Time: 02:00 p.m.
Place: ICAMS² 2013, International Colloquium on Advanced Materials Simulation, Bochum, Germany
John Drain, Department of Materials, University of Oxford, Oxford, United Kingdom
Michael Ford, Department of Materials, University of Oxford, Oxford, United Kingdom
David Pettifor, Department of Materials, University of Oxford, Oxford, United Kingdom
Elemental Mn displays five different phases: the ground-state α-Mn takes the topologically close-packed (TCP) χ-phase (cI58) with β-Mn (cP20), γ-Mn (fcc) and δ-Mn (bcc) stabilised with increasing temperature and ε-Mn (hcp) observed under pressure. Using the DFT binding energy curves for these five phases at T=0K as a database, we have fitted an orthogonal tight-binding (TB) d-band model that qualitatively reproduces the non-magnetic and anti-ferromagnetic DFT curves. Since the magnetic energies are an order of magnitude smaller than those in Fe, the magnetic analytic BOP expansion for Mn requires not 9 but 18 exact moments before converging to the TB results. Interestingly, whereas it is commonly thought that the TCP χ-phase of the Mn ground state is stabilised by size differences between the large and small moment sites, in practice non-magnetic α-Mn is still predicted to be the ground state, so that magnetism is not the critical driving factor. The magnetic moments on the four α-Mn sites are well reproduced. Finally, given the complexity of the non-collinear magnetism (NCLM) that is observed experimentally in α-Mn, we have applied BOP to discuss non-collinear states in the much simpler bcc structure of Fe. The limitations of a purely d-band model are highlighted.