All-optical detection of the spin Hall angle inW/CoFeB/SiO2heterostructures with varying thickness of the tungsten layer

Abstract

The development of advanced spintronics devices hinges on the efficient generation and utilization of pure spin current. In materials with large spin-orbit coupling, the spin Hall effect may convert charge current to pure spin current and a large conversion efficiency, which is quantified by spin Hall angle (SHA), is desirable for the realization of miniaturized and energy efficient spintronic devices. Here, we report a giant SHA in beta-tungsten (β-W) thin films in Sub/W(t)/Co 20 Fe 60 B 20 (3 nm)/SiO 2 (2 nm) heterostructures with variable W thickness.We employed an all-optical time-resolved magneto-optical Kerr effect microscope for an unambiguous determination of SHA using the principle of modulation of Gilbert damping of the adjacent ferromagnetic layer by the spin-orbit torque from the W layer. A non-monotonic variation of SHA with W layer thickness (t) is observed with a maximum of about 0.4 at about t = 3 nm, followed by a sudden reduction to a very low value at t = 6 nm. This variation of SHA with W-thickness correlates well with the thickness dependent structural phase transition and resistivity variation of W above the spin diffusion length of W, while below this length the interfacial electronic effect at W/CoFeB influences the estimation of SHA.2

Determination of Spin-Orbit-Torque Efficiencies in Heterostructures with In-Plane Magnetic Anisotropy

“…From switching measurements, the zero-thermal critical switching current I c0 , thermal stability ∆, and effective damping-like SOT (DL-SOT) efficiency ξ DL eff of these W-based heterostructures can be further determined. |ξ DL eff | ≈0.32 is estimated for devices with 2.0 nm ≤ t Co-Fe-B ≤ 3.5 nm, which is fairly consistent with the magnitude for thin W layer obtained by other approaches [1,[16][17][18]. Fieldlike SOT (FL-SOT) efficiency can be simultaneously estimated and is found to be influenced by placing a MgO layer on top of Co40Fe40B20, with a magnitude much smaller than its damping-like counterpart (|ξ FL eff | ≤ 0.05).…”

Impact of impurities on the spin Hall conductivity in β -W

“…However this fitting procedure based on film thickness can be challenging in the case of β-W, because in thick films, the A15 crystal structure becomes unstable and decomposes into α-W. This can be seen in the work by Mondal et al 41 (black squares) where the transition from β-W to α-W around 5 nm is accompanied by a drop in the SHC to values in the range of our α-W intrinsic prediction. It should be noted that for the case of Pai et al (red circles), the estimated SHC for the thick 15 nm α-W film is higher than our predicted value.…”

“…The magnitude of the Spin Hall conductivity as a function of tungsten film thickness is shown for a variety of experimental measurements 4,5,37,41,42,[50][51][52][53][54] and theoretical predictions 6,47 . The calculated bulk intrinsic SHC for pristine β-W is shown at the Fermi energy (red dashed line) and for maximum possible value with p-type doping (black dashed line).…”

Temperature study of the giant spin Hall effect in the bulk limit of β−W

“…While body-centered-cubic structure α-W with a moderate resistivity of 30~40 µΩcm exhibits a much smaller GSHE, A15 structure β-W with a high resistivity of 100~300 µΩ-cm exhibits a larger GSHE [11,14,22]. The SHA of β-W thin films has been reported to be up to 35% ~ 40% [11,14,23]. Incorporating a large concentration of oxygen into W thin film pushes the SHA even higher to about 50% [10].…”