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.
Pavitra N. Shanbhag, Fabio Orlandi, Pascal Manuel, Martin Etter, Shrikant Bhat and A. Sundaresan
