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Register a new userCorrelation of the angular dependence of spin-transfer torque and giant magnetoresistance in the limit of diffusive transport in spin valves
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Angular variation of giant magnetoresistance and spin-transfer torque in metallic spin-valve heterostructures is analyzed theoretically in the limit of diffusive transport. It is shown that the spin-transfer torque in asymmetric spin valves can vanish in non-collinear magnetic configurations, and such a non-standard behavior of the torque is generally associated with a non-monotonic angular dependence of the giant magnetoresistance, with a global minimum at a non-collinear magnetic configuration.
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We use nanometer-sized point contacts to a Co/Cu spin valve to study the giant magnetoresistance (GMR) of only a few Co domains. The measured data show strong device-to-device differences of the GMR curve, which we attribute to the absence of averaging over many domains. The GMR ratio decreases with increasing bias current. For one particular device, this is accompanied by the development of two distinct GMR plateaus, the plateau level depending on bias polarity and sweep direction of the magnetic field. We attribute the observed behavior to current-induced changes of the magnetization, involving spin transfer due to incoherent emission of magnons and self-field effects.
An anomalous (inverse) spin accumulation in the nonmagnetic spacer may build up when the spin valve consists of magnetic films having different spin symmetries. This leads to wavy-like dependence of spin-transfer torque on the angle between magnetizations, as predicted by spin-dependent diffusive transport model, and also confirmed experimentally. Making use of these predictions, we have numerically studied the magnetization dynamics in presence of such a wavy-torque in Co(8 nm)/Cu(10 nm)/Py(8 nm) nanopillar, considering geometry with extended and etched Co layer. In both cases we specify conditions for the out-of-plane precession to appear in absence of external magnetic field and neglecting thermal fluctuations. We prove the assumption of wavy-like torque angular dependence to be fully consistent with experimental observations. We also show that some features reported experimentally, like nonlinear slope of frequency vs. current behavior, are beyond the applicability range of macrospin approximation and can be explained only by means of full micromagnetic analysis.
The generation of oscillations in the microwave frequency range is one of the most important applications expected from spintronics devices exploiting the spin transfer phenomenon. We report transport and microwave power measurements on specially designed nanopillars for which a non-standard angular dependence of the spin transfer torque (wavy variation) is predicted by theoretical models. We observe a new kind of current-induced dynamics that is characterized by large angle precessions in the absence of any applied field, as this is also predicted by simulation with such a wavy angular dependence of the torque. This type of non-standard nanopillars can represent an interesting way for the implementation of spin transfer oscillators since they are able to generate microwave oscillations without applied magnetic field. We also emphasize the theoretical implications of our results on the angular dependence of the torque.
The spin transfer torque (STT) can lead to steady precession of magnetization without any external applied field in magnetic spin valve where the magnetic layer have very different spin diffusion length. This effect is associated with an unusual angular dependence of the STT, called wavy (WAD-STT), predicted in the frame of diffusive models of spin transfer. In this article, we present a complete experimental characterization of the magnetization dynamics in the presence of a WAD-STT. The results are compared to the prediction of the magnetization dynamics obtained by single domain magnetic simulations (macrospin approximation). The macrospin simulations well reproduced the main static and dynamical experimental features (phase diagram, R(I) curves, dependence of frequency with current and field) and suggest that the dynamical excitations observed experimentally are associated with a large angle out-of-plane precession mode. The present work validates the diffusive models of the spin transfer and underlines the role of the spin accumulation and the spin relaxation effects on the STT.
A large unidirectional magnetoresistance (UMR) ratio of UMR/$R_{xx}sim$ $0.36%$ is found in W/CoFeB metallic bilayer heterostructures at room temperature. Three different regimes in terms of the current dependence of UMR ratio are identified: A spin-dependent-scattering mechanism regime at small current densities $J sim$ $10$$^{9}$A/m$^{2}$ (UMR ratio $propto$ $J$), a spin-magnon-interaction mechanism regime at intermediate $J sim$ $10$$^{10}$A/m$^{2}$ (UMR ratio $propto$ $J$$^{3}$), and a spin-transfer torque (STT) regime at $J sim$ $10$$^{11}$A/m$^{2}$ (UMR ratio independent of $J$). We verify the direct correlation between this large UMR and the transfer of spin angular momentum from the W layer to the CoFeB layer by both field-dependent and current-dependent UMR characterizations. Numerical simulations further confirm that the large STT-UMR stems from the tilting of the magnetization affected by the spin Hall effect-induced spin-transfer torques. An alternative approach to estimate damping-like spin-torque efficiencies from magnetic heterostructures is also proposed.
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