Experimental investigation of off-stoichiometry and 3d transition metal (Mn, Ni, Cu)-substitution in single-crystalline FePt thin films

In L1₀ (fct)-FePt thin films, both tuning Fe and Pt concentrations and substitution with third-metal were studied for magnetic characteristic optimization. We investigated single-crystalline FePt-X (X = Mn, Ni, Cu) thin films grown epitaxially on MgO(001) substrates at a substrate temperature of 350°C by changing Fe, Pt, and X contents, and explored the effects of off-stoichiometry and 3d-metal-substitution. The magnetic moment per atom (m) of FePt-X films as a function of the effective number of valence electrons (n_eff) in 3d metal sites follows the Slater-Pauling-type trend, by which m decreases by the n_eff deviation from n_eff = 8, independently of the X metal and the Pt concentration. The magnetic anisotropy (K_u) exhibits n_eff dependence similar to m. This trend was almost independent of the Pt concentration after compensation using the theoretical prediction on the relation between K_u and Fe/Pt concentrations. Such a trend has been proved for stoichiometric FePt-X films, but it was clarified as robust against off-stoichiometry. The compensated K_u (K u comp) of FePt-Mn and FePt-Cu followed a similar trend to that predicted by the rigid-band model, although the K u comp of the FePt-Mn thin films dropped more rapidly than the rigid band calculation. However, it followed the recent first-principles calculation.