Modelling polycrystalline materials and interfaces

Polycrystalline materials are ubiquitous and dominate the synthetic and natural worlds. They are characterised by the presence of defects such as grain boundaries in the crystal structure. Grain boundaries can significantly influence underlying electrical, magnetic and mechanical properties of materials.

In this thesis interatomic potentials have been used to model grain boundaries in Fe, Cu and Ni. A high throughput computational approach is employed to determine the atomic structure, formation energy and excess volume of a large number of tilt grain boundaries in Fe, Cu and Ni. There is a systematic difference of ~0.2 Å between the excess volumes in Cu and Ni which is in agreement with experiment. It is predicted that the differences in the elastic moduli may give rise to larger differences in excess volume than expected.

Novel plan-view high-resolution transmission electron microscopy and first principles calculations have been employed to provide atomic level understanding of the structure and properties of grain boundaries in the MgO barrier layer of a magnetic tunnel junction. Transmission electron microscopy images reveal grain boundaries in the MgO film including (210)[001] symmetric tilt grain boundaries and (100)/(110)[001] asymmetric tilt grain boundaries amongst others. First principles calculations show how these grain boundaries are associated with locally reduced band gaps (by up to 3 eV).

The knowledge from the modelling of Fe, Cu, Ni and MgO is used to study interfaces of Fe and MgO to further understand magnetic tunnel junctions. The orientational relationship between the Fe and MgO is not known explicitly. Density functional theory is used to predict the energetic stability of Fe/MgO interfaces in different orientational configurations. It is found that the most energetically favourable interface between Fe and MgO is when the atomic columns are in registry.