Magnetic-Field-Induced Spin Reorientation in TmFeO₃ Single Crystals

TmFeO₃ exhibits rich physical properties such as the magneto-optical effect, multiferroicity, and spin reorientation, making it of significant research value in condensed matter physics and materials science. In this study, we utilized a time-domain terahertz magneto-optical spectroscopy system to investigate the change in spin resonance frequency of TmFeO₃ single crystals at T=1.6 K under external magnetic fields 0-7 T. The TmFeO₃ sample was grown in an optical floating zone furnace and its crystallographic orientation was determined using back-reflection Laue X-ray photography with a tungsten target. The measurement setup is a self-built time-domain terahertz magneto-optical spectroscopy system, with a magnetic field range of 0-7 T, a temperature range of 1.6-300 K, and a spectral range of 0.2-2.0 THz. A pair of 1mm-thick ZnTe nonlinear crystals were used to generate and detect terahertz signals through optical rectification and electro-optic sampling techniques. The system's variable temperature and magnetic field are controlled by a superconducting magnet. In experiments, a linearly polarized terahertz wave is incident perpendicularly to the sample surface, and its magnetic component H_THz is parallel to the sample surface. By rotating the sample, the angle (q) between macroscopic magnetic moment M and H_THzcan be tuned, achieving selective excitations of the two modes, that is, q=0 for q-AFM mode or 90° for q-FM mode. Terahertz absorption spectroscopy results indicate that as the magnetic field increases, the quasi-ferromagnetic resonance (q-FM) of TmFeO₃ single crystal shifts towards high frequencies, and quasi-antiferromagnetic resonance (q-AFM) transitions to q-FM at low critical magnetic fields (2.2-3.6 T). Through magnetic structure analysis and theoretical fitting, it is confirmed that the magnetic moment of the single crystal undergoes magnetic field induced spin reorientation. This study contributes to a deeper understanding of the regulatory mechanism of the internal magnetic structure of rare earth ferrite under the combined effects of external magnetic field and temperature field, and the development of related spin electronic devices.