Charge pumping by magnetization dynamics in magnetic and semi-magnetic tunnel junctions with interfacial Rashba or bulk extrinsic spin-orbit couplings

摘要

We develop a time-dependent nonequilibrium Green function (NEGF) approach tothe problem of spin pumping by precessing magnetization in one of theferromagnetic layers within F/I/F magnetic tunnel junctions (MTJs) or F/I/Nsemi-MTJs in the presence of intrinsic Rashba spin-orbit coupling (SOC) at theF/I interface or the extrinsic SOC in the bulk of F layers of finite thickness(F-ferromagnet; N-normal metal; I-insulating barrier). To express thetime-averaged pumped charge current, or the corresponding dc voltage signal inopen circuits that was measured in recent experiments on semi-MTJs [T. Moriyamaet al., Phys. Rev. Lett. 100, 067602 (2008)], we construct a novel solution forthe double-time-Fourier-transformed NEGFs. The two energy arguments of NEGFs inthis representation are connected by the Floquet theorem describing multiphotonemission and absorption processes. Within this fully quantum-mechanicaltreatment of the conduction electrons, we find that: (i) only in the presenceof the interfacial Rashba SOC the non-zero dc pumping voltage in F/I/N semi-MTJcan emerge at the adiabatic level (i.e., proportional to microwave frequency);(ii) a unique signature of this charge pumping phenomenon, which disappears ifRashba SOC is not located with the precessing F layer, is dc pumping voltagethat changes sign as the function of the precession cone angle; (iii) unlikestandard spin pumping in the absence of SOCs, where one emitted or absorbedmicrowave photon is sufficient to match the exact solution in the framerotating with the magnetization, the presence of the Rashba SOC requires totake into account up to ten photons in order to reach the asymptotic value ofpumped charge current; (iv) disorder within F/I/F MTJs can enhance the dcpumping voltage in the quasiballistic transport regime; ...