Spin waves in a semi‐infinite field‐induced metamagnet with three‐ion anisotropic interactions. I. Green function approach

A quartic spin model with three‐ion anisotropic interactions is employed to study surface and bulk spin waves in a layered structure of a semi‐infinite field‐induced metamagnet with a (001) surface at low temperatures, T ≪ T N . The three‐ion surface anisotropy field is allowed to deviate from the bulk value. The calculations involve a derivation of expressions for the spin‐spin Green functions for the semi‐infinite medium. The Green functions are found to have poles at energies corresponding to bulk and surface spin‐wave modes. The theory includes an evolution of the dispersion relations for both the antiferromagnetic and paramagnetic phases.