| 1. |
- Bergman, Anders, et al.
(författare)
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Magnetic interactions of Mn clusters supported on Cu
- 2006
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Ingår i: PHYSICAL REVIEW B. - 1098-0121. ; 73:17, s. 174434-
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Tidskriftsartikel (refereegranskat)abstract
- It is demonstrated that the magnetic interactions can be drastically different for nanosized systems compared to those of bulk or surfaces. Using a real-space formalism we have developed a method to calculate noncollinear magnetization structures and hence exchange interactions. Our results for magnetic Mn clusters supported on a Cu(111) surface show that the magnetic ordering as a rule is noncollinear and cannot always be described by using a simple Heisenberg Hamiltonian. We argue that the use of ab initio calculations allowing for noncollinear coupling between atomic spins constitutes an efficient and reliable way of analyzing nanosized magnets.
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| 2. |
- Bergman, Anders, et al.
(författare)
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Magnetic properties of Fe/Co(001) superlattices from first-principles theory
- 2006
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 74:17
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Tidskriftsartikel (refereegranskat)abstract
- The magnetic properties of Fe/Co(001) superlattices have been studied using fully-relativistic first-principles theories. The average magnetic moment shows a behavior similar to bulk Fe-Co alloys, i.e., an enhanced magnetic moment for low Co concentrations, as described by the Slater-Pauling curve. The maximum of the magnetization curve, however, is lowered and shifted towards the Fe-rich compositions. The increased average magnetic moment for the Fe-rich superlattices, compared to bulk Fe, is due to an enhancement of the Fe spin moment close to the Fe-Co interface. The orbital moments were found to be of the same size as in bulk. The effect of interface roughness on the magnetic properties was investigated, and it was found that-despite local fluctuations due to the varying coordination-the average magnetic moment is only slightly affected. From a mapping of first-principles interactions onto the screened generalized perturbation method, we calculate the temperatures for when Fe/Co superlattices break up into an alloy configuration. Furthermore, the tetragonal distortion of the superlattice structure was found to only have a minor effect on the magnetic moments. Also, the calculated easy axis of magnetization is in the film plane for all compositions studied. It lies along the [100] direction for Fe-rich superlattices and along the [110] direction for Co-rich compositions. The transition of the easy axis occurs around a Co concentration of 50%.
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| 3. |
- Bergman, Anders, et al.
(författare)
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Magnetic structures of small Fe, Mn, and Cr clusters supported on Cu(111) : Noncollinear first-principles calculations
- 2007
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 75:22, s. 224425-
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Tidskriftsartikel (refereegranskat)abstract
- The magnetic structures of small clusters of Fe, Mn, and Cr supported on a Cu(111) surface have been studied with noncollinear first-principles theory. Different geometries such as triangles, pyramids, and wires are considered and the cluster sizes have been varied between two and ten atoms. The calculations have been performed using a real-space linear muffin-tin orbital method. The Fe clusters are found to order ferromagnetically regardless of the cluster geometry. For Mn and Cr clusters, antiferromagnetic exchange interactions between nearest neighbors are found to cause collinear antiferromagnetic ordering when the geometry allows it. If the antiferromagnetism is frustrated by the cluster geometry, noncollinear ordering is found. A comparison between the calculated structures and ground states obtained from simplified Heisenberg Hamiltonians show that the exchange interaction varies for different atoms in the clusters as a result of the different local structure.
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| 4. |
- Bergman, Anders, et al.
(författare)
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Magnetism of Fe clusters embedded in a Co matrix from first-principles theory
- 2004
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 70:17, s. 174446-
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Tidskriftsartikel (refereegranskat)abstract
- We have calculated spin and orbital moments for Fe clusters of sizes up to 700 atoms embedded as impurities in a bcc Co matrix. The calculations have been carried out using relativistic first-principles real-space density functional theory, and we have made a comparison with earlier experimental studies. For Fe atoms close to theFeCo interface, the spin moments are found to increase while atoms far from the interface exhibit bulklike moments. The Co moments remain essentially unchanged and close to the moment of bulk bcc Co. With increasing cluster size, the average moments of the cluster atoms decrease due to the decreased surface to volume ratio. The orbital moments of both Fe and Co are calculated to be small and they stay almost constant regardless of cluster size. Our results for spin moments agree with experimental data but the calculated orbital moments are slightly underestimated. A simplified model indicates that a compound of close-packed Fe clusters surrounded by Co show higher average total moments compared to bulk and multilayer systems with a similar concentration. This increase seems to disappear when cluster-cluster interactions are taken into account. The general trend is that for a given alloy concentration of FexCo1−x, clustering tends to lower the average magnetic moment compared to that of ordered structures and random alloys.
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| 5. |
- Bergman, Anders, et al.
(författare)
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Ultrafast switching in a synthetic antiferromagnetic magnetic random-access memory device
- 2011
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 83:22
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Tidskriftsartikel (refereegranskat)abstract
- The dynamics of a synthetic antiferromagnet (a metallic trilayer) have been explored and are shown to exhibit ultrafast switching on a time scale of tens of ps. This conclusion is based on first-principles, atomistic spin dynamics simulations. The simulations are performed at finite temperature, as well as at T = 0 K (the macrospin limit), and we observe a marked temperature dependence of the switching phenomenon. It is shown that, to reach very high switching speeds, it is important that the two ferromagnetic components of the synthetic antiferromagnet have oppositely directed external fields to one another. Then a complex collaboration between precession switching of an internal exchange field and the damping switching of the external field occurs, which considerably accelerates the magnetization dynamics. We discuss a possible application of this fast switching as a magnetic random access memory device, which has as a key component intrinsic antiferromagnetic couplings and an applied Oersted field.
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| 6. |
- Bergqvist, Lars, 1976-, et al.
(författare)
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Magnetic and electronic structure of (Ga1-xMnx)As
- 2003
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 67:20, s. 205201-
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Tidskriftsartikel (refereegranskat)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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| 7. |
- Bergqvist, Lars, 1976-, et al.
(författare)
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Magnetic properties and disorder effects in diluted magnetic semiconductors
- 2005
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Ingår i: Physical Review B Condensed Matter. - : American Physical Society (APS). - 0163-1829 .- 1095-3795. ; 72:19, s. 195210-
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Tidskriftsartikel (refereegranskat)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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| 8. |
- Bhandary, Sumanta, et al.
(författare)
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Route towards finding large magnetic anisotropy in nanocomposites : Application to a W(1-x)Re(x)/Fe multilayer
- 2011
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 84:9, s. 092401-
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Tidskriftsartikel (refereegranskat)abstract
- We suggest here a nanolaminate, 5[Fe]/2[W(x)Re(1-x)] (x = 0.6-0.8), with enhanced magnetic hardness in combination with a large saturation moment. The calculated magnetic anisotropy of this material reaches values of 5.3-7.0 MJ/m(3), depending on alloying conditions. We also propose a recipe in how to identify other novel magnetic materials, such as nanolaminates and multilayers, with large magnetic anisotropy in combination with a high saturation moment.
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| 9. |
- Björkman, Torbjörn, 1977-, et al.
(författare)
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Theoretical studies of the incommensurate magnetic structure of a heavy fermion system: CeRhIn5
- 2010
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 81:9, s. 094433-
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Tidskriftsartikel (refereegranskat)abstract
- We have investigated the electronic structure and the incommensurate magnetic configuration of the pressure-induced superconductor CeRhIn5. Noncollinear first-principles calculations were performed in the local-density approximation plus U scheme. The observed magnetic configuration is described accurately in our calculations, especially considering the minute energy scale which is relevant (microelectron volt). The band structure and Fermi surfaces were investigated and nesting was found to be responsible for the complex noncollinear magnetic state of CeRhIn5.
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| 10. |
- Bultmark, Fredrik, et al.
(författare)
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Multipole decomposition of LDA+U energy and its application to actinides compounds
- 2009
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 80:3, s. 035121-
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Tidskriftsartikel (refereegranskat)abstract
- A general reformulation of the exchange energy of 5f shell is applied in the analysis of the magnetic structure of various actinides compounds in the framework of LDA + U method. The calculations are performed in a convenient scheme with essentially only one free parameter, the screening length. The results are analyzed in terms of different polarization channels due to different multipoles. Generally it is found that the spin-orbital polarization is dominating. This can be viewed as a strong enhancement of the spin-orbit coupling in these systems. This leads to a drastic decrease in spin polarization in accordance with experiments. The calculations are able to correctly differentiate magnetic and nonmagnetic Pu system. Finally, in all magnetic systems an unusual multipolar order is observed, whose polarization energy is often larger in magnitude than the one of spin polarization.
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