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An ab initio effective Hamiltonian for magnetism including longitudinal spin fluctuations
Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics . Linköping University, The Institute of Technology.
Department of Physics, Uppsala University, Box 530, 751 21 Uppsala, Sweden.
Royal Institute of Technology, SE-100 44, Stockholm, Sweden..
2005 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, Vol. 72, no 10, 104437- p.Article in journal (Refereed) Published
Abstract [en]

We discuss the use of the magnetic force theorem using different reference states upon which the perturbative approach is based. Using a fixed spin disordered local moment state one finds good Curie (or Neél) temperatures, and good energetics for planar spin spirals in the 3d magnets Fe, fcc Co, Ni, Mn, and Cr, though worse agreement for small θ spin spirals. On the other hand, the ferromagnetic reference state provides excellent energetics for small θ spin spirals in Fe, fcc Co, and Ni, and by extension magnon energies under the assumption of adiabacity. However, planar spin spiral energetics and transition temperatures show worse agreement. The reasons for this, and for the case of fcc Co where both approaches work very well, are discussed. We further provide an extension of the mapping of the quantum problem to include longitudinal fluctuations within force theorem based approaches, and discuss the role they will play in magnetic phase transitions. This construction is tested using planar spin spirals where q is fixed but the moment is allowed to relax. It is demonstrated that results from this approach and directly calculated ab initio values agree very well.

Place, publisher, year, edition, pages
2005. Vol. 72, no 10, 104437- p.
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:liu:diva-14275DOI: 10.1103/PhysRevB.72.104437OAI: oai:DiVA.org:liu-14275DiVA: diva2:23069
Available from: 2007-02-01 Created: 2007-02-01 Last updated: 2009-06-04Bibliographically approved
In thesis
1. Development and application of Muffin-Tin Orbital based Green’s function techniques to systems with magnetic and chemical disorder
Open this publication in new window or tab >>Development and application of Muffin-Tin Orbital based Green’s function techniques to systems with magnetic and chemical disorder
2006 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Accurate electronic structure calculations are becoming more and more important because of the increasing need for information about systems which are hard to perform experiments on. Databases compiled from theoretical results are also being used more than ever for applications, and the reliability of the theoretical methods are of utmost importance. In this thesis, the present limits on theoretical alloy calculations are investigated and improvements on the methods are presented.

A short introduction to electronic structure theory is included as well as a chapter on Density Functional Theory, which is the underlying method behind all calculations presented in the accompanying papers. Multiple Scattering Theory is also discussed, both in more general terms as well as how it is used in the methods employed to solve the electronic structure problem. One of the methods, the Exact Muffin-Tin Orbital method, is described extensively, with special emphasis on the slope matrix, which energy dependence is investigated together with possible ways to parameterize this dependence.

Furthermore, a chapter which discusses different ways to perform calculations for disordered systems is presented, including a description of the Coherent Potential Approximation and the Screened Generalized Perturbation Method. A comparison between the Exact Muffin-Tin Orbital method and the Projector Augmented-Wave method in the case of systems exhibiting both compositional and magnetic disordered is included as well as a case study of the MoRu alloy, where the theoretical and experimental discrepancies are discussed.

The thesis is concluded with a short discussion on magnetism, with emphasis on its computational aspects. I further discuss a generalized Heisenberg model and its applications, especially to fcc Fe, and also present an investigation of the competing magnetic structures of FeNi alloys at different concentrations, where both collinear and non-collinear magnetic structures are included. For Invar-concentrations, a spin-flip transition is found and discussed. Lastly, I discuss so-called quantum corrals and possible ways of calculating properties, especially non-collinear magnetism, of such systems within perturbation theory using the force theorem and the Lloyd’s formula.

Place, publisher, year, edition, pages
Institutionen för fysik, kemi och biologi, 2006
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1063
Keyword
Mathematical physics, Theoretical Physics, Muffin-Tin Orbital method, Calculations, Magnetism, Lloyd’s formula
National Category
Other Physics Topics
Identifiers
urn:nbn:se:liu:diva-8231 (URN)91-85643-28-9 (ISBN)
Public defence
2006-12-15, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2007-02-01 Created: 2007-02-01 Last updated: 2009-03-04

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Shallcross, SamKissavos, Andrea E.

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