The relationship between microstructure and field-induced anisotropy in cobalt-rich amorphous alloys after magnetic field annealing

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). Until recently, it was generally assumed that amorphous magnetic alloys respond to field annealing by a process of local directional ordering which leaves the amorphous structure intact. However, striking differences in the microstructural morphology after field annealing were revealed for different glass former ratios B/Si. For high B/Si ratios, the surface crystals are predominantly fcc Co and demonstrate a high density of oxygen faults. For low B/Si ratios, the surface crystals 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 crystals. These observations appear to be related to those of Nesbitt and Heidenreich (E.A. Nesbitt, R.D. Heidenreich, J. Appl. Phys. 30 (1959) 1000–1003) in perminvar alloys where oxygen was found to be necessary for field annealing to be effective.