Ultrastrong coupling phenomenon under anomalous phase transition in HoFeO3

This study investigates the spin dynamics of a b-cut HoFeO3 single crystal using time-domain terahertz magnetospectroscopy (TD-TMS) system. Although the b-axis is the magnetic hard axis (the magnon frequency remains unchanged under an external magnetic field (HDC), an anomalous HDC-induced phase transition was observed at 1.6 K. When HDC exceeded 2.2 T, the Ho3+-Fe3+ exchange interaction progressively weakened, triggering a reorientation of Fe3+ spins from the Γ2-type to the Γ4-type configuration. Then, the formed quasi-ferromagnetic (q-FM) mode meets the electron paramagnetic resonance (EPR) mode, and exhibits an anti-crossing phenomenon. Due to the enormous dipole moment in solids, the normalized coupling strength (η) reached 0.1456, indicating the spin-magnon system entered the ultrastrong coupling regime. These findings not only reveal distinctive spin-coupling dynamics in HoFeO3, but also offer a new platform for exploring ultrastrong light–matter interactions in rare-earth orthoferrites.