An electromagnon in the magnetoelectric (ME) hexaferrite ${\mathrm{Ba}}_{0.5}{\mathrm{Sr}}_{2.5}{\mathrm{Co}}_2{\mathrm{Fe}}_{24}{\mathrm{O}}_{41}$ (${\mathrm{Co}}_2\mathrm{Z}$-type) single crystal is identified by time-domain terahertz (THz) spectroscopy. The associated THz resonance is active on the electric field ($E^{\omega }$) of the THz light parallel to the $c$ axis ($\parallel [001]$), whose spectral weight develops at a markedly high temperature, coinciding with a transverse conical magnetic order below 410 K. The resonance frequency of 1.03 THz at 20 K changes $-8.7\%$ and $+5.8\%$ under external magnetic field ($H$) of 2 kOe along [001] and [120], respectively. A model Hamiltonian describing the conical magnetic order elucidates that the dynamical ME effect arises from antiphase motion of spins which are coupled with modulating electric dipoles through the exchange striction mechanism. Moreover, the calculated frequency shift points to the key role of the Dzyaloshinskii-Moriya interaction that is altered by static electric polarization change under different $H$.

中文翻译：

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室温磁电六方铁氧体中具有灵敏太赫兹磁致变色现象的电磁铁

磁电（ME）六铁氧体中的电磁铁 ${巴}_{0.5}{r}_{2.5}{\mathrm{有限公司}}_{\mathrm{2个}}{铁}_{24}{\mathrm{Ø}}_{41}$ （${\mathrm{有限公司}}_{\mathrm{2个}}\mathrm{ž}$-型单晶通过时域太赫兹（THz）光谱鉴定。相关的太赫兹共振在电场上是活跃的（$E^{\omega }$）的太赫兹光平行于 $C$ 轴（$\parallel [001]$），其频谱权重在明显高温​​下发展，与低于410 K的横向锥形磁阶重合。在20 K时1.03 THz的共振频率发生变化 $-8.7％$ 和 $+5.8％$ 在外部磁场下（$H$）分别沿[001]和[120]为2 kOe。描述圆锥形磁阶的模型哈密顿量阐明了动态ME效应是由自旋的反相运动引起的，这些自旋通过交换约束机制与调制电偶极子耦合。此外，计算出的频移指向Dzyaloshinskii-Moriya相互作用的关键作用，该作用在不同条件下会因静电极化的改变而改变。$H$。