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Title: Development of magnetic bond-order potentials for Mn and Fe-Mn
Author: Drain, John Frederick
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2013
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While group VII 4d Tc and 5d Re have hexagonally close-packed (hcp) ground states, 3d Mn adopts the complex chi-phase which exhibits non-collinear magnetism. Density functional theory (DFT) calculations have shown that without magnetism the chi-phase remains the ground state of Mn implying that magnetism is not the critical factor, as is commonly believed, in driving the anomalous stability of the chi-phase over hcp. Using a tight-binding (TB) model it is found that while harder potentials stabilise close-packed hcp, a softer potential stabilises the more open chi-phase. By analogy with the structural trend from open to close-packed phases down the group IV elements, the anomalous stability of the chi-phase in Mn is shown to be due to 3d valent Mn lacking d states in the core which leads to an effectively softer atomic repulsion between the atoms than in 4d Tc and 5d Re. Subsequently an analytic Bond-Order Potential (BOP) is developed to investigate the structural and magnetic properties of elemental Mn at 0 K. It is derived within BOP theory directly from a new short-ranged orthogonal d-valent TB model of Mn, the parameters of which are fitted to reproduce the DFT binding energy curves of the five experimentally observed phases of Mn, alpha, beta, gamma, delta, and epsilon-Mn. Not only does the BOP reproduce qualitatively DFT binding energy curves of the five different structure types, it also predicts the complex collinear antiferromagnetic (AFM) ordering in alpha-Mn, the ferrimagnetic (FiM) ordering in beta-Mn and the AFM ordering in the other phases that are found by DFT. A BOP expansion including 14 moments is sufficiently converged to reproduce most of the properties of the TB model with the exception of the elastic shear constants which require further moments. Magnetic analytic BOPs are also developed for Fe and Fe-Mn. The Fe model correctly reproduces trends in the structural stabilities of the common metallic structures except that AFM hcp is overstabilised. Reproduction of the elastic constants with a 9-moment BOP is reasonable although as is found for the Mn BOP the elastic shear constants require more moments to converge. Vacancy formation energies are close to those determined by experiment and DFT and the relative stabilities of self-interstitial atom (SIA) defects in ferromagnetic bcc Fe are correctly reproduced. The SIA formation energies are found to be better than those calculated with existing BOP models. The Fe-Mn TB and BOP models were challenging to fit and nonmagnetic face-centred cubic (fcc) structures are overstabilised. Furthermore within BOP an incorrect magnetic solution is predicted for one fcc structure resulting in poor reproduction of the DFT stacking fault energies. Refitting the bond integrals might help to better reproduce the nonmagnetic hcp-fcc energy differences while an environment-dependent Stoner parameter could help provide the flexibility needed to correctly capture the magnetic energy differences.
Supervisor: Pettifor, David G. ; Drautz, Ralf Sponsor: EPSRC (Engineering Physical Science Research Council)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Materials modelling ; Metallurgy ; Alloys ; Atomic scale structure and properties ; Defect analysis ; Physical metallurgy ; Manganese ; Iron ; High Manganese Steels ; Density Functional Theory