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      Spin torque study of the spin Hall conductivity and spin diffusion length in platinum thin films with varying resistivity

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          Abstract

          We report measurements of the spin torque efficiencies in perpendicularly-magnetized Pt/Co bilayers where the Pt resistivity \(\rho_{Pt}\) is strongly dependent on thickness \(t_{Pt}\) . The damping-like spin Hall torque efficiency per unit current density, \(\xi^j_{DL}\) , varies significantly with \(t_{Pt}\), exhibiting a peak value \(\xi^j_{DL}=0.12\) at \(t_{Pt} = 2.8 - 3.9\) nm. In contrast, \(\xi^j_{DL}/\rho_{Pt}\) increases monotonically with \(t_{Pt}\) and saturates for \(t_{Pt} > 5\) nm, consistent with an intrinsic spin Hall effect mechanism, in which \(\xi^j_{DL}\) is enhanced by an increase in \(\rho_{Pt}\) . Assuming the Elliott-Yafet spin scattering mechanism dominates we estimate that the spin diffusion length \(\lambda_s = (0.77 \pm 0.08) \times 10^{-15} \Omega m^2 /\rho_{Pt}\).

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          Spin torque switching with the giant spin Hall effect of tantalum

          We report a giant spin Hall effect (SHE) in {\beta}-Ta that generates spin currents intense enough to induce efficient spin-transfer-torque switching of ferromagnets, thereby providing a new approach for controlling magnetic devices that can be superior to existing technologies. We quantify this SHE by three independent methods and demonstrate spin-torque (ST) switching of both out-of-plane and in-plane magnetized layers. We implement a three-terminal device that utilizes current passing through a low impedance Ta-ferromagnet bilayer to effect switching of a nanomagnet, with a higher-impedance magnetic tunnel junction for read-out. The efficiency and reliability of this device, together with its simplicity of fabrication, suggest that this three-terminal SHE-ST design can eliminate the main obstacles currently impeding the development of magnetic memory and non-volatile spin logic technologies.
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            Observation of the nonlocal spin-orbital effective field.

            The spin-orbital interaction in heavy nonmagnetic metal/ferromagnetic metal bilayer systems has attracted great attention and exhibited promising potentials in magnetic logic devices, where the magnetization direction is controlled by passing an electric current. It is found that the spin-orbital interaction induces both an effective field and torque on the magnetization, which have been attributed to two different origins: the Rashba effect and the spin Hall effect. It requires quantitative analysis to distinguish the two mechanisms. Here we show sensitive spin-orbital effective field measurements up to 10 nm thick ferromagnetic layer and find the effective field rapidly diminishes with the increase of the ferromagnetic layer thickness. We further show that this effective field persists even with the insertion of a copper spacer. The nonlocal measurement suggests that the spin-orbital effective field does not rely on the heavy normal metal/ferromagnetic metal interface.
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              Quantifying interface and bulk contributions to spin-orbit torque in magnetic bilayers.

              Spin-orbit interaction-driven phenomena such as the spin Hall and Rashba effect in ferromagnetic/heavy metal bilayers enables efficient manipulation of the magnetization via electric current. However, the underlying mechanism for the spin-orbit interaction-driven phenomena remains unsettled. Here we develop a sensitive spin-orbit torque magnetometer based on the magneto-optic Kerr effect that measures the spin-orbit torque vectors for cobalt iron boron/platinum bilayers over a wide thickness range. We observe that the Slonczewski-like torque inversely scales with the ferromagnet thickness, and the field-like torque has a threshold effect that appears only when the ferromagnetic layer is thinner than 1 nm. Through a thickness-dependence study with an additional copper insertion layer at the interface, we conclude that the dominant mechanism for the spin-orbit interaction-driven phenomena in this system is the spin Hall effect. However, there is also a distinct interface contribution, which may be because of the Rashba effect.
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                Author and article information

                Journal
                1512.06931
                10.1103/PhysRevLett.116.126601

                Nanophysics

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