Terahertz spectroscopy of spin excitations in magnetoelectric LiFePO4 in high magnetic fields

We investigated the spin excitations of magnetoelectric LiFePO4 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 a four-spin mean-field model for fields applied along the three orthorhombic axes. From the fit of magnetization and magnon frequencies, 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 show a strong shift with the magnetic field, up to 4 cm-1 T-1, when the field is applied along the easy axis. Based on this field dependence, we attribute these high frequency resonances to the excitation of higher spin multipoles and of two magnons, which become THz-active due to the low symmetry of the magnetically ordered state.