Reconsidering the nonlinear emergent inductance: time-varying Joule heating and its impact on the AC electrical response

A nonlinearly enhanced electrical reactance, Z, under a large AC current has been measured to explore emergent inductors, which constitute a new class of inductors based on the spin-transfer torque effect. A nonlinear Z has been observed in conducting magnets that contain noncollinear spin textures and interpreted as the realization of an inductance due to current-induced spin dynamics. However, curious behavior has concomitantly been observed. For instance, the nonlinear Z always has a cutoff frequency of 10⁰--10⁴ Hz, which is much lower than the resonance frequency of a ferromagnetic domain wall, ~10⁷ Hz; furthermore, the temperature and magnetic field variations in Z appear to be considerably correlated with those in the temperature derivative of resistance. This behavior appears to be difficult to understand in terms of the current-induced spin dynamics, and therefore, the earlier interpretation of the nonlinear Z should be further verified. Here, we theoretically and experimentally show that time-varying Joule heating and its impact on the AC electrical response can naturally explain these observations. In the experimental approach, we study the nonlinear AC electrical response of two conducting materials that exhibit no magnetic order, CuIr₂S₄ and 1T'-MoTe₂. Under time-varying Joule heating, a nonlinearly enhanced Z with the curious behavior mentioned above is observed in both systems. Our study implies that the nonlinear Z previously observed in noncollinear magnets includes a considerable contribution of the Joule-heating-induced apparent AC impedance.