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 {HEAD ~ an Fe2O3 phase} to a thickness of 50 nm or more nearer to {TAIL ~ a further surface} on the nitride layer 
nearer to
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 { HEAD ~ an Fe2O3 phase } to a thickness of 50 nm or more nearer to { TAIL ~ a further surface } on the nitride layer
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH