Exceptional field dependence of antiferromagnetic magnons in LiFePO$_4$

Abstract

Low-energy magnon excitations in magnetoelectric LiFePO4 have been investigated by highfrequency high-field electron spin resonance spectroscopy in magnetic fields up to B = 58 T and frequencies up to f = 745 GHz. For magnetic fields applied along the easy magnetic axis, the excitation gap softens and vanishes at the spin-flop field of BSF = 32 T before hardening again at higher fields. In addition, for B BSF we observe a resonance mode assigned to excitations due to Dzyaloshinskii-Moriya (DM)-interactions, thereby evidencing sizable DM interaction of ≈ 150 µeV in LiFePO4. Both the magnetisation and the excitations up to high magnetic fields are described in terms of a mean-field theory model which extends recent zero field inelastic neutron scattering results. Our results imply that magnetic interactions as well as magnetic anisotropy have a sizeable quadratic field dependence which we attribute to significant magnetostriction.