# Spin excitations of magnetoelectric LiNiPO$_4$ in multiple magnetic phases

## TL;DR

This study investigates the spin excitations in magnetoelectric LiNiPO$_4$ across various magnetic phases using infrared spectroscopy, revealing diverse magnon modes, electromagnons, and two-magnon continua, and refining magnetic interaction parameters.

## Contribution

It provides a detailed analysis of spin excitations in LiNiPO$_4$ under magnetic fields, identifying different active modes and refining exchange and anisotropy parameters with a four-sublattice model.

## Key findings

- Identification of six spin resonances and a two-magnon continuum.
- Distinction between magnetic-dipole, electric-dipole, and magnetoelectric excitations.
- Refined values of exchange couplings, anisotropies, and Dzyaloshinskii-Moriya interaction.

## Abstract

Spin excitations of magnetoelectric LiNiPO$_4$ are studied by infrared absorption spectroscopy in the THz spectral range as a function of magnetic field through various commensurate and incommensurate magnetically ordered phases up to 33\,T. Six spin resonances and a strong two-magnon continuum are observed in zero magnetic field. Our systematic polarization study reveals that some of the excitations are usual magnetic-dipole active magnon modes, while others are either electromagnons, electric-dipole active, or magnetoelectric, both electric- and magnetic-dipole active spin excitations. Field-induced shifts of the modes for all three orientations of the field along the orthorhombic axes allow us to refine the values of the relevant exchange couplings, single-ion anisotropies, and the Dzyaloshinskii-Moriya interaction on the level of a four-sublattice mean-field spin model. This model also reproduces the spectral shape of the two-magnon absorption continuum, found to be electric-dipole active in the experiment.