Skyrmionic states in confined helimagnetic nanostructures
Every day, we are producing large amounts of data in the form of multimedia, databases, research data, and many other types that we want to process and eventually store. Irrespective of what we want to do with our data, we always need some type of a memory device. An ever increasing need for data storage creates great challenges for the development of high-capacity storage devices that are cheap, fast, reliable, and robust. Nowadays, hard disk drive technology uses magnetic grains pointing up or down to encode binary data (0 or 1) in so-called perpendicular recording media. Practical limitations are well understood and dubbed the “magnetic recording trilemma”. It defines a trade-off between three conflicting requirements for any memory device (signal-to-noise ratio, thermal stability of the stored data, and the ability to imprint information). Because of these fundamental constraints, further progress requires radically different approaches.
Magnetic skyrmions are topologically protected particle-like whirls in the magnetisation field that can emerge in a special class of magnetic materials - helimagnets. They have the potential to provide solutions for low-power, high-capacity data storage and processing because of their very unique properties. However, one of the major challenges in developing skyrmion-based devices is the skyrmions' magnetic stability. More precisely, all studies so far required an external magnetic field to be applied in order to stabilise skyrmions. Zero-field stability is a crucial requirement for the development of skyrmion-based devices: devices that require external magnetic fields to be stabilised are volatile, harder to engineer, and consume more energy.
We vary several different parameters and at every point in the parameter space run multiple simulations starting from different initial states. In total, this results in thousands of simulations, each taking hours to complete. Because of that, we use IRIDIS High Performance Computing Facility to run dozens of simulations at the same time which significantly reduces the total computation time required to complete this project. Through a systematic study of equilibrium states, we demonstrate that skyrmionic states are the ground states in helimagnetic thin film disk nanostructures at zero external magnetic field . In addition to that, we show that bistable skyrmionic textures undergo hysteretic behaviour between two energetically equivalent skyrmionic states with different core orientation. These findings demonstrate that magnetic skyrmions in confined helimagnetic nanostructures can store data in absence of external magnetic field so that their orientation (up or down) encodes a single information bit (0 or 1). Finally, we study the dynamic properties of skyrmionic states helimagnetic nanostructures which both has importance in fundamental physics as well as for their manipulation . Findings in this project might contribute to the development of future skyrmion-based data storage devices.
This work is financially supported by the EPSRC’s Doctoral Training Centre (DTC) grant EP/G03690X/1. We also acknowledge the use of the IRIDIS High Performance Computing Facility, and associated support services at the University of Southampton.
 Beg, M., Carey, R., Wang, W., Cortés-Ortuño, D., Vousden, M., Bisotti, M.-A., Albert, M., Chernyshenko, D., Hovorka, O., Stamps, R. L., and Fangohr, H. Ground state search, hysteretic behaviour, and reversal mechanism of skyrmionic textures in confined helimagnetic nanostructures. Scientific Reports 5 17137 (2015).
 Beg, M., Albert, M., Bisotti, M.-A., Cortés-Ortuño, D., Wang, W., Carey, R., Vousden, M., Hovorka, O., Ciccarelli, C., Spencer, C. S., Marrows, C. H., and Fangohr, H. Dynamics of skyrmionic states in confined helimagnetic nanostructures. Physical Review B 95 14433 (2017).
 Beg, M. Skyrmionic states in confined helimagnetic nanostructures (Doctoral thesis). University of Southampton (2016).