Lough, Isa Tristan;
(2024)
A dynamic spin model for MnO.
Doctoral thesis (Ph.D), UCL (University College London).
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Abstract
I develop a new three-body interatomic potential (SEIP) to simulate the magnetic phase transition in MnO. Using the formalism proposed by J. Goodenough, the functional form of SEIP is derived to reproduce the effects of virtual electron transfers underlying the superexchange interaction and thus depends not only on the anion-cation distance but also on the Mn-O-Mn angle. In this work, I show that the qualitative effect of the symmetry lowering lattice distortion is captured using the SEIP, parametrised to reproduce the effects of the π and σ orbital interactions associated with magnetic superexchange, and another interatomic potential used to simulate the Mn-Mn direct exchange interaction. The target system of this work is MnO, whose magnetic phase transition is known to be coupled to the lattice dynamics. Surprisingly, there are little to no attempts in the literature at coupling together these two effects, which is the motivation for introducing the SEIP. The SEIP is implemented in a modified version of the simulation package GULP. Secondary to the main aims of this project, is to investigate the magnetic phase transition using the conventional assumption that spin-spin interactions are decoupled from the atomic lattice. Thus, various DFT exchange-correlation functionals are used to calculate the magnitude of the NN and NNN superexchange coupling constants in MnO and NiO. Additionally, NNNN and NNNNN hyperexchange coupling constants are evaluated in MnO. Using this same set of XC functionals, geometry optimisations of the AF2 ground state are used to estimate the size of the symmetry lowering lattice distortions in MnO and NiO, which are then compared to the literature. Finally, predictions of the Néel and Curie-Weiss temperatures are then made using the molecular field approximation (MFA) and Monte Carlo (MC) simulations.
| Type: | Thesis (Doctoral) |
|---|---|
| Qualification: | Ph.D |
| Title: | A dynamic spin model for MnO |
| Open access status: | An open access version is available from UCL Discovery |
| Language: | English |
| Additional information: | © The Author 2024. Original content in this thesis is licensed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC 4.0) Licence (https://creativecommons.org/licenses/by-nc/4.0/). |
| Keywords: | Magnetism, Interatomic Potentials |
| UCL classification: | UCL UCL > Provost and Vice Provost Offices > UCL BEAMS UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Chemistry |
| URI: | https://discovery.ucl.ac.uk/id/eprint/10196461 |
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