Magnon dispersion and terahertz antiferromagnetic resonance frequencies in 3d transition-metal monoxides

Antiferromagnetic insulators 3 d transition-metal monoxides (TMOs), MnO, FeO, CoO, and NiO, are promising materials that open a new avenue for ultra-high speed operation physics in THz regime. Here, we investigate magnon dispersion and antiferromagnetic resonance (AFMR) frequencies of higher and lower modes ( ω α and ω β ) using semi-classical Larmor precession equation, in which magnetocrystalline anisotropy (MCA), magnetic dipole–dipole interaction (MDI), and spin-exchange interactions are considered. The calculated AFMR frequencies for MnO and NiO are in quantitative agreement with the experiments, where the inclusion of rhombohedral distortion is found to be necessary for ω β . In contrast, our results for FeO and CoO are only in qualitative agreement with the experiments. We also discuss the AFMR frequencies under the influence of magnetic parameters, particularly those related to the MCA and MDI. The calculations reveal that the AFMR frequencies exhibit different behavior with respect to magnetic parameters variation, depending on the magnetic easy axis of in-plane within (111) or out-of-plane with respect to (111). By analyzing the AFMR frequencies with respect to the magnetic parameters variation, we identify magnetic interactions that play the role in the AFMR frequencies of 3 d TMOs and partially explain the experimentally observed AFMR frequencies of FeO and CoO. Inclusion of non-negligible spin–orbit coupling effect such as spin-splitting may fully capture the experimentally observed AFMR frequencies of FeO and CoO.