Magnetic phase diagram of multiferroic MnWO₄ probed by ultrasound,

The magnetic phase diagram of multiferroic MnWO₄ is studied in magnetic fields up to 60 T using sound velocity and sound attenuation measurements. Anomalies are observed at temperatures T_N1 = 7.6 K, T_N2 = 12.6 K and T_N3 = 13.6 K that separate commensurate antiferromagnetic (AF1) to helical AF2 and commensurate AF3 to paramagnetic phases, respectively. The anomalies are significantly different and shed light on the spin–phonon coupling and evolution of the various order parameters in this multiferroic material. For temperatures below T_N2 pronounced field hysteresis effects are also observed in the sound velocity, indicating field-induced transformations. In the temperature dependence of the attenuation we observe anomalies distinctively different from the usual maxima related to relaxation effects. They are attributed to the combination of dispersion effects due to domain walls and the discontinuously changing sound velocity. In total, six different magnetic phases, at various temperatures and fields including a novel high-field phase, are revealed and analyzed.