Magnetoelastic coupling and unconventional magnetic ordering in the multiferroic triangular lattice AgCrS2

The temperature evolution of the crystal and magnetic structures of the ferroelectric sulfide AgCrS${}_{2}$ has been investigated by means of neutron scattering. AgCrS${}_{2}$ undergoes at ${T}_{N}=41.6$ K a first-order phase transition, from a paramagnetic rhombohedral $R$3$m$ structure to an antiferromagnetic monoclinic structure with a polar $\mathit{Cm}$ space group. In addition to being ferroelectric below ${T}_{N}$, the low-temperature phase of AgCrS${}_{2}$ exhibits an unconventional collinear magnetic structure that can be described as double ferromagnetic stripes coupled antiferromagnetically, with the magnetic moment of Cr${}^{3+}$ oriented along $b$ within the anisotropic triangular plane. The magnetic couplings stabilizing this structure are discussed using inelastic neutron scattering results. Ferroelectricity below ${T}_{N}$ in AgCrS${}_{2}$ can possibly be explained in terms of atomic displacements at the magnetoelastically induced structural distortion. These results contrast with the behavior of the parent frustrated antiferromagnet and spin-driven ferroelectric AgCrO${}_{2}$.