We investigate the magnetic dynamics of the orthorhombic perovskite ${\mathrm{TmFeO}}_3$ at low temperatures, below the spin reorientation transition at $T_{\mathrm{SR}}\approx 80$ K, by means of time-of-flight neutron spectroscopy. We find that the magnetic excitation spectrum combines two emergent collective modes associated with different magnetic sublattices. The Fe subsystem orders below $T_{\mathrm{N}}\sim 632$ K into a canted antiferromagnetic structure and exhibits sharp, high-energy magnon excitations. We describe them using linear spin-wave theory, and reveal a pronounced anisotropy between in- and out-of-plane exchange interactions, which was mainly neglected in previous reports on the spin dynamics in orthoferrites. At lower energies, we find two crystalline electrical field (CEF) excitations of ${\mathrm{Tm}}^{3+}$ ions at energies of $\sim 2$ and 5 meV. In contrast to the sister compound ${\mathrm{YbFeO}}_3$, where the ${\mathrm{Yb}}^{3+}$ ions form quasi-one-dimensional chains along the $c$ axis, the Tm excitations show dispersion along both directions in the $\left(0KL\right)$ scattering plane. Analysis of the neutron scattering polarization factor reveals a longitudinal polarization of the 2 meV excitation. To evaluate the effect of the CEF on the ${\mathrm{Tm}}^{3+}$ ions, we perform point-charge model calculations, and their results quantitatively capture the main features of Tm single-ion physics, such as energies, intensities, and polarization of the CEF transitions, and the type of magnetic anisotropy.

中文翻译：

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具有非克拉默斯离子的TmFeO3正铁氧体中的低温自旋动力学

我们研究斜方钙钛矿的磁性动力学 ${\mathrm{氧化铁}}_3$ 在低温下，低于自旋重取向转变 ${Ť}_{\mathrm{SR}}\approx 80$ K，通过飞行时间中子光谱学。我们发现，磁激发光谱结合了与不同磁子格相关的两个新兴的集体模式。Fe子系统订单如下${Ť}_{\mathrm{ñ}}〜632$ K变成倾斜的反铁磁结构，并表现出尖锐的高能磁振子激发。我们使用线性自旋波理论描述它们，并揭示了平面内和平面外交换相互作用之间的明显各向异性，这在以前有关正铁氧体自旋动力学的报告中主要被忽略。在较低的能量下，我们发现两个晶体电场（CEF）激发${\mathrm{Tm值}}^{3+}$ 能量为 $〜2$和5 meV。与姊妹院相比${\mathrm{氧化亚铁}}_3$，其中 $b^{3+}$ 离子沿着原子形成准一维链 $C$ Tm激发在轴上显示出沿两个方向的分散 $（0ķ\mathrm{大号}）$散射平面。对中子散射极化因子的分析揭示了2 meV激发的纵向极化。评估CEF对${\mathrm{Tm值}}^{3+}$ 离子，我们执行点电荷模型计算，其结果定量捕获Tm单离子物理学的主要特征，例如CEF跃迁的能量，强度和极化，以及磁各向异性的类型。