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Competition between Magnetic Structures in the Fe-Rich FCC FeNi Alloys
Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
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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. Vol. 76, no 1, 014434- p.
Keyword [en]
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: urn:nbn:se:liu:diva-14277DOI: 10.1103/PhysRevB.76.014434OAI: oai:DiVA.org:liu-14277DiVA: diva2:23071
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
In thesis
1. Thermal Expansion and Local Environment Effects in Ferromagnetic Iron-Based Alloys: A Theoretical Study
Open this publication in new window or tab >>Thermal Expansion and Local Environment Effects in Ferromagnetic Iron-Based Alloys: A Theoretical Study
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:nbn:se:liu:diva-17433 (URN)978-91-7393-682-8 (ISBN)
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
2. 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
3. Configurational and Magnetic Interactions in Multicomponent Systems
Open this publication in new window or tab >>Configurational and Magnetic Interactions in Multicomponent Systems
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis is a theoretical study of configurational and magnetic interactions in multicomponent solids. These interactions are the projections onto the configurational and magnetic degrees of freedom of the underlying electronic quantum mechanical system, and can be used to model, explain and predict the properties of materials. For example, the interactions govern temperature induced configurational and magnetic order-disorder transitions in Heusler alloys and ternary nitrides.

In particular three perspectives are studied. The first is how the interactions can be derived from first-principles calculations at relevant physical conditions. The second is their consequences, like the critical temperatures for disordering, obtained with e.g. Monte Carlo simulations. The third is their origin in terms of the underlying electronic structure of the materials.

Intrinsic defects in the half-Heusler system NiMnSb are studied and it is found that low-energy defects do not destroy the important half-metallic property at low concentrations. Deliberate doping of NiMnSb with 3d-metals is considered and it is found that replacing some Ni with extra Mn or Cr creates new strong magnetic interactions which could be beneficial for applications at elevated temperature. A self-consistent scheme to include the effects of thermal expansion and one-electron excitations in the calculation of the magnetic critical temperature is introduced and applied to a study of Ni1−xCuxMnSb.

A supercell implementation of the disordered local moments approach is suggested and benchmarked for the treatment of paramagnetic CrN as a disordered magnetic phase. It is found that the orthorhombic-to-cubic phase transition in this nitride can be understood as a first-order magnetic order-disorder transition. The ferromagnetism in Ti1−xCrxN solid solutions, an unusual property in nitrides, is explained in terms of a charge transfer induced change in the Cr-Cr magnetic interactions.

Cubic Ti1−xAlxN solid solutions displays a complex and concentration dependent phase separation tendency. A unified cluster expansion method is presented that can be used to simulate the configurational thermodynamics of this system. It is shown that short range clustering do influence the free energy of mixing but only slightly change the isostructural phase diagram as compared to mean-field estimates.

Place, publisher, year, edition, pages
Linköping: Linköpings Universitet, 2010. 98 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1334
Keyword
Magnetic interactions, Configurational thermodynamics, Curie temperature, theoretical physics, magnetism, TiAlN, TiN, AlN, CrN, TiCrN, nitrides, NiMnSb, NiCuMnSb, Heusler alloys, spintronics, half-metallic, spinodal decomposition, first-principles, ab-initio
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-60446 (URN)978-91-7393-330-8 (ISBN)
Public defence
2010-09-09, Planck, Fysikhuset, Campus Valla, Linköping University, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2010-11-17 Created: 2010-10-13 Last updated: 2016-08-31Bibliographically approved

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Abrikosov, IgorKissavos, Andreas E.Liot, FrancoisAlling, BjörnSimak, Sergey

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