V. N. Krivoruchko

Phys. Rev. B 94, 054434 (2016)

The microscopic model of longitudinal spin excitations in a two-sublattice ferrimagnet using the diagrammatic technique for spin operators as been studied. The diagrammatic approach provides us with an efficient procedure to derive graphical representations forperturbation expansion series for different spin Green’s functions and thus to overcome limitations typical for phenomenological approaches. The infinite series involving all distinct loops built from spin wave propagators are summed up. These result in an expression for the longitudinal spin susceptibility χzz(q,ω). It is predicted that both acoustic and exchange longitudinal excitations are energetically above similar modes of transverse spin waves at the same temperature and wave vector.

Fig.1. Graphical representation of the equation for the effective
four-point vortex.