LAUSR

We report a spin-driven ferroelectricity below the helical spin ordering temperature in a nonoxide mineral $\mathrm{MnS}{\mathrm{b}}_{2}{\mathrm{S}}_{4}$, which crystallizes in a centrosymmetric monoclinic $\mathrm{HgB}{\mathrm{i}}_{2}{\mathrm{S}}_{4}$ structure. At $\mathrm{T}\ensuremath{\sim}25\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, the dielectric constant shows an anomaly and the spontaneous electric polarization begins to develop. The polarization obtained from pyroelectric current measurement can be switched by reversing the direction of poling electric field. Interestingly, upon applying magnetic field parallel to the electric field, the polarization is enhanced nearly six times above a critical field $H||E\ensuremath{\sim}5\phantom{\rule{0.16em}{0ex}}\mathrm{T}$, demonstrating large magnetoelectric effect in this system. Below $\mathrm{T}\ensuremath{\sim}25\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, this compound is known to exhibit an unusual magnetic structure where the spiral spins lie in the ac plane but propagate along the $b$ axis with an angle of $\ensuremath{\sim}{133}^{\ensuremath{\circ}}$ between the adjacent magnetic moments. The origin of ferroelectric ordering has been attributed to the inverse Dzyaloshinskii-Moriya type interaction in accordance with local symmetry considerations.