Spin-transport in three-dimensional nanoarchitectures

Curvature-induced effects in nanomembranes will open a new perspective for the technological development of spintronic devices. Spintronics is a vibrant scientific field that has led to technological applications like the magnetic sensors present in modern hard disk drives - its potential impact for future information technologies clearly stands out. The main objective of this work package is the study of spin transport properties of curved nanomembranes in order to probe the curvature-induced spin-orbit coupling. We will use semiconductor or metallic materials with potential use in spintronic devices or with the required properties (e.g. superconductivity) for the implementation of topological quantum computation using Majorana fermions.

Geometry provides a new way to control the strength of the Rashba spin-orbit coupling, and the latter influences the conservation of spin information and the interconversion of spin and charge currents via the inverse spin Hall effect (ISHE). Henceforth, observation of changes in the spintronic properties for an increased amount of curvature in the same material system would constitute a direct demonstration of the emergence of spin-orbit coupling. Demonstration of geometric control over the spin-orbit coupling would represent a breakthrough on the control of spintronic properties.

A following objective is to exploit this control to expand the functionality of spintronic materials and to develop new device architectures where elements with low or high spin-orbit coupling could be implemented within a single material system. To probe the spintronic properties of curved nanomembranes we will use pure spin currents in an all-electrical nonlocal scheme, where spin currents are separated from charge currents. This measurement scheme avoids spurious magnetoresistance effects and allows us to focus on the spin degree of freedom. In this method a charge current is used to drive spins from a magnetic injector into the material under study, and a spin sensitive detector measures the diffusing spins in a part of the circuit outside the path of the charge current.

The most relevant property controlling electronic spin transport is the spin relaxation length λ - the characteristic length scale dictating how the injected spin information decays in a material. The nonlocal geometry allows the reliable extraction of the spin relaxation length by studying the dependence of the spin signals on the separation between the injector and detector. Alternatively, an external magnetic field can be used to induce spin precession and from the resulting precession curves the spin relaxation time and the spin relaxation length can be independently obtained. This method has been successfully applied to mesoscopic devices made of metals, semiconductors, or two-dimensional materials like graphene.

An enhanced spin-orbit coupling due to curvature would enhance spin relaxation processes, and result in a reduction of the spin transport properties. We will measure these properties and interpret their reduction in curved nanomembranes as a signature of induced spin-orbit coupling. Another signature of spin-orbit coupling is the ISHE, which can be used to generate spin currents from charge currents, or to detect spin currents via charge voltages. If ISHE is present, the ferromagnetic electrodes could be replaced by a Hall cross made of the same material as the transport channel, simplifying device architecture. There are several approaches to study spin transport in materials with strong spin-orbit coupling based on the nonlocal technique. Therefore, the appearance (or increase) of spin-Hall effects in curved nanomembranes would also be a signature of curvature induced spin-orbit coupling. Curved geometries remain largely unexplored in spintronics and we are keen to explore the fruitful challenges and opportunities they offer.

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Matias Garcia



The project CNTQC acknowledges the financial support of the Future and Emerging Technologies (FET) programme within the
Seventh Framework Programme for Research of the European Commission, under FET-Open grant number: 618083.