Spin–orbit torque engineering in β-W/CoFeB heterostructures with W–Ta or W–V alloy layers between β-W and CoFeB

Abstract

The spin–orbit torque (SOT) resulting from a spin current generated in a nonmagnetic transition metal layer offers a promising magnetization switching mechanism for spintronic devices. To fully exploit this mechanism, in practice, materials with high SOT efficiencies are indispensable. Moreover, new materials need to be compatible with semiconductor processing. This study introduces W–Ta and W–V alloy layers between nonmagnetic β-W and ferromagnetic CoFeB layers in β-W/CoFeB/MgO/Ta heterostructures. We carry out first-principles band structure calculations for W–Ta and W–V alloy structures to estimate the spin Hall conductivity. While the predicted spin Hall conductivity values of W–Ta alloys decrease monotonically from −0.82\u2009×\u200910 3 \u2009S/cm for W 100 at% as the Ta concentration increases, those of W–V alloys increase to −1.98\u2009×\u200910 3 \u2009S/cm for W 75 V 25 at% and then gradually decrease. Subsequently, we measure the spin Hall conductivities of both alloys. Experimentally, when β-W is alloyed with 20\u2009at% V, the absolute value of the spin Hall conductivity considerably increases by 36% compared to that of the pristine β-W. We confirm that the W–V alloy also improves the SOT switching efficiency by approximately 40% compared to that of pristine β-W. This study demonstrates a new material that can act as a spin current-generating layer, leading to energy-efficient spintronic devices. Changes to materials used in spintronics, an emerging magnetic memory technology, can reduce the energy costs associated with manipulating electron spin. When an electron from a nonmagnetic metal travels into a thin magnetic film, a twisting force known as the spin-orbit torque can emerge. Findings from researchers led by Young Keun Kim, Korea University, Seoul, and Sonny Rhim, University of Ulsan, Ulsan, Korea, may make it easier to use spin-orbit torque to switch between the on/off states of magnetic memory devices. Through a combination of theory and experiment, the team found that the nonmagnetic metal alloys played a key role in torque generation. An optimized heterostructure with an alloy film based on tungsten and smaller amounts of vanadium boosted torque-based switching efficiency by 40 percent compared to those with pristine tungsten films. Based on the first-principles calculation on the β-W based alloy structure using Ta and V, we have introduced the W–Ta and W–V alloy in between the β-W/CoFeB layer. Through the harmonic response method, we confirmed that experimentally obtained spin-Hall conductivity has fairly similar alloy compositional dependence with the theoretically calculated one. Particularly, when W 80 V 20 alloy was placed at the β-W/CoFeB layer, the spin Hall conductivity reached (−2.77\u2009±\u20090.31)\u2009×\u200910 3 \u2009S/cm, which enhanced over 36% compared to the pristine β-W/CoFeB/MgO heterostructure.