Nontrivial temperature dependence of ferromagnetic resonance frequency for spin reorientation transitions

TL;DR

This study reveals an unusual temperature-dependent behavior of ferromagnetic resonance frequency during spin-reorientation transitions in Nd$_2$Fe$_{14}$B, explained through theoretical and computational analysis highlighting the role of transverse magnetization fluctuations.

Contribution

It provides a new theoretical framework linking FMR frequency behavior to magnetization fluctuations during spin reorientation transitions.

Findings

FMR frequency drops to near zero below the transition temperature

Sharp increase of FMR frequency around the transition temperature

Decrease of FMR frequency at higher temperatures after a peak

Abstract

We find unusual temperature dependence of the ferromagnetic resonance (FMR) frequency $f_{\rm R}$ for the spin-reorientation (SR) transition, in which the easy axis changes depending on temperature, observed in the Nd permanent magnet, Nd$_2$Fe$_{14}$B: $f_{\rm R} \sim 0$ below the SR transition temperature ($T_{\rm R}$), drastic increase of $f_{\rm R}$ around $T_{\rm R}$, 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 $f_{\rm R}$ 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.