Magnetic properties of ordered polycrystalline FeRh thin films

Abstract

FeRh alloys with B2-order have been investigated earlier in epitaxial films as the metamagnetic transition temperature (T_AF–FM) depends upon the structure and lattice strain or disorder. Here, we report on an ordered polycrystalline FeRh film on Al₂O₃ (0001) deposited by DC magnetron sputtering. We found that T_AF–FM = 346 K (in 1 T), which can be shifted by a magnetic field to 288 K (in 7 T) with similar rates (dT/dH) of −8.5 K T⁻¹ (heating) and −9.6 K T⁻¹ (cooling) as in epitaxial ones. However, a significant difference in the rate of domain nucleation and propagation with temperature (dM/dT) during cooling and heating procedures is noted. These changes can be ascribed to ordered and disordered polycrystallinity of the film. Furthermore, polarized neutron reflectometry (PNR) shows that the FeRh film is not homogeneous throughout the depth of the film. On the one hand, we found evidence of Fe segregation at the surface of the film extended to within a few nanometers from the top-interface and on the other hand, Rh richness at the substrate-interface. A gradient in Fe at% has resulted in changing magnetic phases from ordered bcc (B2) within the film to a disordered bcc (A2) structure at the surface and a disordered fcc phase (A1) at the bottom. Thus it is possible to grow commercially viable polycrystalline films with similar characteristics to those of epitaxial films.