Magnetic-field-induced spontaneous polarization reversal in multiferroic Mn 0.85 Co 0.15 WO 4

The magnetic and ferroelectric properties of the multiferroic system Mn1-xCoxWO4 (x=0.135, 0.15, and 0.17) are studied in magnetic fields Hc oriented along the monoclinic c axis. Mn0.85Co0.15WO4, which is right at the phase boundary between two helical spin structures, exhibits a spontaneous sign change of the ferroelectric polarization when cooled in fields Hc> 25 kOe. The origin of the ferroelectric polarization is studied and two magnetic exchange interactions contributing to the polarization are identified. In Mn0.85Co0.15WO4, domains of the characteristic helical spin structures, known for x<0.15 and x>0.15, coexist and form domain boundaries. The contributions of the different domains to the global polarization are determined. The polarization reversal in Mn0.85Co0.15WO4 can be explained by a combination of various contributions to the polarization and a strong correlation between magnetic domains of different helical spin orders resulting in a smooth transition across the domain walls, which preserves the chirality of the spin spiral.