Spin excitations of magnetoelectric LiNiPO4 in multiple magnetic phases

In magnetoelectric (ME) materials, magnetic moments and electric polarization are entangled, leading to the static and optical ME effects. This can be used in logical devices or for the manipulation of electromagnetic radiation in the GHz and THz spectral range where the oscillating magnetic moments absorb radiation. The authors use THz spectroscopy to study LiNiPO${}₄$, a four-spin system known to exhibit the static ME effect. Using linearly polarized THz radiation, they show that beside ordinary magnon resonances excited by the magnetic field of radiation there are electromagnons and ME spin excitations activated by the electric field of radiation, thus demonstrating that spin and charge dynamics are coupled in LiNiPO${}₄$. They refine the spin Hamiltonian parameters by mapping the spin resonance spectrum over a broad magnetic field range and fitting it with a mean-field spin model.