New mechanism for field‐induced anisotropy of cobalt‐rich amorphous alloys

Abstract

SUMMARYIt 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 hep 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 hep 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.