Magneto-ionics is a field of spintronics research that aims to manipulate magnetic properties of thin-film materials with the addition of ionic species, such as hydrogen, lithium, oxygen or nitrogen. In many cases, this involves a change in crystal structure and it is experimentally easy to determine and interpret changes in structure and relate them to changes in magnetic behaviour [1]. However, some of the most interesting and applicable materials are amorphous – they lack crystal symmetry and there is no well-defined phase transformation that can be used to understand how the addition of ions affects the magnetic structure [2]. In our experimental work we have investigated the magneto-ionic tuning of ferrimagnetic properties in rare-earth transition-metal alloys, primarily TbCo, with varying compositions [3]. These alloys are extremely susceptible to small changes in the local environment and, due to the rare-earth elements, can show a high affinity for hydrogen ions which are very small and extremely mobile. If the properties can be modified significantly by hydrogen ions alone, then a future device would have an extremely high response curve that can be manipulated relatively rapidly. A key goal here is to understand how the local structure changes and how it relates to the changes in magnetic behaviour. Experimentally, this has been determined using a total x-ray diffraction technique that contains x-ray weighted information about the local pair correlations. But interpretation of this data is challenging. The resulting data is the overlap of all correlations and disentangling them requires a computational model to calculate an ‘ideal’ structure and then compare this to the data that we have already obtained. The aim of this project is to support experimental work with ab-initio simulations of the material structure. We will begin by performing simulations of the simplest structure – a TbCo alloy – to obtain a good first model and then introduce hydrogen to understand how this changes the local structure. Computational pair distribution functions will be generated from the simulated structures, averaged over many stochastic quenches, and compared with the experimental data to verify the applicability of the model. Once we have obtained a good model that agrees with our experimental results, we will extend our understanding of the magneto-ionic interaction by performing simulations of the electronic structure and magnetic properties. [1] Nichterwitz, M., Honnali, S., Kutuzau, M., Guo, S., Zehner, J., Nielsch, K., & Leistner, K. (2021). “Advances in magneto-ionic materials and perspectives for their application”. APL materials, 9(3). [2] Huang, M., Hasan, M. U., Klyukin, K., Zhang, D., Lyu, D., Gargiani, P., ... & Beach, G. S. (2021). “Voltage control of ferrimagnetic order and voltage-assisted writing of ferrimagnetic spin textures”. Nature Nanotechnology, 16(9), 981-988. [3] Hunt, R. G., Moldarev, D., Grassi, M. P., Primetzhofer, D., & Andersson, G. (2025). ”Control of ferrimagnetic compensation and perpendicular anisotropy in Tb x Co (100− x) with H+ ion implantation”. Physical Review Materials, 9(3), 034409.