Stabilising magnetism in two dimensions

University of St Andrews

Two-dimensional (2d) magnets offer promising platforms for the creation of new devices and memory storage media. Unfortunately, nature often conspires to destroy magnetism in two dimensions as thermal fluctuations overcome the tendency to order. For magnetism to survive in two dimensions, either the fluctuations that destroy order must be stopped, or the system must be protected against them. This project will use spectroscopy and modelling to explore how this can be achieved in real materials.

If the magnetic ions that make up a 2d magnet interact over a short distance (as is typically the case), order-destroying fluctuations are present. However, this is not necessarily the case if the interactions are long-range. The first part of this project explores the conditions for magnetic order in a class of composite system where magnetic ions interact over long distances via coupling to an intermediate system. This would open new design possibilities for spintronic devices where the properties of distant spins can be controlled by manipulation of the intermediate spin system.

If the fluctuations cannot be stopped in a given system, perhaps they can be overcome. In the second part, I will investigate how ions in crystals can order magnetically through interactions which fight against the disordering fluctuations. These interactions result from the physics of the magnetic ions themselves and rely heavily on the chemical composition and environment. I will explore these interactions in layered 2d magnets using neutron scattering, then model the data using a formalism that I have developed which captures the interplay of single-ion physics and magnetism.

Whilst neutron scattering is a powerful technique for understanding layered 2d magnets, the toolkit for interrogating single-layer 2d magnets is lacking. The final part of the project will address how scanning tunnelling microscopy can be used to probe single-layer magnets. This technique is yet to be implemented in the UK and there are few predictions of what one might measure. To support its implementation at the University of St Andrews, I will model several 2d magnets, building on the framework that I have developed for analysing neutron data, offering novel predictions.