Competing Magnetic Anisotropy Fields and Double Polarization Flops in Multiferroic Mn1-xCoxWO4

Abstract

We report the cobalt doping dependencies of the ferroelectric polarization and underlying incommensurate spiral magnetic ordering in multiferroic Mn1-xCoxWO4 (0 <= x < 0.18). The spiral ordering changed its rotational plane upon cobalt doping (x similar to 0.1), followed by heliconical ordering at higher doping (x > 0.15). These consecutive changes were accompanied by simultaneous 90 degrees flops of ferroelectric polarizations. Such doping dependencies are driven by the competitions between the anisotropy fields of MnWO4 and CoWO4, the spatial distributions of which are nearly orthogonal to each other. We propose a model that can consistently describe the two consecutive flops based on a combined picture of magnetic anisotropy and biquadratic exchange.