Magnetic ordering and exchange striction stabilized geometric ferroelectricity in multiferroic AgCrS2.

The magnetic and ferroelectric properties of multiferroic AgCrS2 are investigated by the full potential linearized augmented plane wave method. For the low temperature crystal structure, the ground state is found to be in the collinear double ferromagnetic striped state with a spin structure, which is in good agreement with the neutron scattering result. The interlayer magnetic interactions are comparable to the intralayer ones, indicating that AgCrS2 is actually a three-dimensional frustrated antiferromagnet. Moreover, an analysis of Born effective charges and electric polarization reveals that the partial ferroelectricity in AgCrS2 belongs rather to the 'geometric ferroelectric' class, in which exchange striction and/or lattice distortion play an important role in stabilizing the polar structure.