Magneto-electric properties and low-energy excitations of multiferroic FeCr2S4

TL;DR

This study investigates the low-energy electronic excitations in the multiferroic FeCr2S4, revealing new modes below 9K and analyzing their magnetic field dependence, contributing to understanding multiferroic excitations.

Contribution

It provides the first detailed analysis of THz excitations in FeCr2S4, linking observed modes to Fe^{2+} electronic states and modeling their behavior with an effective exchange field.

Findings

New low-energy modes appear below 9K.

Modes are mainly electric-dipole in origin.

Calculated eigenfrequencies match experimental data.

Abstract

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 T_OO=9K 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 Fe^{2+} ions (3d^6, S=2) in an tetrahedral environment. The eigenfrequencies and relative intensities of these absorption lines are satisfactorily reproduced by our calculation assuming an effective exchange field of 12.8 cm-1 at the Fe^{2+}-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 M\"ossbauer studies and the order of magnitude of the electric polarisation induced by orbital and non-collinear spin ordering.