Spin polarized tunneling study on spin Hall effect metals and topological insulator

Abstract

Spin orbit interactions give rise to interesting physics phenomena in solid state materials such as the spin Hall effect (SHE) and topological insulator surface states. Those effects have been extensively studied using various electrical detection methods. However, to date most experiments focus only on characterizing electrons near the Fermi surface, while spin-orbit interaction is expected to be energy dependent. Here we developed a tunneling spectroscopy technique to measure spin Hall materials [1] and topological insulators [2] under finite bias voltages. By electrically injecting spin polarized electrons into spin Hall metals or topological insulators using nonmagnetic material (NM) /oxide/ferromagnet (FM) junctions and measuring the induced transverse voltage (Fig. 1), we are able to study SHE in typical 5d transition metals and the spin momentum locking in topological insulators (Fig. 2). For spin Hall effect metals, the magnitude of the spin Hall angle has been a highly controversial topic in recent studies [3,4]. Results obtained from various techniques (such as non-local spin valves, spin pumping and spin Hall torque) can differ by more than an order of magnitude. With the spin polarized tunneling experiment, we determined the spin Hall angles for heavy element metals (e.g., Ta, Pt, Ir and W). These results turned out to be consistent with the numbers obtained from spin Hall torque measurements, which can help to address this long debating issue. Besides the magnitude, the mechanism of the observed spin Hall effect (or spin orbit torques) is another issue that has attracted a lot of research interest. Competing mechanisms (intrinsic vs extrinsic, surface vs bulk origin) have been proposed to explain the observed phenomena. The voltage dependent spectra from our experiment provide useful information in distinguishing between those potential mechanisms. Finally, because of the impedance matching capability of tunnel junctions, the spin polarized tunneling technique can also be used as a powerful tool to measure resistive materials such as the various newly discovered and proposed topological insulators. In this talk, the results on the typical topological insulator bismuths selenide will be reported.