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Conditions for noncollinear instabilities of ferromagnetic materials
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Theoretical Magnetism.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Theoretical Magnetism.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Theoretical Magnetism.
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2004 (English)In: Physical Review Letters, ISSN 0031-9007, Vol. 93, no 10, 107205- p.Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
2004. Vol. 93, no 10, 107205- p.
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:uu:diva-94356DOI: 10.1103/PhysRevLett.93.107205OAI: oai:DiVA.org:uu-94356DiVA: diva2:168180
Available from: 2006-04-21 Created: 2006-04-21 Last updated: 2012-03-27Bibliographically approved
In thesis
1. Non-collinear Magnetism in d- and f-electron Systems
Open this publication in new window or tab >>Non-collinear Magnetism in d- and f-electron Systems
2006 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis, non-collinear magnetism has been studied by using density functional theory and the augmented plane wave method with local orbitals (APW+lo). Two conditions for non-collinear instabilities have been identified in this thesis. First, the Fermi energy should cut through both spin up and down states. Secondly, strong nesting between the spin up and spin down Fermi surfaces is needed. The two criteria described here can be fulfilled by tuning the exchange-splitting and/or by modifying the volume. Calculations on several elements; bcc V, bcc and fcc Mn, bcc Fe, bcc and fcc Co, and bcc and fcc Ni show that a non-collinear state can be stabilized provided that the criteria discussed above are met. More complex materials have also been analyzed in terms of these two criteria. The substitutional alloys TlCo2Se2-xSx are found in experiments to possess spin spiral structures for x = {0-1.5} and at a concentration x = 1.75 the alloys become ferromagnetic. As S takes the place of Se in the crystal structure the distance between the Co layers is reduced and the turn angle of the spin spiral becomes smaller until it totally vanishes at x = 1.75. This thesis show that the evolution of the magnetic structure in these alloys is the consequence of a modification of the distance between Co layers, which induces a change in the interlayer exchange coupling.

Fermi surfaces have been analyzed in TbNi5 in order to determine nesting features which would be responsible for the magnetic spin spiral observed in this material. The electronic structure of CeRhIn5 is also reported in this thesis. Furthermore, the 3-k magnetic structure of UO2 was investigated and the crystal field levels were calculated. Transition metal systems such as Fe in the superconducting high-pressure hcp phase and in the fcc crystal structure were also studied. The results obtained for fcc Fe are in accordance with previous reports. However the paramagnetic state in hcp Fe is found to be more stable than the antiferromagnetic configurations discussed earlier in the literature as being favored in the volume range where the hcp phase is stable and superconductivity appears (~ 15 GPa). The complex non-collinear magnetic structure in Mn3IrSi was calculated and the results are found to be in good agreement with experiments.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2006. vii + 54 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 169
Keyword
Physics, Non-collinear magnetism, spin spirals, first principles, density functional theory, Fermi surfaces, electronic structure, f-electron systems, Fysik
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-6812 (URN)91-554-6540-4 (ISBN)
Public defence
2006-05-12, Siegbahnsalen, Ångström Laboratory, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2006-04-21 Created: 2006-04-21 Last updated: 2012-03-27Bibliographically approved
2. Electronic Structure and Statistical Methods Applied to Nanomagnetism, Diluted Magnetic Semiconductors and Spintronics
Open this publication in new window or tab >>Electronic Structure and Statistical Methods Applied to Nanomagnetism, Diluted Magnetic Semiconductors and Spintronics
2005 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis is divided in three parts. In the first part, a study of materials aimed for spintronics applications is presented. More specifically, calculations of the critical temperature in diluted magnetic semiconductors (DMS) and half-metallic ferromagnets are presented using a combination of electronic structure and statistical methods. It is shown that disorder and randomness of the magnetic atoms in DMS materials play a very important role in the determination of the critical temperature.

The second part treats materials in reduced dimensions. Studies of multilayer and trilayer systems are presented. A theoretical model that incorporates interdiffusion in a multilayer is developed that gives better agreement with experimental observations. Using Monte Carlo simulations, the observed magnetic properties in the trilayer system Ni/Cu/Co at finite temperatures are qualitatively reproduced.

In the third part, electronic structure calculations of complex Mn-based compounds displaying noncollinear magnetism are presented. The calculations reproduce with high accuracy the observed magnetic properties in these compounds. Furthermore, a model based on the electronic structure of the necessary conditions for noncollinear magnetism is presented.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2005. x+70 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 32
Keyword
Materials science, spintronics, magnetism, Monte Carlo, critical temperature, exchange interactions, percolation, disorder, noncollinear, electronic structure, Materialvetenskap
National Category
Materials Engineering
Identifiers
urn:nbn:se:uu:diva-5732 (URN)91-554-6202-2 (ISBN)
Public defence
2005-04-29, Room 4001, Ångströmlaboratoriet, 10:15
Opponent
Supervisors
Available from: 2005-04-08 Created: 2005-04-08 Last updated: 2013-06-20Bibliographically approved
3. A Theoretical Study of Magnetism in Nanostructured Materials
Open this publication in new window or tab >>A Theoretical Study of Magnetism in Nanostructured Materials
2006 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

A first-principles linear scaling real-space method for investigating non-collinear magnetic behaviour of nanostructured materials has been developed. With this method, the magnetic structures of small supported transition metal clusters have been examined. The geometric constraints imposed on the clusters by the underlying surface is found to cause non-collinear behaviour for V, Cr, and Mn clusters on Cu(111). Fe clusters supported on Cu and Ni have been studied and both spin and orbital moments are found to be enhanced for the Fe atoms, which is attributed to the recuced symmetry present at the surface. Atoms in Co clusters have been found to order antiferromagnetically, and some times in a non-collinear fasion, when deposited on a W surface. Small clusters of fcc Fe embedded in Cu have been examined and a new type of ordering, not present in larger fcc Fe systems was found.

Several theoretical studies of Fe and Co based nanostructures consisting of multilayers or embedded clusters have been conducted, with the aim of predicting high moment materials for use in data storage applications. In agreement with previous experiments an enhancement of the magnetic moment is found compared to the magnetic moment of bcc Fe. The enhancement has been shown to be caused by increased spin moments for Fe atoms in close proximity with Co atoms, and this enhancement depends on the number of Co neighbours. As a result of these studies, a possible method of increasing the magnetic moment of cluster based materials has been proposed.

Fermi surface analysis have been performed both on bulk materials, in order to investigate mechanisms for stabilizing non-collinear magnetic states, and in layered structures where the effect of the Fermi surface on the interlayer exchange coupling has been investigated.

In addition to the development of a real-space electronic structure method for non-collinear magnetism, a density matrix purification method has been implemented in the framework of linear muffin-tin orbitals.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2006. x + 78 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 167
Keyword
Physics, magnetism, clusters, non-collinear, multilayers, first-principles theory, electronic structure, high-moment materials, exchange interactions, linear scaling methods, Fysik
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-6763 (URN)91-554-6527-7 (ISBN)
Public defence
2006-05-05, Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2006-04-12 Created: 2006-04-12 Last updated: 2012-03-27Bibliographically approved

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Nordström, LarsBergqvist, LarsBergman, AndersEriksson, Olle

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