Modern magnetic materials are structured on the nanometre lengthscale to achieve the desired magnetic behaviour. For example, a computer hard drive records information on a storage layer comprised of magnetic grains around 7-8nm in diameter. Information is read back using magnetic sensors which contain films less than 10 atoms thick. Magnetism on this lengthscale, in complex magnetic structures, is a challenging theoretical and experimental problem. Additionally, magnetism poses a difficult problem in terms of the timescales of interest. Currently, experiments using high energy lasers are investigating magnetic properties on a femtosecond timescale. This is an important frontier in solid state physics. At the same time, the data stored in a hard drive must be stable for at least 10 years. Consequently we have to consider dynamic magnetisation processes over some 24 orders of magnitude !!!
These challenges are central to the research of the York computational magnetism group. We specialise in the development of atomistic models of the dynamic properties of nanostructured magnetic materials and are leading the development of multiscale models which link electronic structure calculations to atomistic models as a powerful new approach to unifying the quantum and thermodynamic descriptions of magnetism. The models are applied to the understanding of magnetic recording media, nanoparticle systems for biomedical applications and the investigation of materials for “spin electronic” applications. The latter use the “spin” property of the electron for device applications rather than its charge, and has the potential to replace conventional electronics as this runs into physical limitations later this decade.
Our group in York was in charge of coordinating the €4m project to develop advanced multiscale models of complex magnetic materials with a focus on magnetization dynamics on the femtosecond timescale.
The field of ultrafast magnetization dynamics is a relatively new one and throughout the last decade has gone from begin purely of scientific interest, to see how one might probe magnetism on the timescale of the exchange interaction and subsequently understand the response, to one that has real potential for future technological applications. In order to realise any potential devices based on magnetism on the ultrafast (femtosecond) timescale, an understanding of the underlying physical mechanism and processes is essential.
Schematics of the proposed scanning-mode measurement of EMCD. (a) Schematic drawing of the experimental setup and the data obtained (ADF: annular dark field, PL: projector lens). The detector aperture is placed at the PL cross-over position. In the present STEM mode, the PL cross-over position is on the diffraction plane. (b) ATEM image of the investigated polycrystalline iron film. Scale bar, 50nm. (c) Calculated EMCD signal intensity distribution of a polycrystalline iron film in the diffraction plane. The highlighted area indicates the measured area covered by the detector entrance aperture. The detector entrance aperture (solid circle) is located at the position of 0.4 g(110) away from the origin, and its diameter is 0.5 g(110). The white broken circle represents the possible aperture centre positions in the diffraction plane and blue broken circle corresponds to g(110) ring position for comparison. Scale bar, 2nm-1. The minimum (black) and maximum (white) EMCD values range from -3 to +3%.