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First-principles study of spin-flip transition in Fe-Ni invar alloys
Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
2006 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

More than a hundred years ago, Guillaume discovered that the alloy Fe65Ni35, at equilibrium under normal pressure, was invar. Indeed, its thermal expansion coefficient was anomalously small over a wide temperature interval. It is now known that the invar effect in this alloy is related to magnetism.

Recently, the ground state spin configuration was computed using spin-density functional theory and supercell methods. According to these calculations, the ground state spin configuration was ferromagnetic at atomic volumes bigger than a specific critical atomic volume. At the critical atomic volume, a few spins flipped. The aim of this licenciate project was to study the spin-flip transition which happens in the random fcc Fe50Ni50, using computer codes based on spin-density functional theory and supercell methods.

During the first part of the project, I characterized the chemical environment of the Fe sites where the spins flipped. These sites were in an Fe-rich environment. I computed the effective exchange parameters of the classical Heisenberg hamiltonian of the random fcc Fe50Ni50. The sign of the effective exchange parameter at any site where a spin flipped changed at the critical atomic volume, whereas the sign of the exchange parameter at any other site remained constant. The dispersion in the effective exchange parameters increased, as the atomic volume decreased from a specific atomic volume towards the critical atomic volume.

In the second part of the project, I computed the relaxed ionic positions. For any type of bond, the dispersion in the nearest neighbour bond length was much bigger than the change, due to the ionic positions relaxation, in the average value of the nearest neighbour bond length. Also, I investigated the effect of the ionic positions relaxation on the local magnetic moments. This effect was found to be relatively small.

After I found these results, I wondered what the effect of the ionic positions relaxation on the dispersion of the effective exchange parameters was and if the change of the dispersion in the effective exchange parameters with the atomic volume could provide an explanation for the invar effect in Fe-Ni invar alloys.

Place, publisher, year, edition, pages
Linköping: Linköpings universitet , 2006. , 32 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1232
Series
LIU-TEK-LIC, 13
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-33066Local ID: 19031ISBN: 91-85497-25-8 (print)OAI: oai:DiVA.org:liu-33066DiVA: diva2:253889
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2013-11-25
List of papers
1. Magnetism in systems with reduced dimensionality and chemical disorder: the local environment effects
Open this publication in new window or tab >>Magnetism in systems with reduced dimensionality and chemical disorder: the local environment effects
2006 (English)In: Journal of Magnetism and Magnetic Materials, ISSN 0304-8853, E-ISSN 1873-4766, Vol. 300, no 1, 211-215 p.Article in journal (Refereed) Published
Abstract [en]

We study influence of the local chemical environment, the so-called local environment effects, on the electronic structure and properties of magnetic systems with reduced dimensionality and chemical disorder, and show that they play a crucial role in a vicinity of magnetic instability. As a model, we consider Fe–Ni Invar. We present results obtained from ab initio calculations of the electronic structure, magnetic moments, and exchange interactions in random fcc Fe–Ni alloy, for a single monolayer alloy film on a Cu (0 0 1) substrate as well as in the bulk. We analyze the difference between the film and the bulk magnetization, which is found to be most pronounced for dilute alloys. We also analyze a sensitivity of the individual magnetic moments and effective exchange parameters to the local chemical environment of the atoms.

Place, publisher, year, edition, pages
Elsevier, 2006
Keyword
Local environment effects, Invar alloys, First-principles calculations
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-17428 (URN)10.1016/j.jmmm.2005.10.065 (DOI)
Available from: 2009-03-24 Created: 2009-03-24 Last updated: 2017-12-13
2. Static Ionic Displacements in Fe-Ni Alloys from First-Principles
Open this publication in new window or tab >>Static Ionic Displacements in Fe-Ni Alloys from First-Principles
2006 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 99, 08P906- p.Article in journal (Refereed) Published
Abstract [en]

Static local displacements of ions in disordered face-centered cubic Fe50Ni50 alloy are studied from first principles in the framework of the density functional theory. The disordered alloy is modeled using a 64 atom supercell constructed as a special quasirandom structure. Fully relaxed atomic positions inside the supercell are calculated by means of projected augmented wave method as implemented in Vienna ab initio simulation package. According to our calculation, the relative changes of mean nearest neighbor interatomic distances due to local lattice relaxations are relatively small (0.6%), in agreement with experiment. At the same time, we predict that for all types of pairs, Fe–Fe, Fe–Ni, and Ni–Ni, the dispersion of the nearest neighbor interatomic distances is rather large, and the individual changes of distances between certain pairs of atoms due to local lattice relaxations can be one order of magnitude larger than the mean values for the corresponding pair of atoms

Place, publisher, year, edition, pages
American Institute of Physics, 2006
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-17430 (URN)10.1063/1.2176914 (DOI)
Available from: 2009-03-24 Created: 2009-03-24 Last updated: 2017-12-13Bibliographically approved

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