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本文利用时域太赫兹光谱技术研究了a-cut YbFeO3单晶在温度(1.6-300 K)与磁场(0-7 T)调控下的自旋重取向(spin-reorientation transition，SRT)行为与稀土离子的电子顺磁共振(electron paramagnetic resonance，EPR)跃迁。实验观察到在约7 K附近发生温度诱导的SRT，表现为磁共振模式的突变。在20 K下，沿a轴施加磁场时，样品经历由Γ4相向Γ2相的不完全相变，在Γ24中间相中同时激发准反铁磁模(quasi-AntiFerroMagnetic，q-AFM)与准铁磁模(quasi-FerroMagnetic，q-FM)，并在低频区(<0.8 THz)识别出Yb3+的Zeeman子能级间的EPR跃迁。基于自旋动力学模型与晶体场理论，所有观测模式均获得和实验一致的定量拟合。该模型揭示了Fe3+宏观磁化强度在磁场中的连续转动行为，阐明了SRT的微观机制，SRT过程源于外磁场与Fe3+-Yb3+各向异性交换相互作用之间的竞争与协同，它们共同调制了系统的内部有效场，决定了中间相的稳定性和SRT行为。本研究证实了温度与磁场对YbFeO3自旋构型的有效调控，深化了对Fe3+-Yb3+交换作用机制的理解，为稀土正铁氧体太赫兹磁光器件的开发提供了重要依据。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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