| 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 moments of Fe clusters embedded in an Fe-Co alloy
- 2006
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 74:10, s. 104422-
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Tidskriftsartikel (refereegranskat)abstract
- Motivated by the search for high moment materials, we present first-principles calculations of the magnetic moments for Fe clusters embedded in an Fe-Co alloy as well as in a pure Co matrix. Both spin and orbital contributions have been calculated with the use of a real-space linear muffin-tin orbital method within the atomic sphere approximation. The cluster sizes considered range from 1 to 51 atoms. We find an enhancement of the spin moments of the atoms close to the interface between the cluster and the embedding matrix. The spin moment enhancement is decreasing with increasing Fe content in the surrounding matrix. The slightly lower magnetic moments for the Fe clusters embedded in the Fe rich matrices can nevertheless result in a material with an enhanced saturation magnetization due to the higher moments in the matrix. We argue that the largest average magnetic moments for embedded Fe clusters are obtained for an Fe0.5Co0.5 alloy in the matrix surrounding the clusters.
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| 3. |
- 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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| 4. |
- 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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| 5. |
- 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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| 6. |
- Bergman, Anders, 1978-, et al.
(författare)
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Magnon softening in a ferromagnetic monolayer : A first-principles spin dynamics study
- 2010
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 81:14, s. 144416-
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Tidskriftsartikel (refereegranskat)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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| 7. |
- 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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| 8. |
- Bergqvist, Lars, et al.
(författare)
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Atomistic spin dynamics of low-dimensional magnets
- 2013
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 87
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the magnetic properties of a range of low-dimensional ferromagnets using a combination of first-principles calculations and atomistic spin dynamics simulations. This approach allows us to evaluate the ground state and finite temperature properties of experimentally well characterized systems such as Co/Cu(111), Co/Cu(001), Fe/Cu(001) and Fe/W(110), for different thicknesses of the magnetic layer. We compare our calculated spin wave spectra with experimental data available in the literature, and find a good quantitative agreement. We also predict magnon spectra for systems for which no experimental data exist at the moment, and estimate the role of temperature effects.
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| 9. |
- 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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| 10. |
- 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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