Room temperature spin-orbit torque efficiency in sputtered low-temperature superconductor delta-TaN
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 ${\delta}-$TaN/Co20Fe60B20 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 Co20Fe60B20 layer attributed to the charge-to-spin conversion generated from the ${\delta}-$TaN layer. Remarkably, the spin-torque efficiency determined from ST-FMR and spin pumping measurements is as large as ${\Theta} =$ 0.034 and 0.031, respectively. These values are over two times larger than for ${\alpha}-$Ta, but almost five times lower than for ${\beta}-$Ta, which can be attributed to the low room temperature electrical resistivity $\sim 74{\mu}{\Omega}$ cm in ${\delta}-$TaN. A large spin diffusion length of at least $\sim8$ 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 ${\delta}-$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 (EF). Through additional detailed theoretical analysis, we also found that an isostructural allotrope of the superconducting ${\delta}-$TaN phase, the simple hexagonal structure, ${\theta}-$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 sources.