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Thermal Expansion and Local Environment Effects in Ferromagnetic Iron-Based Alloys: A Theoretical Study
Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics . Linköping University, The Institute of Technology.
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The Nobel Prize for Physics 1920 was awarded to C.-E. Guillaume for his discovery of properties of nickel steels. He had previously observed that certain iron-nickel alloys exhibit the Invar effect i.e. an extremely low thermal expansion coefficient over a wide range of temperature. The decades since then have seen the observation of similar phenomena in other iron-based materials such as iron-platinum and iron-palladium. Moreover, there has been a great deal of theoretical work on the mechanism behind the Invar anomaly in the above-mentioned systems. However, despite many years of intensive research, a widely accepted microscopic theory of the effects is still lacking.

The present thesis aims at providing an insight into the physical nature of the thermal expansion of ferromagnetic random face-centered cubic iron-nickel, ironplatinum and iron-palladium bulk solids.

First, the thermal expansion coefficient is modeled as a function of temperature. The theory relies on the disordered local moment (DLM) formalism. However, contrary to all the previous models, the mapping between equilibrium states and partially disordered local moment (PDLM) states involves the probability that an iron-iron nearest-neighbour pair shows anti-parallel local magnetic moments, and the average lattice constant of the system at a finite temperature is calculated by minimization of an energy. The approach is applied to iron-nickel alloys. The model qualitatively reproduces several experimentally observed properties of disordered fcc iron-nickel solids. This includes Guillaume’s famous plot of the thermal expansion coefficient at room temperature as a function of concentration.

Second, for the purpose of studying the origin of the anomalous expansion, the anomalous and normal contributions to the thermal expansion coefficient are defined, then evaluated for iron-nickel alloys. The results support the idea that the peculiar behaviour of the expansivity, , originates solely from the anomalous contribution,αa.

Subsequently, the anomalous contribution is modeled for iron-nickel systems. In formulating the model, the following observation is taken into account; the average lattice spacing of an Fe100−xNix alloy at temperature T in a partially disordered local moment state is strongly negatively correlated with the probability that a nearest-neighbour pair has each of its two sites occupied by an iron atom and exhibits anti-ferromagnetically aligned magnetic moments (XFFAP). The quantity αa(x, T ) is estimated for several couples of values of the parameters x and T . Model results are found to agree qualitatively and quantitatively well with data obtained from the definition of αa. Thus, the model can successfully explain the basic process leading to the anomalous thermal expansion. It is consistent with the theory that the coefficient αa is controlled by the temperature derivative of XFFAP. Finally, the anomalous contribution to the thermal expansion coefficient of Fe72Pt28 and Fe66Pd34 solids is modeled as that of Fe65Ni35. A good agreement between the model results and experimental data for the expansivity as a function of temperature is noted. In conclusion, the Invar effects in disordered fcc iron-nickel, iron-platinum and iron-palladium alloys may have a common origin.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press , 2009. , 49 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1245
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-17433ISBN: 978-91-7393-682-8 (print)OAI: oai:DiVA.org:liu-17433DiVA: diva2:209337
Public defence
2009-04-03, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2009-03-24 Created: 2009-03-24 Last updated: 2009-03-24Bibliographically approved
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
3. Competition between Magnetic Structures in the Fe-Rich FCC FeNi Alloys
Open this publication in new window or tab >>Competition between Magnetic Structures in the Fe-Rich FCC FeNi Alloys
Show others...
2007 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 76, no 1, 014434- p.Article in journal (Refereed) Published
Abstract [en]

We report on the results of a systematic ab initio study of the magnetic structure of Fe rich fcc FeNi binary alloys for Ni concentrations up to 50 at. %. Calculations are carried out within density-functional theory using two complementary techniques, one based on the exact muffin-tin orbital theory within the coherent potential approximation and another one based on the projector augmented-wave method. We observe that the evolution of the magnetic structure of the alloy with increasing Ni concentration is determined by a competition between a large number of magnetic states, collinear as well as noncollinear, all close in energy. We emphasize a series of transitions between these magnetic structures, in particular we have investigated a competition between disordered local moment configurations, spin spiral states, the double layer antiferromagnetic state, and the ferromagnetic phase, as well as the ferrimagnetic phase with a single spin flipped with respect to all others. We show that the latter should be particularly important for the understanding of the magnetic structure of the Invar alloys.

Place, publisher, year, edition, pages
American Physical Society, 2007
Keyword
Iron alloys, nickel alloys, ferromagnetic materials, magnetic structure, ab initio calculations, density functional theory, linear muffin-tin orbital method, local moments
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-14277 (URN)10.1103/PhysRevB.76.014434 (DOI)
Note
Original Publication: Igor A. Abrikosov, Andreas E. Kissavos, Francois Liot, Björn Alling, Sergey Simak, O. Peil and A. V. Ruban, Competition between Magnetic Structures in the Fe-Rich FCC FeNi Alloys, 2007, Physical Review B Condensed Matter, (76), 1, 014434. http://dx.doi.org/10.1103/PhysRevB.76.014434 Copyright: American Physical Society http://www.aps.org/Available from: 2007-02-01 Created: 2007-02-01 Last updated: 2017-12-13Bibliographically approved
4. Local magnetovolume effects in Fe65Ni35 alloys
Open this publication in new window or tab >>Local magnetovolume effects in Fe65Ni35 alloys
2009 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 79, no 1, 014202- p.Article in journal (Refereed) Published
Abstract [en]

A systematic ab initio study of static ionic displacements in a face-centered-cubic Fe65Ni35 alloy has been carried out. Theoretical results for magnitudes of average Fe-Fe, Fe-Ni, and Ni-Ni < 110 > bond vectors agree well with experimental measurements. In addition, we have observed that in collinear ferrimagnetic states, iron-iron nearest-neighbor pairs with antiparallel local magnetic moments are shorter on average than those with parallel moments. Furthermore, having considered different states (ferromagnetic, nonmagnetic, and collinear ferrimagnetic states) for the same lattice spacing, we have shown that the magnetic structure strongly influences local geometrical properties of the alloy. For example, a transition from a ferromagnetic state to a collinear ferrimagnetic state induces a significant contraction of the volume associated with an iron site where the moment flips. A model system based on a Hamiltonian written as the sum of Lennard-Jones energies and a classical Heisenberg Hamiltonian has been introduced. It yields structural properties which are qualitatively similar to those obtained ab initio. We have found that some of the phenomena can be classified as magnetovolume effects.

Place, publisher, year, edition, pages
American Physical Society, 2009
Keyword
ab initio calculations, ferrimagnetic materials, ferromagnetic materials, Lennard-Jones potential, local moments, magnetic structure, Permalloy
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-16834 (URN)10.1103/PhysRevB.79.014202 (DOI)
Available from: 2009-02-21 Created: 2009-02-20 Last updated: 2017-12-13Bibliographically approved
5. A Simple Theory of the Invar Effect in Iron-Nickel Alloys
Open this publication in new window or tab >>A Simple Theory of the Invar Effect in Iron-Nickel Alloys
(English)Manuscript (Other academic)
Abstract [en]

Certain alloys of iron and nickel (so-called ‘Invar’ alloys) exhibit almost no thermal expansion over a wide range of temperature. It is clear that this is the result of an anomalous contraction upon heating which counteracts the normal thermal expansion arising from the anharmonicity of lattice variations. This anomalous contraction seems to be related to the alloys’ magnetic properties, since the effect vanishes at a temperature close to the Curie temperature. However, despite many years of intensive research, a widely accepted microscopic theory of the Invar effect in face-centered-cubic Fe-Ni alloys is still lacking. Here we present a simple theory of the Invar effect in these alloys based on Ising magnetism, ab initio total energy calculations, and the Debye-Grüneisen model. We show that this theory accurately reproduces several well known properties of these materials, including Guilaume’s famous plot of the thermal expansion coefficient as a function of the concentration of nickel. Within the same framework, we are able to account in the straightforward way for experimentally observed deviations from Vegard’s law. Our approach supports the idea that the lattice constant is governed by a few parameters, including the fraction of iron-iron nearest-neighbour pairs.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-17432 (URN)
Available from: 2009-03-24 Created: 2009-03-24 Last updated: 2010-01-14Bibliographically approved

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