We report on the low-frequency optical excitations in the multiferroic ground state of polycrystalline 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 $T_{OO}=9$~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 Fe${}^{2+}$ ions ($3d^6$, $S=2$) in an tetrahedral S${}^{2-}$ environment. The eigenfrequencies and relative intensities of these absorption lines are satisfactorily reproduced by our calculation assuming an effective exchange field of $12.8c{m}^{-1}$ 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 Mssbauer studies and the order of magnitude of the electric polarisation induced by orbital and non-collinear spin ordering.

中文翻译：

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多铁性FeCr2S4的磁电性质和低能激发

我们报告了在0.3-3 THz频率范围内多晶多铁基态的低频光学激发及其在施加高达7 T的外部磁场时的变化。在低于轨道有序温度的基态下 ${Ť}_{ØØ}=9$〜K我们观察到几种新模式的出现。通过施加与THz辐射的传播方向平行且垂直的外部磁场，我们可以确定最强的吸收主要来自电偶极子。我们将这些模式讨论为铁的低能电子激发${}^{2+}$ 离子（$3d^6$， $\mathrm{小号}=2$）在四面体S中${}^{2-}$环境。假设有效交换场为，通过我们的计算可以令人满意地再现这些吸收线的本征频率和相对强度。$12.8C{米}^{-\mathrm{1个}}$ 在铁${}^{2+}$离子站点。发现交换场的方向略微倾斜$\mathrm{一种}b$飞机。使用我们的方法，我们还可以描述以前由Mssbauer研究报告的结果以及由轨道和非共线自旋排序引起的电极化的数量级。