Spin Torque Efficiency and Spin Transport In β-W and β-W(O_x) Thin Films

Tungsten, in its high resistivity, metastable, A15 form (β-W), has the largest known damping-like spin torque efficiency (|ξ_DL|≥0.3) of the elemental metals, whereas the low resistivity, stable, body-centered cubic structure (α-W) has |ξ_DL| of less than 0.05. We find that slow (≈0.02 nm/sec) sputter deposition of W onto amorphous surfaces, and polycrystalline surfaces with other than the bcc structure, generally results in β-W films, although the β-W phase is only stable to high temperature (≥400 °C) annealing for film thicknesses of less than 5 nm. Previous work has suggested that sputtering W in a partial atmosphere of O₂ can promote β-W growth and lead to even higher values of |ξ_DL|. To investigate this approach further, we have sputtered thin film bilayer stacks of β-W/Fe₆₀Co₂₀B₂₀ onto silicon wafers, varying the O₂ concentration during the deposition of the W, and determined ξ_DL by ST-FMR measurements. In our case, |ξ_DL| steadily decreases from over 0.30 for W films sputtered without O₂ to less than 0.10 for films sputtered with the highest O₂ to Ar ratio (4.0%). We will also report on the interfacial spin transparency of various β-W/FM interfaces and on the spin diffusion length in β-W, which are key parameters for optimization of spin-orbit torque devices.