Optical spectroscopy on multiferroic MnWO4

MnWO4 (Huebnerite) crystallizes in a monoclinic crystal structure composed of alternating layers of Mn2+ and W6+ ions which are octahedrally coordinated by oxygen ions. The partially frustrated spin system exhibits a negative Curie-Weiss temperature TCW∼-75 K in the paramagnetic regime and undergoes a sequence of transitions into complex antiferromagnetic phases below TN∼13.5 K, including transitions from commensurate to incommensurate and collinear to non-collinear magnetic order, which are connected to the onset or decay of ferroelectric polarization. Thus MnWO4 belongs to the class of multiferroics. We present a detailed study of the phonon modes of this compound based on polarized reflectivity measurements in the FIR and MIR regime. In addition, polarized transmission measurements in the sub-mm regime below the phonon frequencies have been carried out in order to investigate the low-energy magnetoelectric excitations, so-called electro-magnons