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.

• Received 30 October 2019

DOI:https://doi.org/10.1103/PhysRevB.101.014432