Atomistic calculations of magnetic properties of rare-earth transition-metal permanent magnets

Rare-earth permanent magnets are becoming increasingly important with increasing emphasis placed on the replacement of fossil fuels in transportation and energy generation. Due to its large (BH)max, Nd2Fe14B has been the permanent magnet material of choice for a wide range of applications since its discovery 40 years ago, including in electric motors and wind turbines. Its excellent magnetic properties come from a combination of high saturation magnetisation and high coercivity. In bulk Nd2Fe14B- sintered magnets however, the coercivity is < 20% of the theoretical Stoner-Wohlfarth limit. In the work presented in this thesis we investigate the possibility of coupling rare-earth permanent magnets with a magnetically soft phase to improve thermal stability and (BH)max using a generic atomistic spin model. We then develop a fully parameterised atomistic spin model for Nd2Fe14B and NdFe12 parameterised from experimental and ab initio data. We use this model to explore more complex magnetic features and materials. The effects of grain boundary interfaces on the local anisotropy are investigated using a Nd2Fe14B/α-Fe interface structure relaxed using molecular dynamics, and the spin dynamics at these interfaces calculated. These reveal Barkhausen-type jumps of domain walls propagating across the interface. Substitution of stabilising elements Ti and Zr into RFe12-type materials and their effects on the inter-sublattice coupling are studied. In addition, the micromagnetic cell size and temperature scaling of the saturation magnetisation and anisotropy are calculated using the atomistic model in an effort to link the models across a multiscale approach to permanent magnet modelling.