Spin dynamics of antiferromagnetically coupled ferromagnetic bilayers—the case of Cr2WO6 and Cr2MoO6

Recent inelastic neutron diffraction measurements on Cr₂(Te, W, Mo)O₆ have revealed that these systems consist of bilayers of spin-3/2 Cr³⁺ ions with strong antiferromagnetic inter-bilayer coupling and tuneable intra-bilayer coupling from ferro (for W and Mo) to antiferro (for Te). These measurements have determined the ground state spin structure and the values of sublattice magnetization, which shows significant reduction of sublattice magnetization from the atomic spin value of 3.0 μ_B for Cr³⁺ atoms. In an earlier paper we theoretically investigated the low temperature spin dynamics of Cr₂TeO₆ bilayer system where both the intra and inter-bilayer couplings are antiferromagnetic. In this paper we investigate Cr₂WO₆ and Cr₂MoO₆ systems where intra-bilayer exchange couplings are ferromagnetic but the inter-bilayer exchange couplings are antiferromagnetic. We obtain the magnon dispersion, sublattice magnetization, two-magnon density of states, longitudinal spin-spin correlation function, and its powder average and compare the results for these systems with results for Cr₂TeO₆.