The domain wall pinning and exchange coupling induced by surface crystallization in field-annealed cobalt-rich amorphous alloys

Attempts were made to correlate magnetic properties of Co-rich amorphous alloys such as domain wall pinning and exchange coupling with microstructural changes during field annealing. It was observed that M-H loops of first annealed material under longitudinal field showed weak exchange anisotropy due to coupling of the amorphous alloy to a semi-hard Co-rich crystalline surface layer. After second annealing without magnetic field, re-entrant magnetization behavior was observed. alloys show appreciable surface crystallization during annealing. Striking differences in the microstructural morphology were revealed for differnt glass former ratios B/Si. For high B/Si ratios, the surface crystallites are predominantly f.c.c. Co and show a high density of oxygen faults. For low B/Si ratios, the surface crystallites are predominantly h.c.p. Co and almost free of faults. gree of exchange coupling is proportional to the B/Si ratio and correlates with the presence of oxygen faults in surface crystallites. It is also observed that there is a good correlation between domain wall pinning field and thickness of crystalline Co layer.