Description
The physical origin of third harmonic magnetoresistance remains debated in nonlinear spintronics. Here we systematically study third harmonic magnetoresistance in Ni-based films with different in-plane anisotropies, including polycrystalline Ni and epitaxial Ni(001) systems. Using in-plane field-rotation measurements together with magnetic-field scaling, we identify thermal anisotropy modulation as a dominant mechanism, distinct from the conventional thermal magnetization modulation. AC Joule heating induces periodic temperature oscillations that modulate magnetic anisotropy constants, driving coherent magnetization oscillations. This mechanism produces distinct angular fingerprints, such as a characteristic sixfold symmetry in Ni(001) films with fourfold magnetic anisotropy. By establishing a quantitative framework based on the 1/H and 1/H^2 scaling relations, we effectively decouple the thermal contributions arising from current-induced modulations of uniaxial and fourfold magnetic anisotropies. Our findings provide a rigorous methodology for interpreting higher-order harmonic transport and highlight thermal–anisotropy coupling as an essential ingredient in nonlinear spintronic measurements.
