Spin excitations of magnetoelectric LiNiPO4 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.
@article{arxiv.1904.06106,
title = {Spin excitations of magnetoelectric LiNiPO$_4$ in multiple magnetic phases},
author = {Laur Peedu and Vilmos Kocsis and Dávid Szaller and Johan Viirok and Urmas Nagel and Toomas Rõõm and Dániel Gergely Farkas and Sándor Bordács and Dmytro Kamenskyi and Uli Zeitler and Yusuke Tokunaga and Yasujiro Taguchi and Yoshinori Tokura and István Kézsmárki},
journal= {arXiv preprint arXiv:1904.06106},
year = {2019}
}