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Anisotropy Distributions in Nanomagnetic Arrays for Patterned Media

Final Report Summary - ANDIST (Anisotropy Distributions in Nanomagnetic Arrays for Patterned Media)

Project objectives
The goal of this research was to characterise and determine the causes of switching field distributions (SFDs) in nominally identical nanomagnetic elements. The knowledge gained then being used to develop materials with lower switching field distributions.
The timeliness and relevance of the proposed research is amply demonstrated by the realisation that the best current materials for Bit Patterned Media (BPM), which is a strong candidate for future ultra-high density data storage, have an intrinsic distribution of anisotropy that is a factor of two greater than modelling shows is required to successfully build a hard disk drive operating at greater than 1 Tbit/in2. BPM provides strong motivation of the research undertaken, but the benefits for other areas of nanomagnetism are also significant, since it is generally true that as the size of nanomagnetic devices decreases the SFD increases. As an example, nanodisks used in magnetic hyperthermia for cancer therapy will require successful development of highly uniform particles where control of the switching properties will be crucial. Hence, if the full potential of nanomagnetism as a practical enabling technology is to be realised, control of SFDs will be a key requirement.

Results
This project has pursued a number of research strands to investigate the underlying causes of SFDs in nanoscale magnetic islands using a range of complementary techniques. The major results are briefly summarised below:
The anomalous Hall effect (AHE) has been used to investigate the switching of patterned Co/Pt multilayer magnetic nanoislands. This work, where the Hall cross has been integrated into the Pt seed layer, is a collaboration with the University of Twente in the Netherlands. The innovation approach to Hall cross fabrication allowed measurements to be made without adding additional structures to the patterned magnetic islands. Using the anomalous Hall output voltage we have observed the magnetic switching of individual islands, allowing the spatial sensitivity across a Hall cross structure to be determined. The experimental results agree well with numerical simulation studies, using a three-dimensional finite element model, and with existing theoretical work, where the spatial sensitivity of two-dimensional Hall cross structures have been found numerically. This approach is scalable to the smallest (< 10 nm) islands as islands with a diameter of 25 nm could be detected using a 1.6 um cross structure. These results have been published in Journal of Applied Physics.

"Spatial sensitivity mapping of Hall crosses using patterned magnetic Nanostructures" M. Alexandrou, P.W. Nutter, M. Delalande, J. de Vries, E.W. Hill, F. Schedin, L. Abelmann and T. Thomson, J. Appl. Phys. 108(4) (2010) 043920 (5).

Magneto-Optic Kerr Effect (MOKE) measurements to determine the switching behaviour of nanoscale island arrays is a key characterization technique and fundamental to understanding SFDs. We have constructed a one-of-a-kind focused MOKE system capable of characterizing islands over areas as small as 2 um2 at multiple wavelengths and using high magnetic fields (>2T) applied at any arbitrary angle. This instrument allows reversal mechanisms to be probed in more detail than was previously possible and results can then be compared with the physical properties of the nanostructures. First results have been presented at the IEEE Magnetics Society Summer School (Assisi, Italy), this will be followed up with a submission to Intermag 2014 (Dresden, Germany). We plan to publish two papers within the next year on the results currently being taken on nanostructures consisting of FePt magnetic exchange spring for BPM. These results will be submitted to appropriate, high quality peer reviewed journals.
Small angle x-ray scattering (SAXS) and Small Angle Neutron Scattering (SANS) measurements have been made on large area arrays of different island sizes and periodicities fabricated by e-beam lithography. This work is a collaboration with the Paul Scherrer Institut (PSI) in Switzerland and is the graduate work of a jointly funded student. The SAXS data show a remarkable 41 orders of scattering peaks. These data have been analysed to give statistically meaningful values for both the mean island diameter and for the diameter variation (assumed Gaussian). The results were presented at MMM/Intermag 2013 in Chicago and are currently being written up for submission to a high quality peer reviewed journal and as part of the student’s PhD thesis. Complementary SANS measurements making use of the ability of neutron scattering to probe both the magnetic and physical structure of the nanomagnetic islands have also been collected. These data are being modeled determine the intra-island magnetic structure and will be correlated with the SAXS results and with results from the MOKE measurements.

Conclusions
This project has generated new results on the magnetic and physical properties of nanoscale islands which have been disseminated through high profile conferences, workshops and invited presentations at universities and research institutes (total 15). One paper has been published and one is currently in review with three others in advanced preparation. The work has produced the most statistically accurate measurement of magnetic island diameter variation using SAXS where many 1000’s of nanoscale islands were measured simultaneously. SANS measurements have allowed the intra-island magnetic structure to be probed. A new design of MOKE magnetometer has been constructed with unique capabilities and used to characterise small array of magnetic islands, whilst the AHE has been shown to be effective in determining the switching properties of individual islands.
This project has enabled two European collaborations to be established with Leon Abelmann’s group at the University of Twente in the Netherlands and with Prof. Heyderman at ETH/Paul Scherrer Institut in Switzerland. In terms of reintegration of the PI into the European research community the project has been instrumental in facilitating the transition.

Potential impact of project
The project will provide significant long term impact through both the development of a new, one-of-a-kind MOKE instrument and via the collaborations established. The direct scientific impact in terms of new instrumentation and results has been described and this will form the basis of on-going work for the next several years. This provides a long-term legacy for the investment made as part of this grant.
In terms of educating the next generation of researchers, the project has made a significant contribution to the graduate studies of two PhD students (Georg Heldt and Smaragda Zygridou) allowing them to collaborate with PSI (SAXS & SANS measurements) and develop new MOKE measurement techniques, respectively.
Additionally, the work done on nanoscale magnetic islands has led directly to the development of a new interdisciplinary project to create lithographically defined nanodiscs for magnetic hyperthermia therapy. Following successful initial results, a proposal in collaboration with Keele University and University Hospital of North Staffordshire, UK and University of Castilla-La Mancha (UCLM), Spain, has been submitted to the UK funding agency (EPSRC).