Optimisation of atomistic model parameters for Nd2Fe14B type rare earth ferromagnets

Rare earth ferromagnets are an important region of study due to their importance in electric motors, low maintenance magnetic resonance imaging scanners, and hard drive actuators, with Neodymium based magnets being of frequent use.
Rare earth ferromagnets sharing the structure of Nd2Fe14B have been previously parameterised for atomistic models manually taking many researcher hours for the parameterisation of one aspect of one alloy. This work provides a framework for evaluating atomistic parameters programmatically, using Python and numerical optimisation methods to automate and vastly speed up the parameterisation process.

Several Curie Temperature Measurement Protocols were implemented and evaluated for their reliability and consistency in calculating Curie temperatures in rare earth alloys from VAMPIRE atomistic model simulation data.

The atomistic spin exchange constants for the Iron-Iron interaction, and the Iron-Rare earth interaction have been parameterised using the Nelder-Mead and Brent-Dekker numerical optimisation methods, using the calculated Curie temperature of the simulation as the optimising objective function. The exchanges parameterised yield a calculated Curie temperature of within 4 Kelvin for the +3 oxidation state rare earths.

The higher order anisotropy constants for Nd2Fe14B were evaluated in a single-spin and multi-spin model and comparisons were made to prior empirical and theoretical work. The First Order Magnetic Phase Transition present at low temperatures and high (>15 Tesla) magnetic field was reproduced qualitatively, but quantitatively the spin direction simulated differs as much as 30% from the empirical behaviour.