Large enhancement of the spin Hall effect in Au by scattering with side-jump on Ta impurities

Abstract : We present measurements of the Spin Hall Eect (SHE) in AuW and AuTa alloys for a large range of W or Ta concentrations by combining experiments on lateral spin valves and Ferromagnetic-Resonance/spin pumping technique. The main result is the identication of a large enhancement of the Spin Hall Angle (SHA) by the side-jump mechanism on Ta impurities, with a SHA as high as + 0.5 (i.e 50%) for about 10% of Ta. In contrast the SHA in AuW does not exceed + 0.15 and can be explained by intrinsic SHE of the alloy without signicant extrinsic contribution from skew or side-jump scattering by W impurities. The AuTa alloys, as they combine a very large SHA with a moderate resistivity (smaller than 85 µΩ.cm), are promising for spintronic devices exploiting the SHE. A goal of spintronics is to generate, manipulate and detect spin currents for the transfer and manipulation of information, thus allowing faster and low-energy consuming operations. Since the discovery of the Giant Magnetoresistance a "classical" way to produce spin currents was to take advantage of ferromagnetic materials and their two different spin channel conductivities [1, 2]. In the last decade the rediscovery and study of spin orbit interaction eects brought new insights into creation mechanisms of spin currents. Among all mechanisms the spin Hall eect (SHE) focused a lot of attention as it allows the generation of spin currents from charge current and vice versa [3]. Despite being observed only a decade ago [46] these eects are already ubiquitous within the Spintronics as standard spin-current generators and detectors [7 9]. The conversion coecient between charge and spin currents is called the spin Hall angle (SHA) and is dened as Θ SHE = ρ xy /ρ xx , ratio of the non-diagonal and diagonal terms of the resistivity tensor. One of the main interests of the SHE is to provide a new paradigm for Spintronics where non-magnetic materials becomes active spin current source and detector. Until now most of the reports focused on single heavy metals and intrinsic SHE mechanisms, the main materials of interest being: Pt, Ta, W, and some oxydes. With intrinsic mechanisms the SHA is typically proportional to the resistivity of the heavy metal, and generally, a large value of the SHA is associated with a high resistivity (i.e.-0.3 for the SHA in β − W is associated with 263 µΩ.cm [10]) which limits the current density and the resulting spin transfer torques on the mag-netisation of an adjacent metallic ferromagnetic material. Extrinsic SHE mechanisms associated with the spin dependent scattering on impurities or defects are an alternative to generate transverse spin currents [11]. Two particular scattering mechanisms have been identied: the skew scattering [12] providing a non-diagonal term of the resistivity tensor proportional to the longitudinal resistivity (ρ xy ∝ ρ xx) and the side jump [13] for which the non-diagonal term is proportional to the square of the resistivity (ρ xy ∝ ρ 2 xx). For instance, the skew scattering mechanism have been observed in CuIr, CuBi, CuPb alloys (SHA=0.02,-0.24,-0.13, resp.) [14, 15]. The intrinsic mechanism from Berry curvature in the conduction band gives the same dependence ρ xy ∝ ρ 2 xx as the side-jump contribution so that, for example the SHE of AuPt (SHA=0.3 at max.) alloys could be explained by a predominant intrinsic eect rather than ascribed to side-jump[16]. In this letter we present a study of Au-based alloys with W and T a impurities. We demonstrate that the side-jump scattering mechanism dominates in AuTa alloys, and generates high spin Hall angles (up to + 0.5) with the additional advantage of resistivities (ρ AuT a < 85µΩ.cm) smaller than in most materials with SHA in the same range. By contrast in AuW alloys the SHE is mainly due to only the intrinsic mechanism and is denitely smaller than in AuTa. This dierence between AuTa and AuW is supported by ab − initio calculations. The alloys were fabricated by DC magnetron sputtering by co-deposition of the two pure materials. The concentration in atomic purcent were determined by chemical analyzes (proton or electron induced X-ray emission) and from the depo-sition rate of each species. We control the alloy-ing through the increase of the resistivity as the
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P Laczkowski, Y Fu, H Yang, J.-C. Rojas-Sánchez, P Noel, et al.. Large enhancement of the spin Hall effect in Au by scattering with side-jump on Ta impurities. Physical Review B : Condensed matter and materials physics, American Physical Society, 2017, 96 (14), pp.140405. ⟨10.1103/PhysRevB.96.140405⟩. ⟨hal-01579001⟩

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