Spin-phonon coupling in highly correlated transition-metal monoxides

We report on the optical response of the highly correlated transition-metal monoxides MnO, FeO, CoO and NiO in the far-infrared regime. The main focus is put on spin-phonon coupling effects, which are found to significantly influence the lattice dynamics in the magnetically ordered phase. Measurements have been performed in the ordered and paramagnetic state for temperatures up to 550 K. A clear splitting of the cubic mode accompanying the transition into long range magnetic order can be identified in MnO, CoO and NiO. In the case of FeO it is argued that an anisotropic phonon response seems to be very likely, though it could not be observed directly. The results are compared to recent experimental and theoretical studies in frustrated magnets, which predict the splitting of zone center phonon modes induced by a non-cubic spin-density distribution.