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