Spin-polarized currents driven by spin-dependent surface screening

Abstract

We have examined spin polarization of the electron current in a ferromagnetic metal induced by the spin-dependent surface screening at the dielectric-ferromagnetic metal (D-FM) interface. In an applied ac voltage, the dynamic band splitting driven by the changes in the screening charge at the D-FM interface develops spin accumulation. The resultant spin accumulation gradient produces a time-dependent spin current. We have derived the rate of the spin accumulation dependent on the rate of the screening charge density accumulation within the Stoner band model. The spin-charge dynamics in the system is then modeled by a set of diffusive equations with the contributions from spin-dependent surface screening and spin-dependent conductivity. We show for MgO-Cu-Co-MgO system that the spin-dependent screening in thin Co film produces spin accumulation 7 times higher than that obtained by the spin-dependent conductivity in thick Co films. We propose an experimental approach to validate our numerical predictions and to distinguish between spin accumulation induced by the spin-dependent conductivity and by the spin-dependent surface screening. *

Nonresonant amplification of spin waves through interface magnetoelectric effect and spin-transfer torque

“…Because the density of the screening charge is spin-dependent 25 – 27 , the dynamic spin-dependent potential produces nonequilibrium spin density at the interfaces, as we have demonstrated in Ref. 13 . Importantly, the strength of the effect depends on the magnetoelectric constant .…”

“…The spin-transfer torque is given by: and it is characterized by spin precession length and spin coherence length where nm/ps is the velocity of an electron at the Fermi level. The source term from spin-dependent surface screening is 13 : The magnetization dynamics is given by the Landau–Lifshitz–Gilbert equation with spin-transfer torque term: where GHz/T is gyromagnetic constant, is magnetic susceptibility and is Gilbert damping constant.…”

“…(10) in Ref. 13 is for Co and for Fe. The exchange constants were taken pJ/m both for Co and Fe, Gilbert damping , and the saturation magnetizations MA/m and MA/m.…”

“…The second method relies on the exchange interaction between spin-polarized conductive electrons and localized magnetic moments through spin-transfer torque (STT). For this method the spin accumulation has to be generated in the system by means of spin-dependent conductivity 11 , 12 , spin-dependent surface screening 13 or various other effects related to the spin–orbit coupling: spin Hall effect 14 – 18 , Rashba–Edelstein effect 19 , generalized spin–orbit torques 20 – 22 and topological effects 23 , 24 .…”