Spin dynamics in magnetic reorientation transitions

The Nd-Fe-B magnet is known as a permanent magnet with strong coercive field and commercially used for motors and generators, etc. It shows a magnetic reorientation transition at around 150 K (magnetic Tc is 600~700 K) [1]. The origin of the reorientation transition is higher order magnetic anisotropy constants, which causes a tilted spin state at low temperatures. We study the temperature dependence of ferromagnetic resonance frequency of an atomistic spin model of the magnet. We estimate the power spectrum by using the stochastic Landau-Lifshitz-Gilbert equation [2]. We find non-monotonic temperature dependence of the peak position of the frequency. We analyze the cause of this phenomenon and show that this non-monotonicity is universal for this type of magnetic reorientation transitions.

[1] M. Nishino et al., Phys. Rev. B 95, 094429 (2017).
[2] M. Nishino and S. Miyashita, Phys. Rev. B 91, 134411 (2015).