THz spectroscopy of spin waves in multiferroic LiFePO₄ in high magnetic fields

LiFePO₄ belongs to the family of multiferroic lithium ortho phosphates compounds where the entanglement of magnetization and electric polarization leads to variety of magnon modes in the THz range that have dynamic electric and magnetic properties. Thus it is important to study THz absorption selection rules of magnons to understand the magneto electric properties of these materials. LiFePO₄ orders antiferromagnetically below 50 K. We used THz absorption spectroscopy to measure magnon excitations in LiFePO₄ single crystals below 5 K in zero and in magnetic fields up to 17 T. Several optically active magnons were observed. From the polarization dependence of absorbed THz radiation we found magnetic- and electric-dipole active magnons, and also a prototypical example of magneto-electric excitation.