Spin current carried by magnons

A spin current is usually carried by electrons and generated due to the imbalance of up-spin and down-spin. Here we investigate another type of spin current, which is carried by magnons. Using nonequilibrium Green's-function technique, we have derived a Landauer-Buttiker-type formula for spin current transport. The spin current satisfies conservation condition and can be expressed in terms of the magnon Green's functions of the mesoscopic ferromagnetic isolating system. As an application of this theory, we study the magnon transport properties of a two-level magnon quantum dot in the presence of the magnon-magnon scattering. By solving the self-consistent equations, we obtain the nonlinear spin current as a function of the magnetochemical potential. The spin current generated by using a parametric quantum pumping mechanism is also discussed.