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Final Report Summary - STIFNANO (Spintronics with Topological Insulator/Ferromagnet Nanodevices)

A topological insulator is a new class of material discovered just a few years ago. It has fascinat-ing properties: it behaves as an insulator in its bulk but contains conducting states at its surface. The charge carriers at the boundary states have their spin locked at a right-angle to their direction of motion (so called spin-momentum locking), which can find applications in sensors, spin-electronics, thermoelectricity, plasmonics, etc. In theory, spin-momentum locking enables the effi-cient conversion of a charge current into a spin density (and vice versa), which is a key functionality for novel spintronics devices. The STIFnano project aims at exploring experimentally this high potential of topological insulators for spintronics.
The project combines instrumentation development, material science and spin-related transport investigations, with the following objectives included in four work packages: (i) fabrication of topological insulator thin films of high crystalline quality, with tailored electronic structure; (ii) advanced characterization of the materials structural, chemical and electronic properties, as an input for growth optimization; (iii) design and fabrication of micro/nano-devices based on multilayered heterostructures containing topological insulators; (iv) investigation of spin-charge conversion effects by magnetotransport measurements.
In order to achieve these objectives, a dual-chamber molecular beam epitaxy equipment was in-stalled and continuously upgraded throughout the project duration. This facility is the first one dedicated to the growth of topological insulators in Spain and one of the few in Europe. This equipment makes it possible to grow topological insulators with a well-defined composition and a thickness controlled down to the atomic level, and enables the fabrication of multilayered heterostructures with sharp interfaces in an ultra-high vacuum environment. The procedures for growing Bi2Se3, Bi2Te3 and (Bi1­xSbx)2Te3 topological insulators were developed during the project in combination with advanced material characterizations. A key achievement is that the optimization of the growth process led to the realization of single-crystalline thin films with a complete suppression of extended crystal defects (twin boundaries) which are commonly observed in these materials and are detrimental to the observation of surface-related phenomena.
In order to evidence and quantify spin-charge conversion, magnetotransport investigations have focused on topological insulator/ferromagnet heterostructures. In principle, passing an electrical current through the topological insulator generates a spin density (through spin-momentum lock-ing), which can be transferred to the ferromagnet, modifying its magnetic behaviour. The device nanofabrication process to measure this effect was fully set up by the fellow, as well as the procedures for radiofrequency magnetotransport experiments with a home-made probe station. As a result, current-induced ferromagnetic resonance measurements were successfully performed for the first time in the host institution. The results are currently being consolidated, with a systematic investigation of the spin-charge conversion efficiency as a function of materials parameter and experimental conditions.
Further magnetotransport studies of topological insulator/ferromagnet bilayers are being per-formed through active collaborations initiated at both the European level (ETHZ, Switzerland) and extra-European level (Osaka University, Japan). The project also resulted in knowledge transfer at the local level, through supervision of a PhD student.
In summary, the project developed a high-level expertise in both material fabrication and physical measurements, which is virtually unique worldwide. It enables fundamental studies of topological insulators for spintronics (currently on-going), in view of realizing novel devices for information and communication technologies.

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