Structural and electronic characterization and spin-orbit effects on IrO2 thin films

Abstract

IrO2 has recently attracted considerable interest for spin-current detection since it exhibits a large spin Hall effect (SHE), a novel phenomenon where the spin-orbit coupling (SOC) converts a charge current into a spin current. In order to make IrO2 suitable for spintronic applications it is essential to grow it in nanostructures with controlled and optimized structure. Surprisingly enough, initial works on IrO2 nanostructures have reported not only a good behavior in the polycrystalline state of such material, but even also a better one in the amorphous state. This is quite an unexpected result that prompts us to understand the effect of structure and low dimensionality on the electronic and magnetic properties. A particular important question to elucidate is the effect of surfaces, interfaces and microstructure in the SOC. Aiming at this; we have grown several IrO2 thin films by means of the sputtering and PLD techniques, obtaining thus, samples with different microstructure, crystallinity and thickness. The strength of the SOC, more specifically 〈L·S〉, was determined by applying sum rule analysis to the XANES spectra. By measuring the HERFD-XANES in the IrO2 films we gained information on how the structural details (surface/bulk ratio, crystallinity, strain, etc.) affect to the strength of the spin-orbit interaction. In all the cases the XANES data showed strong “white lines” at both absorption edges with a significant SOC independently on the structural details. Our data so far indicate that samples with a worse degree of crystallinity have a better SOC, which explains the behavior reported by Fujiwara et al. This is a remarkable result in terms of practical applications. In addition, our data suggest that the crystal orientation and the lattice parameters may be a key factor affecting the strength of the SOC.