Temperature variation by FMR under high power microwave irradiation and effect of external magnetic field change rate

In this study, the effect of the H^ext. variation rate (dH^ext./dt, t: time) on the temperature variation was investigated by applying tens of watts of microwave power at 5.8 GHz with the imposition of an external magnetic field (H^ext.). A substantial temperature increase in ferromagnetic materials was observed under ferromagnetic resonance (FMR) conditions. The external magnetic field causing the temperature peak (H^Tpeak) was almost explained by the resonant magnetic field, considering the effect of the demagnetization field.

In the permalloy foils, the H^Tpeak was almost constant with respect to dH^ext./dt. In the Fe foil and the Fe₃O₄ particles, the H^Tpeak increased with dH^ext./dt during the increasing field sweep runs of H^ext., but the H^Tpeak did not change very much during the decreasing runs.

As for these reasons, the saturation states of magnetization are taken into consideration. FMR could occur under conditions of unsaturated magnetization during increasing field sweep runs of H^ext., resulting in a lower H^Tpeak, due to the lower demagnetization field. However, during the decreasing runs of H^ext., the magnetization was saturated once or the specimen experienced the highest magnetization, which then resulted in a fixed H^Tpeak.

In the YIG particles, the effect of the H^ext. variation rate on the H^Tpeak was small in both increasing and decreasing field sweep runs of H^ext., because of the smaller magnetization compared to the others. It was also pointed out that macroscopic heat transfer processes need to be taken into consideration in order to interpret the observed H^Tpeak and the peak temperature.