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Spin Dynamics in the Magnetoelectric Effect LiCoPO$_4$ Compound

Inelastic neutron scattering (INS) experiments were performed to investigate the spin dynamics in magnetoelectric effect (ME) LiCoPO$_4$ single crystals. Weak dispersion was detected in the magnetic excitation spectra along the three principal crystallographic axes measured around the (0 1 0) magnetic reflection. Analysis of the data using linear spin-wave theory indicate that single-ion anisotropy in LiCoPO$_4$ is as important as the strongest nearest-neighbor exchange coupling. Our results suggest that Co$^{2+}$ single-ion anisotropy plays an important role in the spin dynamics of LiCoPO$_4$ and must be taken into account in understanding its physical properties. High resolution INS measurements reveal an anomalous low energy excitation that we hypothesize may be related to the magnetoelectric effect of LiCoPO$_4$.