| 1. |
- Abrikosov, Igor, et al.
(författare)
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Competition between Magnetic Structures in the Fe-Rich FCC FeNi Alloys
- 2007
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 76:1, s. 014434-
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 2. |
- Abrikosov, Igor, et al.
(författare)
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Magnetism in systems with reduced dimensionality and chemical disorder : the local environment effects
- 2006
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Ingår i: Journal of Magnetism and Magnetic Materials. - : Elsevier. - 0304-8853 .- 1873-4766. ; 300:1, s. 211-215
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 3. |
- Liot, Francois, et al.
(författare)
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A Simple Theory of the Invar Effect in Iron-Nickel Alloys
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 4. |
- Liot, François, 1977-
(författare)
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First-principles study of spin-flip transition in Fe-Ni invar alloys
- 2006
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 5. |
- Liot, Francois, et al.
(författare)
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Local magnetovolume effects in Fe65Ni35 alloys
- 2009
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 79:1, s. 014202-
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 6. |
- Liot, Francois, 1977-, et al.
(författare)
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Static Ionic Displacements in Fe-Ni Alloys from First-Principles
- 2006
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Ingår i: Journal of Applied Physics. - : American Institute of Physics. - 0021-8979 .- 1089-7550. ; 99, s. 08P906-
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Tidskriftsartikel (refereegranskat)abstract
- 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
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| 7. |
- Liot, François
(författare)
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Thermal Expansion and Local Environment Effects in Ferromagnetic Iron-Based Alloys : A Theoretical Study
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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