Elementary excitations in antiferromagnets with degenerate ground state: magnon energies and neutron scattering cross sections for the garnet Fe2Ca3 (GeO4)3

Abstract

Within linear spin wave theory, we discuss the magnetic order of the antiferromagnetic garnet Fe2Ca3 (GeO4)3. We present this as an example of a system with a degenerated mean-field ground state, which consists here of a pair of enantiomorphic structures. While in mean field theory the first neighbour interaction cancels out for the magnetic structure, we show that it leads to a splitting of the acoustical spin wave branch. Stability regions in the space of the exchange constants are predicted. Taking into account the full symmetry of the garnet structure, we obtain additional optical branches not considered in earlier publications. For an inelastic neutron scattering experiment, we have calculated the double differential cross section for one magnon processes and predict regions in reciprocal space where the scattering should be strongest.