The spin-reorientation transition (SRT) in rare-earth orthoferrites provides 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 behaviors and the associated electron paramagnetic resonance (EPR) transitions of Yb³⁺ ions in a-cut YbFeO₃ single crystals by using time-domain terahertz spectroscopy. The temperature-dependent measurements from 1.6 to 300 K show a significant SRT near 7 K, characterized 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-anti ferro magnetic (q-AFM) and quasi-ferro magnetic (q-FM) modes are simultaneously excited as observed in the THz absorption spectra. Notably, even at a 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 range (<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, are 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 jointly modulate the internal effective field and determine the stability of the intermediate Γ₂₄ phase.

In this study, the effective control of spin configurations in YbFeO₃ is confirmed through temperature and magnetic field, deepening the understanding of the Fe³⁺-Yb³⁺ exchange interaction mechanism, and offering important experimental insights for designing terahertz functional devices based on rare-earth orthoferrites.