This thesis firstly investigates spin-orbit torques (SOTs) generated by microwave current and detected by ferromagnetic resonance (FMR) spectroscopy in a uniform ferromagnetic system, namely NiMnSb, a half-Heusler ferromagnet with broken inversion symmetry at room temperature. The spin-orbit coupling (SOC) and the noncentrosymmetric structure of the epitaxial NiMnSb allow microwave current to induce oscillating SOTs on the magnetisation, and by analysing the measured FMR lineshape, the direction and magnitude of the effective spin-orbit fields (SOFs) are determined to characterise the SOTs. Furthermore, the observed current-induced SOFs have the symmetries of both Dresselhaus and Rashba SOC.
The second part of this thesis explores the temperature-dependent behaviour of the current-induced SOFs in NiMnSb from 10 K to room temperature. We found that both Dresselhaus and Rashba types of SOFs are substantially enhanced at low temperature. This work is the first reported temperature-dependent SOFs in epitaxial NiMnSb system which provides more insights into the underlying mechanism that governs the SOFs in the noncentrosymmetric ferromagnets.
Finally, the nonlinear magnetoresistance (MR) effect – unidirectional magnetoresistance (UMR) has been studied in CoFeB/Pt bilayer and epitaxial NiMnSb. In the CoFeB/Pt system, the close correspondence between the asymmetry in resistance and reduction in magnetisation confirmed by the current-induced spin-torque FMR (CI-FMR) measurements demonstrate that observed MR change is due to the creation or annihilation of magnons through spin-flip process and GHz magnetisation excitation by spin-torque effect. Motivated by the experimental approach established in the UMR study of CoFeB/Pt bilayer system, we investigate the resistance asymmetry in NiMnSb extracted from the non-resonating background voltage. The observed resistance asymmetry scales linearly with current density and has a crystallographic dependence. We interpret this resistance asymmetry in NiMnSb as the relative orientation between the magnetisation and the current-induced SOFs.