Nontrivial temperature dependence of ferromagnetic resonance frequency for spin reorientation transitions

We find unusual temperature dependence of the ferromagnetic resonance (FMR) frequency fR for the spin-reorientation (SR) transition, in which the easy axis changes depending on temperature, observed in the Nd permanent magnet, Nd₂Fe₁₄B: fR ～ 0 below the SR transition temperature (TR), drastic increase of fR around TR, and decrease from a peak at higher temperatures. It is nontrivial that the SR transition causes the unusual behavior of the FMR frequency in a wide temperature region. We show the mechanism of the temperature dependence by theoretical and computational analyses. We derive a general relation between fR and magnetizations to help the understanding of the mechanism, and clarify that the fluctuation of the transverse magnetization is a key ingredient for the resonance in all temperature regions.