Room temperature spin-orbit torque efficiency in sputtered low-temperature superconductor δ -TaN

Abstract

In the course of searching for promising topological materials for applications in future topological electronics, we evaluated spin-orbit torques (SOTs) in high-quality sputtered \ensuremath{\delta}-TaN/${\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}$ devices through spin-torque ferromagnetic resonance (ST-FMR) and spin pumping measurements. From the ST-FMR characterization we observed a significant linewidth modulation in the magnetic ${\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}$ layer attributed to the charge-to-spin conversion generated from the \ensuremath{\delta}-TaN layer. Remarkably, the spin-torque efficiency determined from ST-FMR and spin pumping measurements is as large as $\mathrm{\ensuremath{\Theta}}=$ 0.034 and 0.031, respectively. These values are over two times larger than for \ensuremath{\alpha}-Ta, but almost five times lower than for \ensuremath{\beta}-Ta, which can be attributed to the low room temperature electrical resistivity $\ensuremath{\sim}74\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\mathrm{cm}$ in \ensuremath{\delta}-TaN. A large spin diffusion length of at least $\ensuremath{\sim}8\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ is estimated, which is comparable to the spin diffusion length in pure Ta. Comprehensive experimental analysis, together with density functional theory calculations, indicates that the origin of the pronounced SOT effect in \ensuremath{\delta}-TaN can be mostly related to a significant contribution from the Berry curvature associated with the presence of a topically nontrivial electronic band structure in the vicinity of the Fermi level (${E}_{\mathrm{F}}$). Through additional detailed theoretical analysis, we also found that an isostructural allotrope of the superconducting \ensuremath{\delta}-TaN phase, the simple hexagonal structure \ensuremath{\theta}-TaN, has larger Berry curvature, and that, together with expected reasonable charge conductivity, it can also be a promising candidate for exploring a generation of spin-orbit torque magnetic random access memory as cheap, temperature stable, and highly efficient spin current source.