Use this URL to cite or link to this record in EThOS: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.581313
Title: Atomistic modelling of iron with magnetic analytic Bond-Order Potentials
Author: Ford, Michael E.
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2013
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Abstract:
The development of interatomic potentials for magnetic transition metals, and particularly for iron, is difficult, yet it is also necessary for large-scale atomistic simulations of industrially important iron and steel alloys. The magnetism of iron is especially important as it is responsible for many of the element's unique physical properties -- its bcc ground state structure, its high-temperature phase transitions, and the mobility of its self-interstitial atom (SIA) defects. Yet an accurate description of itinerant magnetism within a real-space formalism is particularly challenging and existing interatomic potentials based on the Embedded Atom Method are suited only for studies of near-equilibrium ferritic iron, due to their restricted functional forms. For this work, the magnetic analytic Bond-Order Potential (BOP) method has been implemented in full to test the convergence properties in both collinear and non-collinear magnetic iron. The known problems with negative densities of states (DOS) are addressed by assessing various possible definitions for the bandwidth and by including the damping factors adapted from the Kernel Polynomial Method. A 9-moment approximation is found to be sufficient to reproduce the major structural energy differences observed in Density Functional Theory (DFT) and Tight Binding (TB) reference calculations, as well as the volume dependence of the atomic magnetic moments. The Bain path connecting bcc and fcc structures and the formation energy of mono- and divacancies are also described well at this level of approximation. Other quantities such as the high-spin/low-spin transition in fcc iron, the bcc elastic constants and the SIA formation energies converge more slowly towards the TB reference data. The theory of non-collinear magnetism within analytic BOP is extended as required for a practical implementation. The spin-rotational behaviour of the energy is shown to converge more slowly than the collinear bulk energy differences, and there are specific problems at low angles of rotation where the magnitude of the magnetic moment depends sensitively on the detailed structure of the local DOS. Issues of charge transfer in relation to magnetic defects are discussed, as well as inadequacies in the underlying d-electron TB model.
Supervisor: Pettifor, David G.; Drautz, Ralf Sponsor: EPSRC
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.581313  DOI: Not available
Keywords: Physical Sciences ; Physical & theoretical chemistry ; Structural chemistry ; Materials Sciences ; Alloys ; Atomic scale structure and properties ; Materials modelling ; Physical metallurgy ; Surfaces ; Condensed Matter Physics ; iron ; interatomic potentials ; magnetism ; non-collinear magnetism ; electronic structure ; bond-order potentials ; tight-binding ; density functional theory
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