Microstructural and magnetic studies of Mn–Al thin films

Mn–Al thin films with high coercivity and high saturation magnetization were successfully fabricated by rf magnetron sputtering with properly controlled chemical composition, substrate temperature, and annealing temperature. A high coercivity of about 3000 Oe and a saturation magnetization of about 420 emu/cc have been achieved. We have observed that during annealing at 410 °C, the nonmagnetic ε phase with a grain size of roughly 100 nm transforms into a metastable ferromagnetic τ phase with a platelike grain size of roughly 300 nm. From the continuous measurement of the stress of the films in vacuum as a function of temperature, we observed a compression stress during heating below 220 °C, and a tension stress above 220 °C during cooling. The structure phase transformation from ε to τ phases was related to the stress variation from compression to tension. The high coercivity can be explained by the high magnetocrystalline anisotropy constant of the τ phase and the magnetoelastic energy arises from the residual stress of Mn–Al films after the shear transformation.