Enhancement of spin-orbit torque in WS2/Co/Pt trilayers via spin-orbit proximity effect

Enhancement of spin-orbit torque (SOT) efficiency in ferromagnet/heavy-metal bilayer is promising for the realization of low-power spintronic devices. Here we show that inserting a single-layer ${\mathrm{WS}}_{2}$ between the substrate and Co/Pt layers, can reduce Co coercivity by $\ensuremath{\sim}28%$ and increase dampinglike SOT efficiency by $\ensuremath{\sim}30%$, up to 35.07 Oe/$({10}^{7}\mathrm{A}\text{/}{\mathrm{cm}}^{2})$. When inserting ${\mathrm{WS}}_{2}$ with different layers, we further demonstrate that these phenomena only exist for odd ${\mathrm{WS}}_{2}$ layers, i.e., monolayer and trilayer, while they disappear for even ${\mathrm{WS}}_{2}$ layers, i.e., bilayer. Theoretical analysis based on the first-principles calculations suggests that the results originate from the thickness-controlled charge transfer between ${\mathrm{WS}}_{2}$ and Co, which is consistent with the spin-orbit proximity effect.