Large Spin Hall Angle in β-W Thin Films Grown on CoFeB without Oxygen Plasma

We report on growth optimization and magnetodynamic properties of beta-W/Co20Fe60B20 system. We show that a relatively low growth rate of similar to 0.17 angstrom/s is essential for the stabilization of the beta-phase of tungsten. The low growth rate allows for the residual oxygen present in the chamber to get incorporated into the growing film, which helps in the stabilization of beta-phase tungsten as evidenced by X-ray diffraction and X-ray photoelectron spectroscopy. Using these optimized growth conditions, we achieved the beta phase in tungsten thin films up to a thickness of 60 nm. The ferromagnetic resonance measurements of beta-W/Co20Fe60B20 show a linear behavior of the Gilbert damping constant with the inverse of the thickness of the CoFeB layer, from which, we calculated spin mixing conductance to be (1.62 +/- 0.15) x 10(18)m(-2). Using the inverse spin Hall effect measurements, we obtained a large spin Hall angle of -0.44 +/- (0.04) in beta-W, which is achieved without using oxygen plasma during growth of tungsten.