Anomalous magnetic field-dependent spin dynamics in Gd0.3Ho0.7FeO3 single crystal near the ordering temperature of rare-earth ions

Understanding the magnon dynamics near the rare-earth ion ordering temperature is crucial for revealing the complex exchange interactions between R3+ and Fe3+ ions in RFeO3 (where R is a rare-earth element). With Ho3+ substitution, Gd0.3Ho0.7FeO3 undergoes a Γ4–Γ2 spin reorientation transition in the range 48–34 K, which induces complex magnetic competition between the Fe3+ and R3+ sublattices. Here, we employ low-temperature terahertz time-domain spectroscopy under magnetic field to investigate the magnetic field-dependent spin dynamics of Gd0.3Ho0.7FeO3 single crystals. At 2–6 K, with the magnetic field applied along the c-axis, two magnon resonance peaks are observed, which split between 1 and 6 T, merge at 7 T, and re-split at 8 T. This anomalous behavior, which distinctively differs from the simple monotonic splitting in typical canted antiferromagnets, is attributed to the strong Gd3+-Fe3+ antiferromagnetic coupling that drives the reversal of Fe3+ magnetic moments at a critical field. These results provide in-depth insight into the intricate R3+-Fe3+ coupling in orthoferrite antiferromagnets and offer potential avenues for manipulating magnons excitations.