Bridging magnetic and electronic structure techniques via atomistic approaches

New capability of constructing structure-property phase diagrams of materials with magnetic properties. One of conundrums in the last century of research in this class of materials was a distinct barrier between total energy calculations, nowadays based on the Density Functional Theory (DFT), and magnetic structure calculations using model Hamiltonian approaches. In the former field it is often unclear what roles the magnetic components play in the structure property evolution with temperature, pressure or chemical potentials. The magnetism from the DFT point of view is immediately apparent as a manifestation of spin or the magnetic moment, but a particular phase transition may be "driven" by other properties of the system such as thermal expansion. From the magnetic perspective the information available from the electronic structure calculations in turn can be frustratingly opaque in that it does not clarify which types of the physical interactions are dominant for a particular set of conditions. To bring these two methods together, we propose to abstract from many details of the electronic structure and concentrate only on those that will directly affect the magnetic properties. The remaining interactions of the system can be accounted for using traditional interatomic potentials. The power of the atomistic approaches that treat atoms as (potentially polarisable) point charges will be enhanced by augmenting this description with point-like magnetic moments. A combined total energy description therefore will include semi-classical atomistic contribution and a magnetic (spin) model term that can be obtained also semi-classically or with an appropriate quantum-mechanical approach. The latter will typically require extensive lattice Monte Carlo simulations that will need to exploit High Performance Computers. The student's application project will concern ternary manganese based oxides initially concentrating on modelling CaMnO3, LaMnO3 and their solid solution. The work will involve three aspects: (i) DFT calculations studying magnetic ordering in a small unit cell aimed at obtaining parameters of magnetic interactions that will be used in (ii) magnetic Monte Carlo simulations that will aim to derive order parameter information from model Hamiltonians and the experimental data; and (iii) atomistic GULP simulations of the two pure systems including both types of interactions. When major model parameters will be established - the work will be moved to the solid solution and construction of the corresponding phase diagram.