Magneto-optical Kerr effect with non-uniform optical polarisation

The magneto-optical Kerr effect (MOKE) is widely used to characterise the magnetic behaviour or state of materials. Uniform beam polarisations are invariably used in MOKE measurements, whether as a parallel beam for thin film measurements or a focussed beam for measurements of small magnetic structures. Observing MOKE from in-plane magnetisation requires an oblique angle of incidence, with higher signals generally obtained with larger angles of incidence. This leads to a compromise being reached between spatial resolution and signal. Tightly focussed light beams are not purely transverse and can have a component of longitudinal electric field in the vicinity of the focus. Radially polarised light beams can significantly increase this effect and have a much reduced focal spot size for the longitudinal field component. We propose to use the longitudinal electric field associated with tightly focussed radially-polarised laser beams to perform normal-incidence MOKE measurements on materials with in-plane magnetisation. This will allow exquisite spatial resolution of magnetic thin films and nanostructures, plus the unprecedented ability to depth-resolve magnetisation. Optimising this will require a new framework to relate the relevant physics of MOKE with non-uniform optical polarisation. The three advantages of radial-polarisation MOKE (R-MOKE) are:1. An order of magnitude increase in the longitudinal component of electric field (which couples to the in-plane magnetisation to generate a Kerr signal)2. A reduction of the transverse spot size affording higher spatial resolution3. The unprecedented ability to depth-resolve magnetisation in MOKE measurements.

The main beneficiaries of this feasibility study into the normal incidence magneto-optical Kerr effect (R-MOKE) for in-plane magnetisation will be academics, as described elsewhere. A successful project will have a wider impact later as improved magneto-optical analytical techniques become more widely available to both academia and industry. This will then assist our understanding and the development of magnetic materials and devices, leading to new products that benefit industry and society. The versatile, high resolution and rapid observations that the R-MOKE configuration will allow could impact several commercial sectors, including permanent magnets, data storage and emerging 'spintronics'. The main way of ensuring that these benefits feed through to industry and the marketplace is to ensure that the scientific outcomes are well publicised, primarily through publishing academic papers and giving presentations at conferences and workshops. We have requested funds to travel to a significant numbers of conferences towards the end of the grant period. In particular, the Magnetism and Magnetic Materials (MMM) and Metallic Multilayer (MML) conferences attract a large number of academic and industrial delegates. We will also organise a one-day meeting in the final month of this project to allow interested parties to learn about R-MOKE background and magnetometry techniques, and have requested support for this. We have investigated the potential of intellectual property resulting from this project but have concluded that it is not worth pursuing as the MOKE magnetometry/microscopy sector is very small and specialised, while magneto-optical (MO) recording has now been superceded by other technologies and is no longer being developed commercially. The PDRA and undergraduate (UG) summer students will benefit significantly through training and diverse experience. The PDRA will gain exposure to computer modelling, optics, magnetic materials and nanofabrication techniques, providing a balanced and relevant portfolio of skills for his/her future career. The two summer students will benefit from a relatively extended project for UG level and gain experience of the research environment. Summer students tend to be self-selectively of high calibre and we would hope that this project would provide encouragement for them to continue in post-graduate scientific research. The portable magneto-optical (PMO) system developed as part of this project will be the main vehicle for disseminating our research to secondary school teachers and students. It will be designed primarily to demonstrate magneto-optical effects but easily adapted to show optical activity of simple solutions and crystals. The PMO system will support talks in local secondary schools and to some of the many groups of interested visitors the Sheffield and Durham departments receive annually. This will not only have inherent educational and outreach value but will also provide an opportunity to discuss the more advanced aspects of this research project. To reach a wider audience, the PMO system will be described in a suitable education journal (e.g. Physics Education) and details published on the project website in order to allow the system to be replicated at low cost by teachers or with student projects. We are requesting funding for the PMO system construction and for the occasional transport of the system between Sheffield and Durham.