Spin dynamics in the unconventional multiferroic AgCrS 2

The magnetic excitation spectrum of AgCrS${}_{2}$, a layered multiferroic compound with a collinear four-sublattice antiferromagnetic order, has been investigated by means of time-of-flight inelastic neutron scattering between 5 and 300 K. Coupling mean-field and spin-wave calculations, the experimental spectrum is well reproduced introducing, in the model Hamiltonian, a ferromagnetic exchange interaction between nearest neighbors and a non-negligible antiferromagnetic in-plane next-nearest-neighbor interaction. This study shows that the sign of the magnetic interaction between chromium ions depends on the Cr-Cr distance with a threshold value above which it becomes ferromagnetic and emphasizes the role of the magnetoelastic coupling in the understanding of the physical properties of AgCrS${}_{2}$.