Magnetoelectric properties and low-energy excitations of multiferroic FeCr2 S4

© 2020 American Physical Society. We report on the low-frequency optical excitations in the multiferroic ground state of polycrystalline FeCr2S4 in the frequency range 0.3-3 THz and their changes upon applying external magnetic fields up to 7 T. In the ground state below the orbital-ordering temperature TOO=9 K we observe the appearance of several new modes. By applying the external magnetic field parallel and perpendicular to the propagation direction of the THz radiation, we can identify the strongest absorptions to be of predominantly electric-dipole origin. We discuss these modes as the low-energy electronic excitations of the Fe2+ ions (3d6, S=2) in an tetrahedral S2- environment. The eigenfrequencies and relative intensities of these absorption lines are satisfactorily reproduced by our calculation assuming an effective exchange field of 12.8cm-1 at the Fe2+-ions sites. The direction of the exchange field is found to be slightly tilted out of the ab plane. With our approach we can also describe previously reported results from Mössbauer studies and the order of magnitude of the electric polarization induced by orbital and noncollinear spin ordering