Mn₃Ge-based tetragonal Heusler alloy thin films with addition of Ni, Pt, and Pd

We have investigated substitution effects of Ni, Pt, and Pd on phase formation and magnetic properties of D0₂₂-Mn₃Ge thin films. We prepared (Mn_1-x M _x )₃Ge thin films (M  =  Ni, Pt, Pd) at 650 °C by magnetron sputtering on MgO(0 0 1) substrates with x varying from 0.03 to 0.6. For improving the film quality, a Cr(0 0 1) seed layer was employed. The D0₂₂ structure formed only for the lowest concentrations of Ni and Pt. Nevertheless, the doped samples showed strong perpendicular magnetic anisotropy up to x  =  0.1. For high Ni concentrations, we observed the formation of a soft ferromagnetic Mn _x Ni _yGe phase with a Curie temperature of about 230 K, while in samples with high Pt content the antiferromagnet L1₀-MnPt phase is formed along with GePt. In contrast, for Pd substitution, the D0₂₂ structure is preserved up to x  =  0.2, exhibiting strong perpendicular magnetic anisotropy and low saturation magnetization. Interestingly, the coexistence of the D0₂₂-Mn₃Ge and a novel D0₂₂-(Mn_1-x Pd _x )₃Ge phase was revealed, which might have been facilitated by the low lattice mismatch to the Cr(0 0 1) seed layer. With further increase of the Pd concentration, the D0₂₂ structure vanishes and mainly the GePd and GePd₂ phases are present. Overall within the investigated sample series, the saturation magnetization strongly decreases with increasing dopant concentration, offering the possibility to adjust the saturation magnetization in the range between 20 and 100 emu cm^-3, while still preserving strong perpendicular magnetic anisotropy, which is important for spintronic applications.