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Exploring the Spin Physics at the Boundaries of Materials with Strong Spin-Orbit Interaction

Final Report Summary - SPINBOUND (Exploring the Spin Physics at the Boundaries of Materials with Strong Spin-Orbit Interaction)

SPINBOUND addressed a new research frontier on spin physics at the boundaries of materials with strong spin-orbit interaction (SOI). Although the properties of these materials have been studied for more than half a century, researchers are just starting to grasp the richness of SOI phenomena taking place at them. SOI leads to surface states with unusually large spin splitting in simple heavy elements such as gold, platinum or bismuth. It can also produce a nontrivial topology in band insulators, such as known thermoelectric material bismuth selenide (Bi2Se3), which brings about metallic boundary states with exotic spin textures that are protected by time reversal symmetry. Despite their fundamental and technological interest, information on the nature of charge and spin transport at such surface states is scarce and controversial. Within SPINBOUND we studied the transport properties of topological insulator (TI) (Bi,Sb)2(Se,Te)3. The devices were fabricated by mechanical exfoliation from bulk crystals and by molecular beam epitaxy (MBE). Notably it was found that, at low temperatures (< 50 K) and when the chemical potential lies inside the bulk bandgap, the crystal resistivity is strongly temperature dependent, reflecting inelastic scattering due to the thermal activation of optical phonons. The dominant phonon mode strongly interacts with the surface states of the material and represents the main source of scattering at the surface at high electric fields. We have also systematically investigated TI/metal(M) and TI/ferromagnetic-insulator(FMI) interfaces by combining spectroscopy characterization and spin-torque ferromagnetic resonance. For TI/M interfaces, we found that Te diffusion out of the TIs generally occurs upon metal deposition and that Te reduces the transmission of the current-induced spin accumulation across interfaces. By selecting the proper metallic spacer at a TI/Permalloy (Py) interface, the Te out-diffusion can be drastically reduced, which results in a large increase of a field-like torque opposed to the Oersted torque, providing an unambiguous evidence for a large Edelstein effect and information on the nature of the observed spin torques. Magnetic proximity effect at TI/FMI interfaces are fundamental for the observation of exotic phenomena, such as the quantum anomalous Hall effect, which has implications in quantum metrology. We have performed element selective magnetometry to study proximity effects at TI/EuS interfaces by searching for induced magnetic moments on the TI. Different TIs of the (Bi,Sb)2(Se,Te)2 family were used in this study. However, a negligible magnetic signal (below the detection limit of the magnetometry technique) was found on Bi, Sb, Te or Se atoms of the TIs in contact with EuS, at temperatures and conditions where the latter is magnetic and could polarize these non-magnetic atoms. Finally, we have studied the SOI proximity effects in (light) materials, such as graphene, with low SOI. Theoretical work demonstrated that the spin relaxation in graphene with, for example, gold impurities can be described by means of an entanglement between spin and pseudospin driven by random SOI, which is unique to graphene, and also lead to large spin Hall effects. Follow-up experimental work in Pt-graphene have shown unprecedented spin-to-charge conversion efficiencies in non-local devices. We have further developed novel ways to characterize the dominant spin-orbit fields in these systems, by using spin relaxation anisotropy, and hot electron transport in non-local electronic devices. With these experiments we identified the nature of spin relaxation in pristine graphene and demonstrated proximity-induced SOI when contacting graphene with a transition-metal dichalcogenides. Further we demonstrated that a remote spin signal can be enhanced, and even sustained, over long distances with temperature gradients.