Mapping Spin Polarisation in Quasi-One-Dimensional Channels

The Magneto-Optic Kerr Effect (MOKE) results from the interaction of a polarised beam of light (usually from a laser) with a set of magnetic moments. For example, the spins of the atoms in a ferromagnetic material such as iron or cobalt. Specifically, the polarisation of the beam is rotated slightly as it reflects from the "spin polarised" surface of the ferromagnet. The size of the angle depends on the strength of the magnetism, and is known as the Kerr rotation. In this way, it is possible to obtain a direct measurement of the magnetism of a material, or more generally of the spin polarisation of a nanostructure if the beam can be finely focused. We propose to use MOKE to study the spin polarisation of electrons travelling along quantum wires, which are narrow channels (~500nm) defined by metallic surface gates in a semiconductor heterostructure. One of the many interesting properties of quantum wires is the possibility that they permit fully spin-polarised transport, in which the magnetic moments of the electrons travelling along the quantum wire are all aligned. This property, and its control by applying voltage to the metal surface gates is of great interest to researchers who want to use quantum wires as the building blocks of future quantum spintronic devices. However, this property has not been directly measured, only inferred from very careful measurements of the conducting properties of the quantum wire. We believe that MOKE could potentially measure the spin polarisation of these electrons directly, which would provide the information required to drive forward the field of solid-state quantum spintronics.

A similar system which holds great promise uses channels made from graphene, which promises to make possible a new generation of extremely reliable and energy-efficient spintronic devices. Apart from its excellent conduction properties, graphene is hoped to provide the required control of spin polarisation of the electrons travelling through it if the edges of the graphene channel are themselves spin-polarised and therefore magnetic. No experimental evidence for this widely-predicted and crucial property has yet been obtained, but we believe that again, focused MOKE can resolve the uncertainty and pave the way for graphene-based components to appear in ultra-efficient future electronic devices.

The impact on the academic community from our research will come in two forms. Firstly we will have developed a new focussed magneto-optic Kerr effect (FMOKE) measurement system that will allow the non-destructive, sub-micron analysis of spin accumulation in spintronic devices at an unrivalled combination of spatial resolution and magnetic field strength. Secondly, we will have used this apparatus to discover the underlying physical mechanism behind spin transport in potentially important quantum structures such as semiconductor quantum wires and graphene nanoribbons and dots.

Success in our investigations will therefore provide answers to fundamental questions of considerable interest to the quantum computation, device physics and materials science communities. Further impact will follow in spintronics, as the new physical understanding gained will aid the development of new devices based on spin currents rather than charge flow. This feasibility study's main beneficiaries will therefore be academics, as detailed above. However, the new tools and understanding that this work aims to provide could lead after further development to innovative commercial products of benefit to both future academia/industry and society. With this in mind, we intend to convey our results to potential beneficiaries via publications in research journals and presentations at both international and UK conferences and workshops.

A potentially more immediate impact may be forthcoming on the scientific instrumentation industry. Durham Magneto Optics Ltd. (DMO), founded by Prof. Cowburn, who is a Co-Investigator on this proposal, supplies complete MOKE systems based on his groundbreaking design which are used throughout the world. If our equipment is successful, we can develop in partnership with DMO a new generation of such systems with both sub-micron resolution and the ability to work at low temperature and high magnetic field.

The PDRA/Researcher Co-I and the undergraduate summer student will both benefit from training and experience during this cross-disciplinary project. The PDRA will gain experience in new techniques in graphene-based device preparation, as well as further development of skills in magneto-optical and electrical measurements at low temperatures, and in presentation of the results at conferences. The summer student will develop skills and depth of knowledge in a diverse range of basic and advanced laboratory and measurement techniques, and gain experience of the research environment, which will hopefully encourage them to pursue postgraduate research.