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

Magnetoelectric multiferroics feature crosscoupling between magnetic and charge order. Here, the authors study the absorption of electromagnetic radiation by spin waves in magnetoelectric LiFePO${}_{4}$ in the terahertz (THz) spectral range. Using linearly polarized THz radiation, they establish that there are spin waves that interact with magnetic $a\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}d$ electric components of radiation. This is unusual for electron spins since they are susceptible to magnetic fields only. Another striking observation is that the number of spin-wave modes in the THz spectrum exceeds four times the number of modes expected for a four-spin system. Using the mean-field model and the magnetic field dependence of the THz spectra, the authors identify four conventional spin-wave modes in LiFePO${}_{4}$. It is proposed that other THz-active spin-wave modes are more exotic quadrupolar excitations, such as spin-stretching modes and two-magnon excitations.