High-resolution polarised Raman scattering study on spin-phonon coupling in multiferroic MnWO4

MnWO4 passes three antiferromagnetic phase transitions below the Neel temperature T-N = 13.5 K. One of these phases is also ferroelectric and MnWO4 is therefore referred to as multiferroic. We investigated the effect of magnetic and electric ordering on the lattice dynamics by high-resolution polarized Raman spectroscopy. With an accuracy of about 0.02 cm(-1), anomalous shifts of nearly all Raman-active phonon wave numbers were observed below T-N. High-energy metal-oxygen stretching modes soften on cooling, while low-energy vibrations become harder. Due to their linear temperature dependence, these anomalies can be attributed to a higher-order spin-phonon coupling process. The phonon line widths are unaffected by the magnetic phase transitions and continuously increase with temperature as expected for three-phonon decay processes.