We measure the spin Hall angle in Au layers of 5 to 100 nm thicknesses by spin pumping from Y 3 Fe 5 O 12 epitaxial films grown by ultrahigh vacuum, off-axis sputtering. We observe a striking increase in the spin Hall angle for Au layers thinner than the measured spin diffusion length of 12.6 nm. In particular, the 5 nm Au layer shows a large spin Hall angle of 0.087, compared to those of 0.016 and 0.017 for the 50 and 100 nm Au layers, respectively, suggesting that the top surface plays a dominant role in spin Hall physics when the spin current is able to reach it. Other spin pumping related parameters, including Gilbert damping enhancement, interfacial spin mixing conductance, and spin current are also determined for Au layers of various thicknesses.Given the pervasive role ultrathin films in electrical and spin transport applications, this result emphasizes the importance of considering the impact of the top surface and reveals the possibility of tuning critical spin parameters by film thickness. PACS: 72.25.Ba, 76.50.+g, 72.25.Mk, 75.70.Ak
2The spin Hall effect (SHE) and its reciprocal process, the inverse spin Hall effect (ISHE), have generated intense interest in recent years as a means of producing, manipulating, and detecting spin currents in nonmagnetic materials, opening new routes to spin-based electronic applications.1-13 The ability to convert an unpolarized electrical current into a spin current can be quantitatively described by the spin Hall angle (θ SH ).14,15 θ SH is a material-specific quantity that arises from spin-orbit coupling (SOC). Its magnitude and sign are primarily determined by atomic number, and for transition metals, by the d-orbital filling. [16][17][18][19] Au is a transition metal with a large atomic number of 79, which should lead to strong SOC and hence a large θ SH . As a result, both SHE and ISHE in Au have been extensively studied by various techniques.
20-28These studies have reported values of θ SH for Au between 0.25% (0.0025) and 11% (0.11 during film growth and the substrate rotates at 10 degrees/sec to achieve optimal film uniformity.A radio-frequency power of 60 W is used for YIG sputtering, which gives a deposition rate of 0.51 nm/min. The Au is grown in-situ on the YIG film at room temperature by off-axis DC sputtering at a deposition rate of 2.24 nm/min. The crystalline quality of the YIG films and YIG/Au bilayers are examined by X-ray diffraction (XRD) and X-ray reflectivity (XRR) using a 3 Bruker D8 Discover high-resolution triple-axis X-ray diffractometer, and scanning transmission electron microscopy (STEM) using an FEI probe-corrected Titan 3 80-300 S/TEM. where e is the electron charge, σ Au is the Au conductivity, λ SD is the spin diffusion length in Au, is the effective interfacial spin mixing conductance, L is the sample length, P = 1.21 is a factor due to the ellipticity of magnetization precession, 5 γ is the gyromagnetic ratio, h rf = 0.25Oe is the rf magnetic field in the EPR cavity at P rf = 200 mW, and α is the Gilbert damping constant of ...