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Nonequilibrium Spin-transfer Torque in SFNFS Junctions

By Erhai Zhao


Spin-polarized current passing through a ferromagnet can transfer spin angular momentum to the ferromagnet and exert a torque on the magnetic moment [1, 2]. This mechanism offers unique opportunities to manipulate the magnetic state of nanomagnets. Experimentally spin-transfer torque driven magnetization precession and magnetization reversal have been observed in magnetic multilayers [3, 4]. A well studied multilayer system is the magnetic nanopillar which consists of a ferromagnetnormal metal-ferromagnet (FNF) trilayer connected to normal metal electrodes. The typical thickness of each layer is several nm, and the diameter of the pillar is of the order of 50 nm [4]. When sandwiched between superconducting electrodes, the FNF trilayer can mediate finite Josephson coupling to form a SFNFS Josephson junction [5]. In such junctions, scattering of quasiparticle at the magnetic interfaces is sensitive to the phase shift in each F layer as well as the condensate phase difference φ across the junction. This indicates the spin momentum transfer between the quasiparticles and ferromagnets can be tuned by varying φ. Waintal and Brouwer calculated the phase-sensitive equilibrium torque in SFNFS junctions [6]. They also showed the nonequilibrium torque in NFNFS junctions acquires novel features, e.g. it can favor perpendicular configuration of the two moments [7]

Year: 2006
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