Resolving the spin reorientation and crystal-field transitions in TmFeO₃ with terahertz transient

Rare earth orthoferrites (RFeO₃) exhibit abundant physical properties such as, weak macroscopic magnetization, spin reorientation transition, and magneto-optical effect, especially the terahertz magnetic response, have received lots of attention in recent years. In this work, quasi-ferromagnetic (FM) and quasi-antiferromagnetic (AFM) modes arising from Fe sublattice of TmFeO₃ single crystal are characterized in a temperature range from 40 tO₃00 K, by using terahertz time-domain spectroscopy (THz-TDS). The magnetic anisotropy constants in ac-plane are estimated according to the temperature-dependent resonant frequencies of both FM and AFM modes. Here, we further observe the broad-band absorptions centered ∼0.52, ∼0.61, and ∼1.15 THz below 110 K, which are reasonably assigned to a series of crystal-field transitions (R modes) of ground multiplets (6 H 3) of Tm³⁺ ions. Specially, our finding reveals that the spin reorientation transition at a temperature interval from 93 to 85 K is driven by magnetic anisotropy, however, which plays negligible role on the electronic transitions of Tm ions in the absence of applied magnetic fields.