It is preferred that the cage of the present invention has an oxide layer comprising an Fe3O4 phase as a main ingredient and not containing an Fe2O3 phase to a thickness of 50 nm or more nearer to {HEAD ~ a further surface} on {TAIL ~ the nitride layer} 
on
It is preferred that the cage of the present invention has an oxide layer comprising an Fe3O4 phase as a main ingredient and not containing an Fe2O3 phase to a thickness of 50 nm or more nearer to { HEAD ~ a further surface } on { TAIL ~ the nitride layer }
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