Domain structures of nanocrystalline Fe₉₀Zr₇B ₃ alloy studied by Lorentz microscopy

As-quenched Fe₉₀Zr₇B₃ alloy has been crystallized at 773, 923 and 973 K, it is found that the specimens annealed at 773 and 923 K have grains with the same size, but the one annealed at 773 K has a thick amorphous matrix and a broad grain size distribution. Big domains and smooth domain walls are observed in the specimen annealed at the optimum condition (923 K), and excellent magnetic softness is obtained. On the other hand, the one annealed at 973 K has very large grains (40 nm). Very small domains with irregular walls are observed, indicating a weak intergranular exchange coupling. Through in situ Lorentz microscopy, a relaxation of internal stress in the specimen annealed at 773 K is observed at an elevated temperature (333 K). The internal stress observed is considered to be one of the important factors that degrade the soft magnetic properties. Based on the results of differential thermal analysis, a two-step annealing, where the as-quenched specimen is pre-heated at 723 K and subsequently annealed at 773 K, is utilized. The domain structure of the specimen treated by the two-step annealing is analyzed. It is found that the size of the domain is larger than that of the specimen annealed at 773 K, suggesting the possibility of control of the soft magnetic properties.