Competing magnetic anisotropy fields and double polarization flops in multiferroic mn_1-xCo_xWO₄

We report the cobalt doping dependencies of the ferroelectric polarization and underlying incommensurate spiral magnetic ordering in multiferroic Mn _1-xCo_xWO₄ (0 ≤ x < 0:18). The spiral ordering changed its rotational plane upon cobalt doping (x ~ 0:1), followed by heliconical ordering at higher doping (x > 0:15). These consecutive changes were accompanied by simultaneous 90° 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.