NEW MECHANISM FOR FIELD-INDUCED ANISOTROPY OF COBALT-RICH AMORPHOUS-ALLOYS

It is generally assumed that amorphous magnetic alloys respond to field annealing by a process of local directional ordering which leaves the amorphous structure intact. We have made a comparative microstructural study of field-annealed Co95-xFe5(BSi)(x) amorphous alloys using transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) with thin sections parallel to the ribbon surface. Field annealing response was measured from anisotropy in low-field hysteresis loops. These alloys show appreciable surface crystallization for annealing as much as 80 K below the bulk crystallization temperature. The surface crystallization proceeds by a known mechanism (selective oxidation) to which we have added a more detailed understanding. Three steps are involved: (1) formation of an amorphous borosilicate surface oxide layer during annealing; (2) depletion of glass stabilizing elements (boron and silicon) from the underlying amorphous metal substrate; (3) primary crystallization of the destabilized, metal-enriched subsurface layer to an fee or hcp cobalt-rich phase, Striking differences in the microstructural morphology were revealed for different glass former ratios B/Si. For high B/Si ratios, the surface crystallites are predominantly fee Co and show a high density of oxygen faults. For low B/Si ratios, the surface crystallites are predominantly hcp Co and almost free of faults, Response to field annealing is proportional to the B/Si ratio and correlates with the presence of oxygen faults in surface crystallites. Electron diffraction and microprobe analysis indicate that the surface oxide in silicon-rich alloys is a dense silica glass which appears to be an effective diffusion barrier to oxygen. The surface oxide in boron-rich alloys is a more porous oxide richer in B2O3. These observations appear to be related to those from perminvar alloys where oxygen was found to be necessary for field annealing to be effective.