Magnetic properties of Fe-Pd magnetic shape memory alloys: Density functional calculations and epitaxial films

Abstract

The magnetic properties of Fe-Pd magnetic shape memory alloys with different Pd contents are investigated in epitaxial films and by means of density functional calculations. The magnetocrystalline anisotropy energy (MAE) of ordered and disordered Fe-Pd alloys is calculated as a function of $c/a$ ratio and Pd concentration. Disorder is found to be essential for the experimentally observed easy $a$-axis anisotropy in fct-${\text{Fe}}_{70}{\text{Pd}}_{30}$. The MAE of disordered fct-Fe-Pd not only increases with decreasing $c/a$ but also increases with increasing Pd concentration for fixed $c/a$. Epitaxial Fe-Pd films deposited on MgO (100) exhibit (with increasing Pd concentration from 25 to $37\text{ }\text{at}\text{.}\text{ }%$ Pd) bcc, bct, and fct structures at room temperature. Temperature dependent magnetic properties are studied by using hysteresis, saturation magnetization, and susceptibility measurements. The coercivity and MAE at room temperature in the fct films is higher than in bct and/or bcc films. Increasing MAE is observed with increasing Pd concentration within the fct phase. Due to the nanocscale microstructure, the magnetocrystalline anisotropy energy in the fct films is reduced compared to bulk. Furthermore, it is observed that the fct Fe-Pd films with $\text{Pd}<33\text{ }\text{at}\text{.}\text{ }%$ continuously transform to the bct phase during cooling. In contrast to bulk materials, this transformation is found to be reversible in thin films.