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The field of spin electronics (spintronics) was initiated by the discovery of giant magnetoresistance (GMR) for which Fert[1] and Grunberg[2] were awarded the 2007 Nobel Prize for Physics. GMR arises from differential scattering of the majority and minority spin electrons by a ferromagnet (FM) so that the resistance when the FM layers separated by non-magnetic (NM) spacers are aligned by an applied field is different to when they are antiparallel. In 1996 Slonczewski[3] and Berger[4] predicted that a large spin-polarised current could transfer spin-angular momentum and so exert a spin transfer torque (STT) sufficient to switch thin FM layers between stable magnetisation states[5] and, for even higher current densities, drive continuous precession which emits microwaves[6]. Thus, while GMR is a purely passive phenomenon which ultimately depends on the intrinsic band structure of the FM, STT adds an active element to spintronics by which the direction of the magnetisation may be manipulated. Here we show that highly non-equilibrium spin injection can modify the scattering asymmetry and, by extension, the intrinsic magnetism of a FM. This phenomenon is completely different to STT and provides a third ingredient which should further expand the range of opportunities for the application of spintronics.
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Spin-transfer torque and current induced spin dynamics in spin-valve nanopillars with the free magnetic layer located between two magnetic films of fixed magnetic moments is considered theoretically. The spin-transfer torque in the limit of diffusive spin transport is calculated as a function of magnetic configuration. It is shown that non-collinear magnetic configuration of the outermost magnetic layers has a strong influence on the spin torque and spin dynamics of the central free layer. Employing macrospin simulations we make some predictions on the free layer spin dynamics in spin valves composed of various magnetic layers. We also present a formula for critical current in non-collinear magnetic configurations, which shows that the magnitude of critical current can be several times smaller than that in typical single spin valves.
Recent experimental measurements of magnetoresistance in dual spin valves [A. Aziz et al., Phys. Rev. Lett. 103, 237203 (2009)] reveal some nonlinear features of transport, which have not been observed in other systems. We propose a phenomenological model describing current-dependent resistance (and giant magnetoresistance) in double spin valves. The model is based on a modified Valet-Fert approach, and takes into account the dependence of bulk/interface resistance and bulk/interface spin asymmetry parameters for the central magnetic layer on spin accumulation, and consequently on charge current. Such a nonlinear model accounts for recent experimental observations.
Giant magneto-Seebeck (GMS) effect was observed in Co/Cu/Co and NiFe/Cu/Co spin valves. Their Seebeck coefficients in parallel state was larger than that in antiparallel state, and GMS ratio defined as (SAP-SP)/SP could reach -9% in our case. The GMS originated not only from trivial giant magnetoresistance but also from spin current generated due to spin polarized thermoelectric conductivity in ferromagnetic materials and subsequent modulation of the spin current by spin configurations in spin valves. Simple Mott two-channel model reproduced a -11% GMS for the Co/Cu/Co spin valves, qualitatively consistent with our observations. The GMS effect could be applied simultaneously sensing temperature gradient and magnetic field and also be possibly applied to determine spin polarization of thermoelectric conductivity and Seebeck coefficient in ferromagnetic thin films.
The spin absorption process in a ferromagnetic material depends on the spin orientation relativelyto the magnetization. Using a ferromagnet to absorb the pure spin current created within a lateralspin-valve, we evidence and quantify a sizeable orientation dependence of the spin absorption inCo, CoFe and NiFe. These experiments allow determining the spin-mixing conductance, an elusivebut fundamental parameter of the spin-dependent transport. We show that the obtained valuescannot be understood within a model considering only the Larmor, transverse decoherence and spindiffusion lengths, and rather suggest that the spin-mixing conductance is actually limited by theSharvin conductance.
For the technologically relevant spin Hall effect most theoretical approaches rely on the evaluation of the spin-conductivity tensor. In contrast, for most experimental configurations the generation of spin accumulation at interfaces and surfaces is the relevant quantity. Here, we directly calculate the accumulation of spins due to the spin Hall effect at the surface of a thin metallic layer, making quantitative predictions for different materials. Two distinct limits are considered, both relying on a fully relativistic Korringa-Kohn-Rostoker density functional theory method. In the semiclassical approach, we use the Boltzmann transport formalism and compare it directly to a fully quantum mechanical non-equilibrium Keldysh formalism. Restricting the calculations to the spin Hall induced, odd in spatial inversion, contribution in the limit of the relaxation time approximation we find good agreement between both methods, where deviations can be attributed to the complexity of Fermi surfaces. Finally, we compare our results to experimental values of the spin accumulation at surfaces as well as the Hall angle and find good agreement for the trend across the considered elements.
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