Impact of Chemical and Morphological Changes on the Phase Stability of Magnetic Materials

Abstract

Permanent magnetic materials are of fundamental importance to the modern world, utilised in fields as broad as computers, cars, and MRI machines. Their importance is set to increase as the world move towards sustainable energy and away from fossil fuels. A seamless switch requires an increase in magnet production, and an improvement in performance. Rare-earth reduced permanent magnets are considered a solution to these two problems.

This thesis investigates the impact of chemical and morphological changes on the phase stability of rare-earth reduced hard permanent magnets. New methodologies for investigating the position preference of atomic substitutions and dopants have been applied to the RT12 (R = Rare-Earth, T = Transition metal) phase group. This work demonstrates that substitution of the transition metal for titanium in NdFe12, SmFe12, and SmCo12, decreases the cohesive energy, and therefore increases the stability of the structure up to 8Ti at.%. Through analysis of substitution positions it is demonstrated this is tied to a structural effect, derived from a switch in the symmetry of preferential substitution positions.

To gauge the manufacturing feasibility of one of these phases, computational investigations of the melting temperature of NdFe12 at various pressures were performed using a Solid Liquid coexistence methodology applied in Molecular Dynamics. Pair potentials used for this work were generated by a genetic algorithm potential fitting methodology, which has application beyond the RT12 phase group.

Finally, a new methodology for understanding grain morphology is presented, which takes into consideration the shape, surfaces, and interfaces of cyrstalline grain structures. This methodology is tested on the FePt L10 structure, which is able to produce stable magnetic grains at nanometer sizes, due to it’s magnetic anisotropy of Ha = 6-10 MJ/m . This work shows that at grain sizes between 3-9nm, the morphology of the grains is dominated by surface energy, and will result in structures with {111} planes as their primary faces. This result has implications for the design of next generation hard drives.