Magnon-Phonon Coupling in Two-Dimensional Ferrimagnetic Fe _0.75 Cu _0.25 NbO ₄

We report on the direct observation of two-magnon excitations and strong magnon-phonon coupling in the paramagnetic state of the two-dimensional ferromagnetic Fe0.75Cu0.25NbO4, using temperature-dependent Raman spectroscopy. Furthermore, we investigate the spin reorientation process followed by a magnetic transition from ferrimagnetic to paramagnetic. The spin reorientation phenomenon was evidenced through the Hopkinson effect observed in thermomagnetic curves and temperature-dependent Raman spectroscopy. The magnetic transition (TC = 908 K) was observed through temperature-dependent magnetization (M–T) measurements and confirmed by temperature- and field-dependent in situ Raman spectroscopy. Above TC = 860 K, two magnon modes emerge in the Raman spectrum at 104 and 968 cm–1. In addition, we observe strong interactions between magnons and phonons for the modes at 144 and 597 cm–1. The temperature-dependent Raman results show that magnons transfer their spin angular momentum to phonons and generate phonon spins via strong coupling. These findings suggest that magnon-phonon coupling can be directly observed in the temperature-dependent Raman scattering, providing insights into these interactions in 2D polycrystalline magnetic materials.