Temperature-dependent Raman and IR studies of multiferroic MnWO4 doped with Ni2+ ions

Temperature-dependent Raman and IR studies of MnWO(4) crystal doped with Ni(2+) ions were performed in the 4.2-300 K range. These studies were complemented by magnetization and specific heat measurements in the 2-100K range, which revealed that MnWO(4) crystal doped with Ni(2+) ions exhibits two phase transitions at 13.9 and 12.5K. Temperature evolution of Raman wavenumbers and linewidths revealed anomalous behaviour at low temperatures. These anomalies have been attributed to spin-phonon coupling, which appear due to onset of antiferromagnetic spin ordering. The observed anomalies extend above T(N)=13.9 K. This behaviour is consistent with the fact that MnWO(4) is a moderately magnetically frustrated material.