Exceptional field dependence of antiferromagnetic magnons in LiFePO$_4$

TL;DR

This study investigates the magnetic excitations in LiFePO4 under high magnetic fields, revealing field-dependent magnon behavior, a Dzyaloshinskii-Moriya interaction, and significant magnetostriction effects.

Contribution

It provides the first high-field ESR measurements of LiFePO4, demonstrating the field dependence of magnons and quantifying the DM interaction and magnetostriction effects.

Findings

Magnon gap softens and vanishes at 32 T, then hardens again.

Detection of a DM interaction of approximately 150 μeV.

Magnetic interactions and anisotropy show quadratic field dependence.

Abstract

Low-energy magnon excitations in magnetoelectric LiFePO$_4$ have been investigated by high-frequency 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 smaller than BSF we observe a resonance mode assigned to excitations due to Dzyaloshinskii-Moriya (DM)-interactions, thereby evidencing sizable DM interaction of approx 150 micro eV 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 sizable quadratic field dependence which we attribute to significant magnetostriction.