Spin Hall conductivity enhancement of tungsten by copper alloying

Abstract

Spin currents generated by the spin Hall effect in e.g. heavy metals like tungsten (W) are interesting candidates for next generation information storage applications. The spin Hall effect has intrinsic and extrinsic contributions of which the latter can be controlled by adding impurities into the material. This enhances the spin Hall angle (SHA) θSH, which is the ratio of charge to spin current density [1]. When considering energy efficient spin current generation, it is necessary to also consider the spin Hall conductivity, which is the SHA divided by the charge resistivity. While W in its high resistive phase (β-W) has the highest spin Hall angle among pure heavy metals, it has a relatively low spin Hall conductivity (SHC). In contrast, α-W is a low resistivity phase with a low SHA but high SHC [2].In this work, we investigated the changes in SHA and SHC in α-W with copper (Cu) impurities. We chose to measure the SHA with spin-torque ferromagnetic resonance (ST-FMR), as it also provides information about the ferromagnetic effective damping αeff. The effective damping parameter is essential when optimizing the stack for energy efficient applications because it is inversely proportional to the critical switching current density.Multilayer stacks of Ti(2)|Fe(5)|Cu100-xWx(5)|Ti(10) were fabricated with the Cu100-xWx layers deposited via co-sputtering of Cu and W at different RF and DC-powers to obtain alloys of different concentrations. The ST-FMR measurements were carried out for 6 different frequencies (10-15 GHz) and at each frequency the field was swept from 0 to 5 kOe. The measured DC voltage contains a symmetric and anti-symmetric Lorentzian function, whose prefactors can be used to obtain the spin Hall angle.We found that the SHA increases with increasing W concentration until it reaches a maximum at around 60% W. The SHA changes from 0.054 ± 0.004 to 0.20 ± 0.02 , which corresponds to an enhancement of 270%. For the SHC, the trend is similar, with an increase of 120%. Results are shown in Fig 1 (a) and (b), respectively. As the resistivity also increases with impurity content, the SHC shows a smaller enhancement than the SHA. The large SHA at 60% W could be attributed to a high crystallinity as revealed by XRD measurements.As ST-FMR offers the determination of the effective damping, we can determine the ratio of αeff/θSH, which is proportional to the critical magnetization switching current density [3]. We find that it increases only slightly with increasing W concentration (Fig 2 (a)). At 60% W, αeff/θSH decreases by a factor of 4 as compared to pure W. The ratio of αeff/θSH almost follows the inverse of the SHA (Fig 2 (b)), suggesting that the change in damping affects the ratio less than changes in SHA. Our findings here reveal that the Cu40W60 alloy has excellent spin-orbit torque switching characteristics, allowing it to be utilized in energy-efficient spintronic devices. **