Magnetisation & Transport Dynamics of Nanomagnets with Toroidal Ground States

Abstract

The study of molecular nanomagnets is important not only for fundamental scientific reasons, but also for applications in the miniaturisation of technology - the prospect of storing data on individual nanomagnets is particularly exciting. By removing unwanted interactions between dipole moments on neighbouring nanomagnets, non-dipolar toroidal moments may allow for even denser data storage. Motivated by this potential application (and by scientific curiosity for as-yet unknown benefits of toroidal moments), my thesis covers a range of topics related to the modelling of nanomagnets capable of producing toroidal magnetic states.

Firstly, I provide the context for my research by introducing the fields of single-molecule magnetism, toroidal moments and molecular spintronics. Secondly, I analyse the toroidal states arising from spin-frustration in a triangular nanomagnet of three spin-1/2 centres with C3 symmetry. One breakthrough of this study, is that by including antisymmetric exchange interactions arising from weak spin-orbit coupling, non-dipolar toroidal states can be prepared in a ground-energy doublet, separated from an excited doublet of non-toroidal dipolar states.

I then discuss a hypothetical experiment where the nanomagnet is placed in a tunnelling spintronics device to split the steady-state, non-equilibrium populations of toroidal states. I describe the optimal energetic regimes and device geometries to achieve the best population splitting, and show how the splitting could be monitored by measuring the extent to which a spin current's polarisation is reversed by the toroidal states.

Thirdly, I extend the investigation to a spin-frustrated triangular nanomagnet with C2v symmetry, such that two sites have general spin S and the third site has spin S3 = 1/2. The resulting toroidal states are intriguing for their complete lack of reliance on spin-orbit coupling, which is typically required to produce toroidal moments in isolated molecules. Again, I characterise the nanomagnet’s spin textures and performance in a tunnelling spintronics device, describing the optimal energetic regimes. Additionally, I explore the effects of changing S and delve deeper into the mechanism by which the toroidal states’ populations are split.

Fourthly, I build on a previous study by Soncini, Langley, Murray, Rajaraman and co-workers (Nat. Commun. 2017) to analyse a family of double-triangle complexes of the type {Dy3M(III)Dy3}X, where M = Cr, Mn, Fe, Co, Al and X = NO3, Cl. Based on crystallographic data and ab initio calculations, I use parameter-free models to calculate the complexes' energy spectra, powder magnetisations and direct-current magnetic susceptibilities. Confirming predicted trends and discovering new trends in the complexes' ferrotoroidic/antiferrotoroidic coupling, I describe the first toroido-structural correlations ever to be reported. I then simulate the dynamical magnetisation of these complexes, and by analysing their Zeeman spectra and various relaxation mechanisms, I suggest how to improve their magnetic hysteresis.

Much work still needs to be done to explore the fundamental properties of nanomagnets with toroidal ground states, particularly after these discoveries of toroido-structural correlations, and non-dipolar toroidal moments in ground-energy manifolds of triangles with zero/weak spin-orbit coupling. Thus, I outline the next steps towards implementing toroidal moments in nanotechnology, and end this thesis with a summary of the progress made.