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Magnetic properties of Fe/Co(001) superlattices from first-principles theory
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
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2006 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 74, no 17, 174409- p.Article in journal (Refereed) Published
Abstract [en]

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%.

Place, publisher, year, edition, pages
2006. Vol. 74, no 17, 174409- p.
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:uu:diva-94290DOI: 10.1103/PhysRevB.74.174409ISI: 000242409000080OAI: oai:DiVA.org:uu-94290DiVA: diva2:168091
Available from: 2006-04-12 Created: 2006-04-12 Last updated: 2017-12-14Bibliographically approved
In thesis
1. 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
2. Materials for Magnetic Recording Applications
Open this publication in new window or tab >>Materials for Magnetic Recording Applications
2005 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In the first part of this work, the influence of hydrogen on the structural and magnetic properties of Fe/V(001) superlattices was studied. The local structure of the vanadium-hydride layers was determined by extended x-ray absorption fine structure (EXAFS) measurements. The magnetic ordering in a weakly coupled Fe/V(001) superlattice was investigated using the magneto-optical Kerr effect (MOKE). The interlayer exchange coupling is weakened upon alloying with hydrogen and a phase with short-range magnetic order was observed.

The second part is concerned with first-principles calculations of magnetic materials, with a focus on magnetic recording applications. The uniaxial magnetic anisotropy energy (MAE) of Fe, Co, and Ni was calculated for tetragonal and trigonal structures. Based on an analysis of the electronic states of tetragonal Fe and Co at the center of the Brillouin zone, tetragonal Fe-Co alloys were proposed as a material that combines a large uniaxial MAE with a large saturation magnetization. This was confirmed by experimental studies on (Fe,Co)/Pt superlattices. The large uniaxial MAE of L10 FePt is caused by the large spin-orbit interaction on the Pt sites in connection with a strong hybridization between Fe and Pt. Furthermore, it was shown that the uniaxial MAE can be increased by alloying the Fe sublattice with Mn. The combination of the high-moment rare-earth (RE) metals with the high-TC 3d transition metals in RE/Cr/Fe multilayers (RE = Gd, Tb, Dy) gives rise to a strong ferromagnetic effective exchange interaction between the Fe layers and the RE layer. The MAE of hcp Gd was found to have two principal contributions, namely the dipole interaction of the large localized 4f spins and the band electron magnetic anisotropy due to the spin-orbit interaction. The peculiar temperature dependence of the easy axis of magnetization was reproduced on a qualitative level.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2005. viii + 65 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 53
Keyword
Physics, magnetic materials, magnetic anisotropy, saturation magnetization, magnetic data storage, superlattice, hydrogen, density functional theory, first-principles calculations, two-dimensional magnetism, FP-LMTO, EXAFS, MOKE, Fysik
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-5800 (URN)91-554-6252-9 (ISBN)
Public defence
2005-05-20, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15
Opponent
Supervisors
Available from: 2005-04-28 Created: 2005-04-28 Last updated: 2011-05-05Bibliographically approved

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Bergman, AndersSanyal, BiplabNordström, LarsEriksson, Olle

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