Enhanced Spin Hall Conductivity in Low-Resistivity Metal/Semimetal Thin Film Heterostructure

The sustainability of numerous data-driven artificial intelligence applications will benefit from low-power nonvolatile memory [1]. Magnetic memory devices based on spin-orbit-torque (SOT)-driven switching offer fast switching, and high endurance [2]. However, high electrical resistivity of the traditional SOT-generating materials e.g., heavy metals and topological insulators often leads to large switching power in such devices [2,3]. To address this challenge, a SOT material system with simultaneously low electrical resistivity and large SOT efficiency together can result in an enhanced spin Hall conductivity required for low-power magnetic memory [4].<br/>In this work, we uncover a large spin Hall conductivity at room temperature in Ta/TaP metal/topological semimetal thin film heterostructures. Using harmonic Hall measurement in a 4 nm/ 2 nm Ta/TaP bilayer film (with 4 nm Co as the ferromagnet), we measured a spin Hall conductivity of ~ 2.1 × 10⁵(h/2π/2e) Ω^-1m^-1. This value is ~2× larger compared to that of control 6 nm metal Ta, and larger than most SOT materials (e.g., heavy metals, topological insulators) at similar thicknesses [4].<br/>The enhanced spin Hall conductivity in Ta/TaP bilayer is primarily enabled by the low electrical resistivity of the 2 nm thin TaP semimetal, which we attribute to a proportionally higher conduction through a surface channel in the ultrathin topological semimetal films [5]. We note that, the SOT efficiency in our Ta/TaP bilayer heterostructure is comparable to our control heavy metal Ta (≈ - 0.1).<br/>We sputter deposited these Ta/TaP heterostructures at relatively low temperatures of 400 °C on MgO or sapphire substrate yielding non-crystalline semimetal thin films. Remarkably these non-crystalline films lead to an enhanced spin Hall conductivity in our work that is otherwise expected from topological semimetals in their crystalline form because of their strong spin polarization and large momentum relaxation times of their surface states [6,7].<br/>In summary, we find an enhanced spin Hall conductivity in non-crystalline ultrathin TaP semimetal on a metal Ta seed layer. The results and the fundamental insights obtained here could inspire the ultrathin topological semimetal family as the next-generation SOT materials for high-density, low-power magnetic memory technology.<br/><b>References</b>: <b>1</b>. S. Salahuddin <i>et al</i>., Nature Electronics 1, 442–450 (2018). <b>2</b>. L. Liu <i>et al</i>., Science 336, 555–558 (2012). <b>3</b>. Y. Yang <i>et al</i>., Nature Communications 8, 1364 (2017). <b>4</b>. Q. Shao <i>et al</i>., IEEE Transactions on Magnetics 57, 7, 2021. <b>5</b>. M. Breitkreiz <i>et al</i>., Physical Review Letters 123, 066804 (2019). <b>6</b>. A. Johansson <i>et al</i>., Physical Review B 97, 085417 (2018). <b>7</b>. Y. Sun <i>et al</i>., Physical Review Letters 117, 146403 (2016).