Successive ferroelectric transitions induced by complex spin structures in MnBi2S4

Our study reveals the emergence of ferroelectricity induced by incommensurate spin orders in the magnetic sulfide $\mathrm{Mn}{\mathrm{Bi}}₂{\mathrm{S}}₄$, which has a $\mathrm{Hg}{\mathrm{Bi}}₂{\mathrm{S}}₄$-type centrosymmetric monoclinic structure ($C2/m$). This compound reveals multiple magnetic transitions at ${T}_{\mathrm{N}1}=27\phantom{\rule{0.16em}{0ex}}\mathrm{K}, {T}_{\mathrm{N}2}=23\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and ${T}_{\mathrm{N}3}=21.5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, associated with three distinct incommensurate spin structures (ICM1, ICM2, and ICM3). ICM1 is described as an antiferromagnetic spin density wave with an incommensurate modulation vector ${\mathbit{k}}₁=(0,\ensuremath{\beta},\frac{1}{2})$ and magnetic superspace group $C\overline{1}.1{}^{\ensuremath{'}}(\ensuremath{\alpha},\ensuremath{\beta},\ensuremath{\gamma})0s$. While ICM2 has a similar propagation vector as ICM1, pyrocurrent measurements suggest the polar nature of this phase. This is further confirmed by neutron diffraction, which indicates a cycloidal structure with a magnetic superspace group $Bm.1{}^{\ensuremath{'}}(0,\frac{1}{2},\ensuremath{\gamma})0s$. The magnetic ground state (ICM3) is a polar helical spin structure with magnetic space group $B2.1{}^{\ensuremath{'}}(0,0,\ensuremath{\gamma})ss$ which is incommensurately modulated with propagation vector $\phantom{\rule{4pt}{0ex}}{\mathbit{k}}₂=(0,0,\ensuremath{\gamma})$ where $\ensuremath{\gamma}=0.3793(1)$ (at 1.5 K).