Incomplete softening of the antiferromagnetic resonance mode at the spin-reorientation transition in multiferroic GdFe3(BO3)4

Khoroshiy, I.N., Skorobogatov, S.A., Nikitin, S.E., (...), Titova, V.R., Pankrats, A.I.// Physical Review B//

The multiferroic ferroborate GdF⁢e3⁢(B⁢O3)4 with huntite-type structure exhibits magnetic ordering below 𝑇N=38 K and contains two magnetic subsystems associated with Gd and Fe ions. Competing anisotropies of these subsystems drive a spin-reorientation transition at 𝑇SR=10.7 K, switching the ground state from easy-axis to easy-plane. Using antiferromagnetic resonance, we investigate the spin dynamics across this transition. The observed incomplete softening of a magnon mode during both field- and temperature-induced spin-reorientation transitions indicates the first-order nature of the phase transition, which is accompanied by a discontinuous jump in the effective anisotropy field. We reproduce this behavior using a simple model that attributes the jump in the anisotropy field to the presence of an effective fourth-order anisotropy constant, responsible for the discontinuous character of the transition. Remarkably, for in-plane magnetic fields, we identify a new AFMR mode that persists from 12 K up to 𝑇N. This mode likely corresponds to the dynamics of a long-period incommensurate state, previously detected by resonant elastic x-ray scattering.