Refining magnetic interactions from the magnetic field dependence of spin-wave excitations in magnetoelectric LiFePO$_4$

TL;DR

This study uses high-field THz spectroscopy to analyze spin excitations in LiFePO4, revealing electromagnons, refining magnetic interaction parameters, and identifying higher-order excitations influenced by magnetic field and symmetry.

Contribution

We refined the magnetic interaction parameters in LiFePO4 by analyzing its spin excitations under high magnetic fields, revealing new electromagnon and multipole excitations.

Findings

Identification of electromagnons and magnetoelectric resonances.

Refined exchange couplings, anisotropies, and Dzyaloshinskii-Moriya interactions.

Observation of high-frequency excitations shifting with magnetic field.

Abstract

We investigated the spin excitations of magnetoelectric $\text{LiFePO}_4$ by THz absorption spectroscopy in magnetic fields up to 33$\,$T. By studying their selection rules, we found not only magnetic-dipole, but also electric-dipole active (electromagnons) and magnetoelectric resonances. The magnetic field dependence of four strong low-energy modes is reproduced well by our four-sublattice spin model for fields applied along the three orthorhombic axes. From the fit, we refined the exchange couplings, single-ion anisotropies, and the Dzyaloshinskii-Moriya interaction parameters. Additional spin excitations not described by the mean-field model are observed at higher frequencies. Some of them shows a strong shift with magnetic field, up to 4$\,\text{cm}^{-1}/\text{T}$, when the field is applied along the easy axis. Based on this field dependence, we attribute these high frequency resonances to excitation of higher spin multipoles and of two magnons, which become THz-active due to the low symmetry of the magnetically ordered state.