Current-induced magnetization switching in Pt/Co/W and Pt/Co/W_0.82Pt_0.18 structures with perpendicular magnetic anisotropy

Magnetization switching caused by current-generated spin-orbit torques in heavy metal/ferromagnetic metal/heavy metal heterostructures has become a significant issue due to its excellent performance in low-power and high-density storage applications. In our work, based on Pt/Co/W and Pt/Co/W_0.82Pt_0.18 structures with perpendicular magnetic anisotropy (PMA), the effective spin Hall angles (${\theta }_{SH}^{eff}$) of the Pt/Co/W and Pt/Co/W_0.82Pt_0.18 structures with different thicknesses of W and W_0.82Pt_0.18 alloy were determined by the harmonic measurements. We observed that the spin Hall angles were related to the thickness of the top-heavy metal layer. The magnetization switching efficiency is improved by the combined action of the bottom Pt and the top W due to the opposite signs of the spin Hall angles for the two heavy metals. When the thickness of the top-heavy metal layer reaches 3 nm, both Pt/Co/W and Pt/Co/W_0.82Pt_0.18 show their maximum effective spin Hall angles of 0.244 ± 0.008 and 0.223 ± 0.007, respectively. Nevertheless, the effective spin Hall angles were decreased with the thickness beyond 4 nm, which can be explained by the phase of W becomes a pure α-W from the mixed-phase of α-W and β-W. It indicates that the spin Hall angles of β-W were larger than α-W. Our results confirm an effective method to enhance the spin Hall angles and decrease the switching current density. (paper)