It is preferred that the cage of the present invention has an oxide layer comprising an Fe3O4 phase as {TAIL ~ a main ingredient} and not containing an Fe2O3 phase to {HEAD ~ 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 an Fe3O4 phase as { TAIL ~ a main ingredient } and not containing an Fe2O3 phase to { HEAD ~ 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 OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH