Successive ferroelectric transitions induced by complex spin structures in MnBi2S4
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 MnBi2S4, which has a HgBi2S4-type centrosymmetric monoclinic structure (C2/m). This compound reveals multiple magnetic transitions at TN1=27K, TN2=23K, and TN3=21.5K, 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 k1=(0,β,12) and magnetic superspace group C¯1.1′(α,β,γ)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′(0,12,γ)0s. The magnetic ground state (ICM3) is a polar helical spin structure with magnetic space group B2.1′(0,0,γ)ss which is incommensurately modulated with propagation vector k2=(0,0,γ) where γ=0.3793(1) (at 1.5 K).
FIG. (a) The Mn2+ spin arrangement in MnBi2S4 (at 24 K) with eight-unit cells showing AFM SDW propagating along the b axis. (b) The cycloidal Mn2+ spin configuration in MnBi2S4 (at 21 K) with eight-unit cells propagating along the c axis and spins lying in the ac plane in monoclinic cycloidal phase. (c) Helical arrangement of Mn2+ spins propagating along the c axis, where the adjacent spins are rotating at an angle of 137◦ in MnBi2S4.
