Pressure induced spin-flop transition in multiferroic Mn$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$WO$_{4}$*

The effect of cobalt substitution in Mn$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$WO$_{4}$ results in the most complex multiferroic phase diagram with multiple polarization flops upon increasing Co content. Two critical concentrations occur at x$=$0.075 and x$=$0.15 that separates multiferroic phases with different spin structures and orientation of the ferroelectric polarization. For 0.075 \textless x \textless 0.15, neutron diffraction has revealed that the spin configuration forms an \textit{ac} spiral with associated electric polarization lying in the \textit{ac}-plane. With application of hydrostatic pressure, we have examined the evolution of spin structure for x$=$0.135 sample. We have observed a spin-flop transition to a conical spin structure similar to that of x\textgreater 0.15 samples. Our diffraction results are consistent with the pressure polarization measurement where a polarization flop to the $b$-axis is observed. The origin of pressure induced spin-flop transition is discussed.