The effect of oxidation on magnetic properties of amorphous Co75.26−xFe4.74(BSi)20 + x alloys

A comparative oxidation study of several amorphous Co75.25−xFe4.74(BSi)20+x alloys was carried out, A reentrant magnetization behavior and field-induced anhotropy were obtained after oxidation of the amorphous Co-rich ribbons. During this oxidation, the ribbons were found to develop surface oxides which are primarily non-magnetic borosilicate or a combination of borosilicate and magnetic oxides such CoO or FeO. Beneath this lies about 100–1000 Å thick Co-rich magnetic alloy which may be either hcp or fcc in its crystal structure. The thickness of Co crystallized layer is determined by the type of the surface oxides. idation products such as surface and internal oxides (oxygen impurity faults) appear to affect magnetization behavior of Co rich amorphous alloys significantly. We have determined the amount of metalloids (a critical concentration) which is necessary to form a continuous layer of the most thermodynamically stable oxide, in our case borosilicate, on the surface. We also observed that there is a good correlation between reentrant magnetization and thickness of Co layer. The best reentrant M-H loop was obtained in ribbons with a surface borate-rich borosilicate since this ensures the conditions such as (1) metalloid depletion in the substrate and (2) formation of oxygen impurity faults in Co grains, that are required for strong reentrant magnetization behavior.