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The spin-reorientation transition (SRT) in rare-earth orthoferrites offers an important platform for exploring the coupling and manipulation of spin dynamics, which is crucial for developing high-frequency spintronic and terahertz (THz) magneto-optical devices. In this work, we systematically investigate the temperature- and magnetic-field-induced SRT behavior and the associated electron paramagnetic resonance (EPR) transitions of Yb3+ ions in a-cut YbFeO3 single crystals using time-domain terahertz spectroscopy (THz-TDS). The temperature-dependent measurements from 1.6 to 300 K reveal a distinct SRT near 7 K, marked by a sudden shift of the magnetic resonance mode frequency. This indicates a transition of the Fe3+ spin configuration from the low-temperature Γ2 phase to the high-temperature Γ4 phase, driven primarily by the temperature evolution of the anisotropic Fe3+-Yb3+ exchange interaction.
Under an external magnetic field applied along the a-axis at 20 K, the system exhibits an incomplete field-induced SRT from the Γ4 phase toward the Γ2 phase. In the intermediate Γ24 phase, both the quasi-AntiFerroMagnetic (q-AFM) and quasi-FerroMagnetic (q-FM) modes are simultaneously excited, as observed in the THz absorption spectra. Notably, even at the maximum field of 7 T, the transition remains incomplete, indicating the stabilization of the intermediate phase over a wide field range. In the low-frequency region (<0.8 THz), two absorption peaks exhibiting clear blue shifts with increasing magnetic field are identified as EPR transitions between Zeeman sublevels of the crystal-field-split Kramers doublets of Yb3+ ions.
All experimental observations, including the temperature- and magnetic-field-dependent frequency responses of the q-AFM and q-FM modes as well as the evolution of the electron paramagnetic resonance signals with magnetic field, have been quantitatively described by coupling a spin dynamics model with crystal field theory. The model successfully reproduces the continuous rotation of the macroscopic Fe3+ magnetization vector within the ac plane under an applied magnetic field, revealing the microscopic mechanism of the field-induced SRT. The analysis demonstrates that the SRT process results from the competition and synergy between the external magnetic field and the anisotropic Fe3+-Yb3+ exchange interaction, which collectively modulate the internal effective field and determine the stability of the intermediate Γ24 phase.
This study confirms the effective control of spin configurations in YbFeO3 via both temperature and magnetic field, provides a deeper understanding of the Fe3+-Yb3+ exchange interaction mechanism, and offers important experimental insights for the design of terahertz functional devices based on rare-earth orthoferrites.-
Keywords:
- Terahertz Spectroscopy /
- Rare-earth Orthoferrites /
- SRT /
- Exchange Interaction
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