Thickness and composition-dependent spin-orbit torque behaviors in perpendicularly magnetized Ta/W (t)/CoFeB and Ta_1-xW_x/CoFeB junction structures

The spin-orbit torque (SOT) generated in normal metal (NM)/ferromagnet (FM) junctions is of technological interest as it can reduce the critical current density required for magnetization switching in memory devices. Ta and W, 5d NMs, exhibit large spin-orbit couplings and are compatible with the semiconductor processing. In this study, we investigate variations in SOT properties of Ta/W(t)/CoFeB/MgO/Ta (bilayer NM structures, where t is the W thickness) and Ta_1-xW_x/CoFeB/MgO/Ta (alloyed NM structures, where x is the W concentration in at%) junctions prepared on Si substrates with thermal oxides. In the former structure, the effective spin-orbit torque (SOT) field gradually increases with the W thickness reaching its maximum at 2.0 nm, followed by a decrease. In the latter structure, the effective SOT field behaves differently in the Ta- and W-rich regions. In-plane-current-induced switching measurements show similar trends, where the minimum and maximum switching current densities for the bilayer and alloyed NM structures are 3.4–10.3 and 0.81 to 6.3 × 10⁷ A/cm², respectively. Microstructural analyses by X-ray diffraction and transmission electron microscopy suggest that the crystal phase of W changes from the amorphous to the α phase in the bilayer NM structure with the increase in the W thickness and the alloyed NM structure with the change in the concentration.