| 1. |
- Bergman, Anders, 1978-, et al.
(author)
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Magnon softening in a ferromagnetic monolayer : A first-principles spin dynamics study
- 2010
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 81:14, s. 144416-
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Journal article (peer-reviewed)abstract
- We study the Fe/W(110) monolayer system through a combination of first-principles calculations and atomistic spin dynamics simulations. We focus on the dispersion of the spin-waves parallel to the [001] direction. Our results compare favorably with the experimental data of Prokop et al. [Phys. Rev. Lett. 102, 177206 (2009)] and correctly capture a drastic softening of the magnon spectrum, with respect to bulk bcc Fe. The suggested shortcoming of the itinerant electron model, in particular that given by density functional theory, is refuted. We also demonstrate that finite-temperature effects are significant, and that atomistic spin dynamics simulations represent a powerful tool with which to include these.
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| 2. |
- Bergqvist, Lars, 1976-, et al.
(author)
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Magnetic and electronic structure of (Ga1-xMnx)As
- 2003
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 67:20, s. 205201-
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Journal article (peer-reviewed)abstract
- We present theoretical calculations of the magnetic and electronic structure of Mn-doped GaAs (in the zinc-blende structure). The magnetic properties are shown to be very sensitive to structural defects, in particular, As antisite defects and Mn at interstitial positions. Only when considering such defects can the experimental magnetic moments be reproduced by first-principles theory. We present a simple model for understanding the connection between the magnetic ordering and the As antisites, and the way in which the defects help to stabilize a partial disordered local-moment state. The connection between the energetics of the Mn substitution and the As antisite concentration is also analyzed. In addition, we compare the calculated magnetic properties and electronic structures of Mn situated on substitutional sites (Mn replacing a Ga atom) and on interstitial sites, where in agreement with observations the interstitial site is found to be less favorable. Finally, combining our first-principles calculations of the spin-wave excitation energies with a classical Heisenberg Hamiltonian we have calculated interatomic exchange interactions, and using Monte Carlo simulations we present theoretical values of the critical temperature as a function of Mn concentration.
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| 3. |
- Bergqvist, Lars, 1976-, et al.
(author)
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Magnetic properties and disorder effects in diluted magnetic semiconductors
- 2005
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In: Physical Review B Condensed Matter. - : American Physical Society (APS). - 0163-1829 .- 1095-3795. ; 72:19, s. 195210-
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Journal article (peer-reviewed)abstract
- We present calculations of the exchange interactions and critical temperatures for several diluted magnetic semiconductor systems. It is shown that the exchange interactions are dominated by short-ranged interactions that have a strong directional dependence. Using a combination of first-principles calculations of the exchange interactions together with Monte Carlo simulations of the classical Heisenberg model, in which the positional disorder and spin fluctuations are properly included, the calculated critical temperatures are in good agreement with experimantal observations. It is shown that agreement between theory and experiment, as regards ordering temperatures, is obtained only when the magnetic atoms are randomly positioned in a simulation cell which proves that disorder effects play a very important role. The effect of strong electron-electron interaction has been studied by means of the LSDA+U scheme. We investigate in detail the nature of the anisotropic exchange interactions by means of a Fermi surface analysis.
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| 4. |
- Eriksson, Olle, et al.
(author)
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Magnetism of Fe/V and Fe/Co multilayers
- 2003
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In: Journal of Physics. - : IOP. - 0953-8984 .- 1361-648X. ; 15, s. 599-
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Journal article (peer-reviewed)abstract
- We discuss in this paper the magnetic and structural parameters of Fe/V and Fe/Co multilayers. The electronic structure, magnetic moments (spin and orbital) and Curie temperatures as well as the magneto-crystalline anisotropy are calculated using first principles theory. Although theory is fairly successful in reproducing the experimental data we argue that the observed difference between theory and experiment most likely is due to lattice imperfections and that the interface between e.g. Fe and V is not perfectly sharp. We also present a model, based on the theory of elasticity, for analysing the structural properties of multilayers.
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| 5. |
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| 6. |
- Lizzaraga, R., et al.
(author)
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Conditions for Noncollinear Instabilities of Ferromagnetic Materials
- 2004
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In: Physical Review Letters. - : American Physical Society (APS). - 0031-9007 .- 1079-7114. ; 93:10, s. 107205-
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Journal article (peer-reviewed)abstract
- Two criteria have been identified here which determine whether a magnetic metal orders in a collinear (e.g., ferromagnet) or noncollinear (e.g., spin-spiral) arrangement. These criteria involve the ratio between the strength of the exchange interaction and the width of the electron bands, as well as Fermi-surface nesting between spin-up and spin-down sheets of the Fermi surface. Based on our analysis we predict that even typical ferromagnetic materials (e.g., Fe, Co, and Ni) should be possible to stabilize in a noncollinear magnetic order in, e.g., high pressure experiments.
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| 7. |
- Atodiresei, N., et al.
(author)
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Controlling the Magnetization Direction in Molecules via Their Oxidation State
- 2008
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In: Physical Review Letters. - : American Physical Society (APS). - 0031-9007 .- 1079-7114. ; 100:11
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Journal article (peer-reviewed)abstract
- By means of ab initio calculations we predict that it is possible to manipulate the magnetization direction in organic magnetic molecules by changing their oxidation state. We demonstrate this novel effect on the Eu2(C8H8)3 molecule, in which the hybridization of the outer π ring states with the Eu4f states causes a redistribution of the orbitals around the Fermi level leading to a strong ferromagnetism due to a hole-mediated exchange mechanism. As a key result, we predict an oscillatory behavior of the easy axis of the magnetization as a function of the oxidation state of the molecule—a new effect, which could lead to new technological applications.
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| 8. |
- Belhadji, Brahim, et al.
(author)
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Trends of exchange interactions in dilute magnetic semiconductors
- 2007
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In: Journal of Physics. - : IOP. - 0953-8984 .- 1361-648X. ; 19:43, s. 436227-
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Journal article (peer-reviewed)abstract
- We discuss the importance of different exchange mechanisms like double exchange, p–d exchange and anti-ferromagnetic as well as ferromagnetic superexchange in dilute magnetic semiconductors (DMSs). Based on the coherent potential approximation for the electronic structure of the DMSs we show that the different mechanisms exhibit different dependences on the concentration of the magnetic impurities, on the hybridization with the wavefunctions of neighbouring impurities and on the position of the Fermi level in the band gap. However, common to all mechanisms is that, as long as half-metallicity is preserved, they are determined by the hybridization with the orbitals of neighbouring impurities and of the resulting energy gain due to the formation of bonding and anti-bonding hybrids. By calculating the exchange coupling constants Jij(EF) as a function of the position of the Fermi level we obtain a universal trend for the exchange interactions with band filling.
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| 9. |
- Bergqvist, Lars, 1976-, et al.
(author)
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A theoretical study of half-metallic antiferromagnetic diluted magnetic semiconductors
- 2007
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In: Journal of Physics. - : IOP. - 0953-8984 .- 1361-648X. ; 19:21, s. 216220-
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Journal article (peer-reviewed)abstract
- Based on electronic structure calculations and statistical methods, we investigate a new class of materials for spintronic applications: half-metallic antiferromagnetic diluted magnetic semiconductors (HMAF-DMSs). As shown recently by Akai and Ogura, these DMS systems contain equal amounts of low-valent and high-valent transition metal impurities, such that their local moments exactly compensate each other. We present ab initio calculations using the KKR-CPA and the PAW-supercell methods, and show that quite a few half-metallic antiferromagnets should exist. Our calculations demonstrate that the exchange coupling parameters in these systems are dominated by a strong antiferromagnetic interaction between the two impurities. The Néel temperatures are calculated by Monte Carlo simulations and in mean-field approximation. It is shown that the latter method strongly overestimates the critical temperatures and that the more realistic values obtained by Monte Carlo techniques are rather low.
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| 10. |
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