Quantitative study of spin Hall effects in mesoscopic thin films

Abstract

Spin Hall effect (SHE) can convert longitude charge current into a transverse spin current, owing to the strong spin-orbit coupling in heavy nonmagnetic metals, such as Pt. Its reciprocal process, inverse spin Hall effect (ISHE), converts spin current into charge current. [...] ☐ In this work, the SHE and ISHE of Pt films with mesoscopic dimensions are explored by using nonlocal spin injection/detection method at 10 K. We fabricate nonlocal SHE/ISHE structure, which consists of a Pt stripe, a ferromagnetic spin injector/detector bridged by a Cu channel. All relevant physical quantities are determined in-situ on the same substrate, and a quantitative approach is developed to characterize all processes effectively. Extensive measurements with various Pt thickness values reveal an upper limit for the Pt spin diffusion length: λ_pt ≤ 0.8 nm. The average product of spin Hall angle α_H and λ_pt is substantial: α_H λ_pt = (0.142 ± 0.040) nm for 4 nm thick Pt, though a gradual decrease is observed at larger Pt thickness. It’s noteworthy that the resistivity of the Pt film is 150 – 300 μΩ•cm because of mesoscopic lateral confinement, and this value is substantially larger than that of an extended film. The high resistivity enhances the energy efficiency of the spin Hall effects. ☐ Anomalous Hall effect (AHE) in ferromagnets shares the same physics and mechanism of SHE. Due to the ferromagnetic nature and the presence of charge current, it complicates the direct detection of the spin currents that accompanies AHE. By using nonlocal AHE/ISHE structures, we detect the spin accumulation generated by AHE in mesoscopic ferromagnetic Ni81Fe19 (permalloy or Py) films electrically. By exploring a series of devices with various Py thicknesses, the Py spin diffusion length λ_Py is found to be much shorter than the film thicknesses. The product of α_H and λ_Py is determined to be independent of thickness and resistivity: α_H λ_Py= (0.066 ± 0.009) nm at 5 K and (0.041 ± 0.010) nm at 295 K. These values are comparable to those obtained from mesoscopic Pt films. It makes Py an intriguing alternative to Pt, because of its lower cost and existing ferromagnetic properties.