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Electronics and Spintronics of Topological Insulator/MAgnetic Insulator heterostructures

Periodic Reporting for period 1 - ESTIMA (Electronics and Spintronics of Topological Insulator/MAgnetic Insulator heterostructures)

Reporting period: 2019-02-27 to 2021-02-26

Novel materials and developments for spintronic applications are among the most active and successful research topics in condensed matter physics. Topological insulators (TIs) have been on the spotlight for such potential applications as they promise dissipationless carrier transport through their spin-momentum locked surface states. The principal strategy for making practical use of TIs is believed to be by combination with magnetic materials due to the emergence of novel magneto-electric effects, either by doping TIs with magnetic impurities or at interfaces of thin film heterostructures. However, the mechanisms for magnetic proximity and electronic transport at these interfaces are not completely understood and therefore not well controlled. ESTIMA project was designed to investigate such magnetic proximity effects in heterostructures incorporating TIs and magnetic insulators (MI). The research program involved growth of the TI/MI structures with state-of-the art molecular beam epitaxy, device design, nanofabrication, and magneto-electrical characterization.
ESTIMA was devoted to the detailed investigation of magnetic proximity effects (MPE) in heterostructures incorporating topological insulators (TI) and magnetic insulators (MI). During these two years, the experienced researcher has worked on the growth and characterization of these TI/MI heterostructures. The TI films were the (Bi,Sb)2(Se,Te)3 family of compounds, whereas the selected magnetic materials for the project were the insulators EuS, YIG or Tm3Fe5O12 (TIG), which were chosen based on published results. Such previous reports showed significant evidence of MPE in the TI, albeit with indirect techniques, such as charge transport and spin-polarized neutron scattering. During the implementation of ESTIMA, we focused in the use of x-ray absorption spectroscopy techniques including x-ray magnetic circular dichroism (XMCD) to understand the origin of MPE in TI/MI heterostructures. Because of its element-sensitive nature, the XMCD results are free of ambiguities of magnetism induced by material interface diffusion, providing a definite answer on the origin of magnetism. These investigations revealed that there is actually no induced magnetic moment originating at the surface of the (Bi,Sb)2(Se,Te)3 topological insulator material in contact to any of the selected MIs, contrary to previous reports on such structures.

Results of ESTIMA were presented by the experienced researcher in 5 contributed talks in national and international conferences. They also yielded 4 scientific peer-review publications in high impact journals, and a couple more are in preparation. Press releases related to these results were publicized in the ESTIMA twitter account and ICN2 webpage.
The overall results of ESTIMA suggest that the magnetic signatures previously reported for TI/MI heterostructures using, for example, electronic transport or neutron scattering experiments, do not originate from induced ferromagnetic order in the TI atoms at the TI/MI interface. Our observations are very important and actually explain why novel and interesting phenomena in these materials, such as the quantum anomalous Hall effect, was yet not observed even when the magnetic proximity effect was believed to be significant. They also highlight the difficulties that are inherent to draw definitive conclusions using electronic transport, even if one of the materials is insulating, while neutron scattering experiments, which rely on detecting magnetism on nanometer precision in depth at the interface, have no means of ruling out the origin in element diffusion (such as Eu, or Fe). Our results are, therefore, highly relevant for the design of TI heterostructures that could be implemented in future electronic and spintronic devices.
Molecular beam epitaxy system used to fabricate the structures for ESTIMA together with an illustrat