41b8-af05-50006a2a422e" xlink:href="magn-20241228.xsd#magn_PreviousAcquisitionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_254bbc7c-3f3c-4a8b-9b14-b7e06281b4d8" xlink:to="loc_magn_PreviousAcquisitionMember_cc2e38c2-c521-41b8-af05-50006a2a422e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_EnvironmentRemediationAgreementTerm_c7fcd6f3-45a6-4ea3-83b8-c51733892a09" xlink:href="magn-20241228.xsd#magn_EnvironmentRemediationAgreementTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_c985dc06-debc-41ed-a301-e52f40a322d7" xlink:to="loc_magn_EnvironmentRemediationAgreementTerm_c7fcd6f3-45a6-4ea3-83b8-c51733892a09" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccrualForEnvironmentalLossContingencies_ff841eef-6ce9-4933-9ca1-9ff8683a4292" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccrualForEnvironmentalLossContingencies"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_c985dc06-debc-41ed-a301-e52f40a322d7" xlink:to="loc_us-gaap_AccrualForEnvironmentalLossContingencies_ff841eef-6ce9-4933-9ca1-9ff8683a4292" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_AccrualForEnvironmentalLossContingenciesEscrowAsset_56d019bf-a88b-4ac0-bcbb-2bce39688074" xlink:href="magn-20241228.xsd#magn_AccrualForEnvironmentalLossContingenciesEscrowAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_c985dc06-debc-41ed-a301-e52f40a322d7" xlink:to="loc_magn_AccrualForEnvironmentalLossContingenciesEscrowAsset_56d019bf-a88b-4ac0-bcbb-2bce39688074" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_BusinessCombinationContingentConsiderationLiabilityAccretionRate_83d25968-c61b-470f-ad5f-be9ff77eb61e" xlink:href="magn-20241228.xsd#magn_BusinessCombinationContingentConsiderationLiabilityAccretionRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_c985dc06-debc-41ed-a301-e52f40a322d7" xlink:to="loc_magn_BusinessCombinationContingentConsiderationLiabilityAccretionRate_83d25968-c61b-470f-ad5f-be9ff77eb61e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationLiabilityNoncurrent_193c7622-3c14-454e-92d6-23f7e1ebbc26" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BusinessCombinationContingentConsiderationLiabilityNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_c985dc06-debc-41ed-a301-e52f40a322d7" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationLiabilityNoncurrent_193c7622-3c14-454e-92d6-23f7e1ebbc26" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magn.com/role/CorporateExpenseAllocationDetails" xlink:type="simple" xlink:href="magn-20241228.xsd#CorporateExpenseAllocationDetails"/>
  <link:definitionLink xlink:role="http://www.magn.com/role/CorporateExpenseAllocationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_magn_CorporateExpenseAllocationsLineItems_a0bd537b-df5d-4a74-ad13-df39b45c7f92" xlink:href="magn-20241228.xsd#magn_CorporateExpenseAllocationsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_CorporateExpenseAllocationsTable_d421bba7-e96e-4c96-8ae1-8dfe639fc60c" xlink:href="magn-20241228.xsd#magn_CorporateExpenseAllocationsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_magn_CorporateExpenseAllocationsLineItems_a0bd537b-df5d-4a74-ad13-df39b45c7f92" xlink:to="loc_magn_CorporateExpenseAllocationsTable_d421bba7-e96e-4c96-8ae1-8dfe639fc60c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_b54cba4c-4292-4951-bb6d-405d06a95634" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_magn_CorporateExpenseAllocationsTable_d421bba7-e96e-4c96-8ae1-8dfe639fc60c" xlink:to="loc_dei_LegalEntityAxis_b54cba4c-4292-4951-bb6d-405d06a95634" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_b54cba4c-4292-4951-bb6d-405d06a95634_default" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_b54cba4c-4292-4951-bb6d-405d06a95634" xlink:to="loc_dei_EntityDomain_b54cba4c-4292-4951-bb6d-405d06a95634_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_520ca4da-0006-4030-9b3f-46824ccb6960" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_b54cba4c-4292-4951-bb6d-405d06a95634" xlink:to="loc_dei_EntityDomain_520ca4da-0006-4030-9b3f-46824ccb6960" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_TreasureHoldcoInc.Member_4ba13ccf-7b6a-48e7-bda2-899526de1e32" xlink:href="magn-20241228.xsd#magn_TreasureHoldcoInc.Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_520ca4da-0006-4030-9b3f-46824ccb6960" xlink:to="loc_magn_TreasureHoldcoInc.Member_4ba13ccf-7b6a-48e7-bda2-899526de1e32" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_53a58705-192b-41b9-ab99-26f1a3569deb" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_magn_CorporateExpenseAllocationsTable_d421bba7-e96e-4c96-8ae1-8dfe639fc60c" xlink:to="loc_srt_RangeAxis_53a58705-192b-41b9-ab99-26f1a3569deb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_53a58705-192b-41b9-ab99-26f1a3569deb_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_53a58705-192b-41b9-ab99-26f1a3569deb" xlink:to="loc_srt_RangeMember_53a58705-192b-41b9-ab99-26f1a3569deb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_5761beff-27eb-4d32-8228-39feb69a07aa" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_53a58705-192b-41b9-ab99-26f1a3569deb" xlink:to="loc_srt_RangeMember_5761beff-27eb-4d32-8228-39feb69a07aa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_1861f36f-b5de-4c92-bf11-d90ab78216f7" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_5761beff-27eb-4d32-8228-39feb69a07aa" xlink:to="loc_srt_MinimumMember_1861f36f-b5de-4c92-bf11-d90ab78216f7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_364cfe73-0fee-4d2a-95b8-a439adbab88a" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_5761beff-27eb-4d32-8228-39feb69a07aa" xlink:to="loc_srt_MaximumMember_364cfe73-0fee-4d2a-95b8-a439adbab88a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_CorporateExpenseAllocation_4698a366-9a41-446e-9982-c8268ae10f1a" xlink:href="magn-20241228.xsd#magn_CorporateExpenseAllocation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_magn_CorporateExpenseAllocationsLineItems_a0bd537b-df5d-4a74-ad13-df39b45c7f92" xlink:to="loc_magn_CorporateExpenseAllocation_4698a366-9a41-446e-9982-c8268ae10f1a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_CorporateExpensesOnStandaloneBasis_09b48adb-7e20-4a3b-a170-ad5d653a28ab" xlink:href="magn-20241228.xsd#magn_CorporateExpensesOnStandaloneBasis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_magn_CorporateExpenseAllocationsLineItems_a0bd537b-df5d-4a74-ad13-df39b45c7f92" xlink:to="loc_magn_CorporateExpensesOnStandaloneBasis_09b48adb-7e20-4a3b-a170-ad5d653a28ab" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magn.com/role/SubsequentEventsDetails" xlink:type="simple" xlink:href="magn-20241228.xsd#SubsequentEventsDetails"/>
  <link:definitionLink xlink:role="http://www.magn.com/role/SubsequentEventsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventLineItems_4e9d20d8-e18a-4fc3-bb95-7366d50fdd40" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SubsequentEventLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTable_fca5b6c0-1de9-416e-8712-b685ac1b66b5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SubsequentEventTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SubsequentEventLineItems_4e9d20d8-e18a-4fc3-bb95-7366d50fdd40" xlink:to="loc_us-gaap_SubsequentEventTable_fca5b6c0-1de9-416e-8712-b685ac1b66b5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_3d172a4c-3161-44f8-a17c-3005d9921c44" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_fca5b6c0-1de9-416e-8712-b685ac1b66b5" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_3d172a4c-3161-44f8-a17c-3005d9921c44" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_3d172a4c-3161-44f8-a17c-3005d9921c44_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_3d172a4c-3161-44f8-a17c-3005d9921c44" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_3d172a4c-3161-44f8-a17c-3005d9921c44_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_1fab6e95-dfd0-4344-a24e-ff0303d3f6cb" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_3d172a4c-3161-44f8-a17c-3005d9921c44" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_1fab6e95-dfd0-4344-a24e-ff0303d3f6cb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CurrencySwapMember_c09a6fd4-2646-4cac-ad74-5370fa10d305" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CurrencySwapMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_1fab6e95-dfd0-4344-a24e-ff0303d3f6cb" xlink:to="loc_us-gaap_CurrencySwapMember_c09a6fd4-2646-4cac-ad74-5370fa10d305" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_CrossCurrencySwapMaturingNovember2027Member_2bf9d0b7-ab4f-452f-95d1-a6b32218f985" xlink:href="magn-20241228.xsd#magn_CrossCurrencySwapMaturingNovember2027Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_1fab6e95-dfd0-4344-a24e-ff0303d3f6cb" xlink:to="loc_magn_CrossCurrencySwapMaturingNovember2027Member_2bf9d0b7-ab4f-452f-95d1-a6b32218f985" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_magn_CrossCurrencySwapMaturingNovember2029Member_19152250-e14f-477f-8287-de5574a4591b" xlink:href="magn-20241228.xsd#magn_CrossCurrencySwapMaturingNovember2029Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_1fab6e95-dfd0-4344-a24e-ff0303d3f6cb" xlink:to="loc_magn_CrossCurrencySwapMaturingNovember2029Member_19152250-e14f-477f-8287-de5574a4591b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_37b6ddbe-2e4d-4bea-812f-4c3fb7358cc5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_fca5b6c0-1de9-416e-8712-b685ac1b66b5" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_37b6ddbe-2e4d-4bea-812f-4c3fb7358cc5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_37b6ddbe-2e4d-4bea-812f-4c3fb7358cc5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_37b6ddbe-2e4d-4bea-812f-4c3fb7358cc5" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_37b6ddbe-2e4d-4bea-812f-4c3fb7358cc5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_e21bdcb8-d767-461d-8192-0d41f86750c0" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_37b6ddbe-2e4d-4bea-812f-4c3fb7358cc5" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_e21bdcb8-d767-461d-8192-0d41f86750c0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_b816a95c-6393-404c-b835-f98078af5780" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_e21bdcb8-d767-461d-8192-0d41f86750c0" xlink:to="loc_us-gaap_SubsequentEventMember_b816a95c-6393-404c-b835-f98078af5780" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeCashReceivedOnHedge_e05fe037-9594-45cd-aa2e-b9b01eaeec7f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeCashReceivedOnHedge"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_4e9d20d8-e18a-4fc3-bb95-7366d50fdd40" xlink:to="loc_us-gaap_DerivativeCashReceivedOnHedge_e05fe037-9594-45cd-aa2e-b9b01eaeec7f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeNotionalAmount_778a10de-840a-442c-8cd1-fc5b51203857" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeNotionalAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_4e9d20d8-e18a-4fc3-bb95-7366d50fdd40" xlink:to="loc_us-gaap_DerivativeNotionalAmount_778a10de-840a-442c-8cd1-fc5b51203857" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fasb.org/us-gaap/role/eedm/ExtensibleEnumerationLists" xlink:type="simple" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-roles-2024.xsd#eedm"/>
  <link:definitionLink xlink:role="http://fasb.org/us-gaap/role/eedm/ExtensibleEnumerationLists" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrentAbstract" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LiabilitiesCurrentAbstract"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedLiabilitiesCurrent" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccruedLiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract" xlink:to="loc_us-gaap_AccruedLiabilitiesCurrent" xlink:type="arc" order="41"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsAbstract" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AssetsAbstract"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization" xlink:type="arc" order="101"/>
  </link:definitionLink>
</link:linkbase>
