Ab initio scale-bridging modelling and design of magnetic systems for memory applications
Abstract
Magnetic materials have many uses, e.g. for electricity production, in the transport sector and for information technology. In the latter case, it is common to use oppositely oriented domains of a ferromagnet to store sequences of "0" and "1", the natural the building blocks behind the binary code in hard drives. Miniaturization of this technology has shown impressive results but has almost approached the physical limit of domain stability. In some magnets, however, domains can wrap around single points in space to form so-called skyrmions, which consist of atomic spin and have a size in the nanometer range. Important here is that they show non-trivial topology which makes them more stable compared to regular magnetic domains and offers an alternative memory storage with potentially higher energy efficiency and information density, which motivates the ongoing research. Unfortunately, topological magnets often rely on rare elements and complicated energy-intensive synthesis, while also not fulfilling all criteria to be suitable for real-life application, e.g. showing stable magnetism at room temperature and skyrmions with a size below 100 nm as well as being able to host 2 types of skyrmions at the same time to encode the information bits.
Aiming to address some of these challenges, the proposed project will focus on the theoretical modelling of the magnetic and multiferroic properties of different topologically non-trivial systems, such as lacunar spinels, B20 compounds, layered nanostructures where unexpected behavior may be observed at atomic interfaces, rare-earth compounds (e.g. GdRu2Si2 and GdRu2Ge2 where magnetic frustration stabilizes skyrmions) and a large variety of organic magnets with a potential for future sustainable applications in energy and information technologies.
Multiscale approach will allow to model the system behavior on different length scales, starting with a few Angstrom and proceeding to several hundred nanometers, based on the first principles of quantum mechanics. In step 1, state-of-the-art electronic structure methods will address the electronic properties which correspond to the shortest length scale. The electronic structure will be calculated using density functional theory and whenever necessary the description of electronic correlations will be improved by means of the dynamical mean-field theory. In step 2, the magnetic interactions of Heisenberg and Dzyaloshinskii-Moriya types will be calculated based on the electronic properties obtained in step 1 and will provide the necessary information for large-scale modelling of magnetic textures. Finally, the spin dynamics and micromagnetic simulations based on the calculated magnetic interactions will address temperature-dependent magnetic properties and possibility of stabilizing topological textures in external magnetic field.
A common problem of skyrmionic systems is a low maximal temperature for skyrmion stability which limits their applications. If a system with skyrmions at room temperature is discovered within the project, it can lead to important technological advances. Possible strategies for inducing such robust magnetic skyrmions will be the application of mechanical pressure and chemical substitution which will be modelled in this project from first principles. Machine-learning analysis will be applied to the obtained numerical results, in order to reveal important factors for skyrmionic functionality.
