Spin pumping from finite-sized electron systems in ballistic and diffusive transport regimes

This paper presents a theory for spin pumping from finite-sized conductors at paramagnetic resonance. The spin generation and transport in the system exposed to a rotating magnetic field are analyzed by using the nonequilibrium Green’s-function formalism, showing that a time-independent spin current is produced with no charge current when the electrochemical potentials in all leads attached to the system are identical. A physically transparent formulation for the spin pumping is derived in terms of the spin-current continuity equation with the source term due to the rotating field. The characteristics of spin-pumping systems are investigated by numerical calculation. In the ballistic transport regime, the pumped spin at resonance is augmented normally with the system size and the pumping intensity, insofar as the system contacts ideal leads that transport spin to the outside world and the pumping field is perturbatively weak. In the diffusive transport regime, the spin pumping is strongly enhanced with the degree of static disorder. The mechanism of enhancement is discussed in view of the weak localization.