Phase ordering and its effect on magnetic and structural properties of FeRh ultrathin films

Abstract

Applications using FeRh for controllable exchange coupling of two magnetic layers with in-plane and out-of-plane anisotropies require ultrathin (∼10 nm) films with pure antiferromagnetic (AF) FeRh α″ phase at room temperature (RT). However, it is also well known that the antiferromagnetic-ferromagnetic (AF-FM) transition of FeRh is sensitive to composition and deteriorates at low thicknesses. Hence, in this work, we study the composition-dependent phase ordering of co-sputtered FeRh thin films at ultrathin thicknesses of ∼10 nm. As the ultrathin films get richer in Rh, the appearance of α″ phase is typically characterized magnetically by a sudden drop in RT moment, and structurally by a slight decrease in degree of B2 chemical ordering with a sharp decrease in c-axis lattice constant. These observations are consistent with the FeRh phase diagram where FeRh abruptly enters the AF α″ phase once it becomes slightly disordered. Dependences of magnetic transition parameters on composition were also described. Moreover, higher sputtering powers possibly allow the formation of purer α″ phase with less γ-face centered cubic phase impurities. Consequently, a composition optimized 10 nm film shows a relatively low residual moment (13.5 emu/cc), thus suggesting good AF phase formation. In addition, correlation of the magnetic transition parameters with the crystal structural parameters reveal that the maximum rate of AF-FM transition (Rmax,h) and the corresponding total change in magnetization (ΔMh) interestingly shows linear dependence on the c-axis lattice constant, but would depart from this linearity under certain conditions, i.e., when grain sizes were large, crystallinity was improved and Fe content was high.