Magnetic force microscopy study of heat-treated Fe₈₁Ga ₁₉ with different cooling rates

Phase constitution and domain structures of heat-treated Fe ₈₁Ga₁₉ cooled from 800 °C at different rates have been studied by X-ray diffraction (XRD) and magnetic force microscopy (MFM). Heat-treated samples were annealed at 800 °C for 3 h, followed by water quenching, air cooling ( ∼ 160 °C/min), furnace cooling ( ∼ 5 °C/min) and slow cooling (0.1 °C/min). Both the as-quenched and air-cooled Fe₈₁Ga₁₉ had a single disordered bcc A2 phase, but exhibited obvious difference in alignment of stripe domains. Dendrite domains were observed in the furnace-cooled sample with the formation of the ordered DO₃ phase in the A2 matrix. For the slow-cooled Fe81Ga19 sample, L12 phase was found to be precipitated and showed a plate domain structure with a relatively weaker contrast. The great thermal history dependence of the initial domain structure of Fe₈₁Ga₁₉ samples is considered to be responsible for the change in their magnetostrictive behaviors.