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

Abstract

MnWO4 passes three antiferromagnetic phase transitions below the Neel temperature TNQ=Q13.5 K. One of these phases is also ferroelectric andMnWO4 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 wavenumbers were observed below TN. 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 linewidths are unaffected by the magnetic phase transitions and continuously increase with temperature as expected for three-phonon decay processes.