It is preferred that the cage of the present invention has {HEAD ~ an oxide layer} comprising {TAIL ~ an Fe3O4 phase} as 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 
comprising
It is preferred that the cage of the present invention has { HEAD ~ an oxide layer } comprising { TAIL ~ an Fe3O4 phase } as 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 HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL 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 OTH OTH