Large magnon dichroism and other optical properties of the hexagonal ferrite h-Lu_0.6Sc_0.4FeO₃ with altermagnetic A₂ spin ordering

Multiferroic hexagonal h-Lu_0.6Sc_0.4FeO₃ single crystals with noncollinear spins were studied using the THz and Raman scattering spectroscopies and ellipsometry. Antiferromagnetic resonances, or magnons, were found at about 0.85 and 1.2 THz. These magnons harden as temperature increases and disappear above 130 K. This behavior is consistent with the magnetic susceptibility and a phase transition to a previously reported weak ferromagnetic state. A strong dichroism at the resonance with the antiferromagnetic doublet has been observed at zero external magnetic field using both conventional circular polarization and THz vector vortex beams. This observation is attributed to the strong altermagnetic properties of h-Lu_0.6Sc_0.4FeO₃ with broken P·T symmetry. The splitting of the magnon doublet in an external magnetic field applied along the c axis yields a g factor of 3.0 for the Fe³⁺ ions. Raman spectra of the optical phonons revealed a Fano-type asymmetry due to their interaction with a continuum of polar excitations. Electronic transitions were studied with ellipsometry and the results were compared with the modeled using density functional theory combined with embedded dynamical mean-field theory.