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Electronic structure and chemical and magnetic interactions in ZnO doped with Co and Al: Experiments and ab initio density-functional calculations
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and condensed matter physics.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and condensed matter physics.
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2008 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 78, no 8, 085319- p.Article in journal (Refereed) Published
Abstract [en]

We present results of electronic structure and magnetization measurements of Co:ZnO and Co:ZnO codoped with Al thin-film samples fabricated by solution-based methods together with ab initio electronic structure calculations. Electronic structure measurements indicate that the Co states lie close to the valence-band edge with pinning of the Fermi level primarily due to native defects yielding a heavily n-doped material. The findings in the electronic structure measurements are corroborated by results from theoretical calculations. We find that it is necessary to go beyond the local-density approximation to achieve agreement with experiments. Moreover, the theoretical calculations indicate a tendency for the formation of Co clusters, giving rise to an antiferromagnetic exchange interaction between the Co atoms. The magnetization measurements are well in line with the theoretical predictions, showing a dominating superparamagnetic behavior arising from small antiferromagnetic clusters containing uncompensated spins.

Place, publisher, year, edition, pages
2008. Vol. 78, no 8, 085319- p.
National Category
Physical Sciences Engineering and Technology
Identifiers
URN: urn:nbn:se:uu:diva-107891DOI: 10.1103/PhysRevB.78.085319ISI: 000259406900078OAI: oai:DiVA.org:uu-107891DiVA: diva2:233451
Available from: 2009-09-01 Created: 2009-08-31 Last updated: 2017-01-25
In thesis
1. Density Functional Theory Applied to Materials for Spintronics
Open this publication in new window or tab >>Density Functional Theory Applied to Materials for Spintronics
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The properties of dilute magnetic semiconductors have been studied by combined ab initio, Monte Carlo, and experimental techniques. This class of materials could be very important for future spintronic devices, that offer enriched functionality by making use of both the spin and the charge of the electrons. The main part of the thesis concerns the transition metal doped ZnO.

The role of defects on the magnetic interactions in Mn-doped ZnO was investigated. In the presence of acceptor defects such as zinc vacancies and oxygen substitution by nitrogen, the magnetic interactions are ferromagnetic. For dilute concentrations of Mn (~ 5%) the ordering temperature of the system is low, due to the short ranged character of the exchange interactions and disorder effects.

The clustering tendency of the Co atoms in a ZnO matrix was also studied. The electronic structure, and in turn the magnetic interactions among the Co atoms, is strongly dependent on the exchange-correlation functional used. It is found that Co impurities tend to form nanoclusters and that the interactions among these atoms are antiferromagnetic within the local spin density approximation + Hubbard U approach.

The electronic structure, as well as the chemical and magnetic interactions in Co and (Co,Al)-doped ZnO, was investigated by joined experimental and theoretical techniques. For a good agreement between the two, approximations beyond the local density approximation must be used. It is found that the Co atoms prefer to cluster within the semiconducting matrix, a tendency which is increased with Al co-doping. We envision that it is best to describe the system as superparamagnetic due to the formation of  Co nanoclusters within which the interactions are antiferromagnetic.

The magnetic anisotropy and evolution of magnetic domains in Fe81Ni19/Co(001) superlattices were investigated both experimentally, as well as using model spin dynamics. A magnetic reorientation transition was found.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2010. 67 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 721
Keyword
spintronics, dilute magnetic semiconductors, density functional theory, exchange interactions, magnetic percolation, ordering temperature, disorder, electronic structure
National Category
Physical Sciences
Research subject
Materials Science
Identifiers
urn:nbn:se:uu:diva-119887 (URN)978-91-554-7737-0 (ISBN)
Public defence
2010-04-16, Polhemsalen, 751 20 Uppsala, Ångström Laboratory, 10:15 (English)
Opponent
Supervisors
Available from: 2010-03-26 Created: 2010-03-02 Last updated: 2010-03-26Bibliographically approved
2. Fundamental Properties of Functional Magnetic Materials
Open this publication in new window or tab >>Fundamental Properties of Functional Magnetic Materials
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Magnetic properties of powders, thin films and single crystals have been investigated using magnetometry methods. This thesis provides analysis and conclusions that are supported by the results obtained from spectroscopic and diffraction measurements as well as from theoretical calculations.

First, the magnetic behavior of transition metal (TM) doped ZnO with respect to doping, growth conditions and post annealing has been studied. Our findings indicate that the magnetic behavior stems from small clusters or precipitates of the dopant, with ferromagnetic or antiferromagnetic interactions. At the lowest dopant concentrations, the estimated cluster sizes are too small for high resolution imaging. Still, the clusters may be sufficiently large to generate a finite spontaneous magnetization even at room temperature and could easily be misinterpreted as an intrinsic ferromagnetic state of the TM:ZnO compound.

Second, influence of lattice strain on both magnetic moment and anisotropy has been investigated for epitaxial MnAs thin films grown on GaAs substrates. The obtained magnetic moments and anisotropy values are higher than for bulk MnAs. The enhanced values are caused by highly strained local areas that have a stronger dependence on the in-plane axis strain than out-of plane axis strain.

Finally, spin glass behavior in Li-layered oxides, used for battery applications, and a double perovskite material has been investigated. For both Li(NiCoMn)O2 and (Sr,La)MnWO6, a mixed-valence of one of the transition metal ions creates competing ferromagnetic and antiferromagnetic interactions resulting in a low temperature three-dimensional (3D) spin glass state. Additionally, Li(NiCoMn)O2 with large cationic mixing exhibits a percolating ferrimagnetic spin order in the high temperature region and coexists with a two-dimensional (2D) frustrated spin state in the mid temperature region. This is one of the rare observations where a dimensional crossover from 2D to 3D spin frustration appears in a reentrant material.

Place, publisher, year, edition, pages
Uppsala: acta universitatis upsaliensis, 2010. 74 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 790
Keyword
Magnetic anisotropy, magnetic aging, magnetic semiconductors, exchange interactions, magnetic oxides, spin glasses, lithium-ion batteries, short-range magnetic order
National Category
Physical Sciences
Research subject
Engineering Science with specialization in Solid State Physics
Identifiers
urn:nbn:se:uu:diva-133257 (URN)978-91-554-7953-4 (ISBN)
Public defence
2010-12-17, Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Note
Felaktigt tryckt som Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 720Available from: 2010-11-25 Created: 2010-11-04 Last updated: 2011-03-21Bibliographically approved
3. New Materials for Spintronics: Electronic structure and magnetism
Open this publication in new window or tab >>New Materials for Spintronics: Electronic structure and magnetism
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Materials exhibiting new functionalities due to interdependent electric (e.g. conductivity) and magnetic properties are potentially interesting for spintronics applications. We have investigated electronic and magnetic properties by means of x-ray spectroscopies and SQUID magnetometry in several magnetic materials, often in the form of thin films, which have shown promising properties for applications.

One of the main subjects has been studies of inter-diffusion between layers in multilayer structures, which is an important factor for spin-dependent transport and magnetic properties. These studies have been performed by high kinetic (HIKE) photoemission spectroscopy where high photon energies increase the bulk sensitivity in comparison to soft x-ray photoemission spectroscopy. Cu/Ni multilayers were studied mainly as a model system and revealed a diffusion process that was dependent on layer thicknesses and capping materials. CoFeB/MgO/CoFeB, which is used as a magnetic field sensor in hard drives, has recently been shown to exhibit a perpendicular magnetic anisotropy (PMA) switchable by electric fields. We have studied both the interface quality and magnetic properties of thin CoFeB layers exhibiting PMA. Layered structures of full Heusler alloys Co2MnGe/Rh2CuSn have been proposed as a promising candidate for current-perpendicular-to-plane giant magneto-resistance sensors. Using HIKE,we have shown that diffusion of atoms, mainly Mn, occurs at temperatures lower than what is used in device fabrication, which likely contributes to the limited magneto-resistance values obtained.

Lately, a large body of research has been performed on semiconductors doped with transition metal elements with the hope to find a ferromagnetic semiconductor at room temperature, a foundation for new devices combining spin and charge in their functionality. We have investigated Co and Fe doping in ZnO for different concentrations of the dopants and different annealing temperatures. The Co and Fe atoms are shown to forms clusters for which antiferromagnetic interactions are dominating.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2012. 66 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 897
Keyword
Spintronics, X-ray photoemission, XPS, XMCD, XAS, magnetic semiconductors, HIKE, HAXPES, multilayer, monte carlo, magnetism
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-167415 (URN)978-91-554-8275-6 (ISBN)
Public defence
2012-03-16, Polhemsalen, Ångström Laboratory, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2012-02-24 Created: 2012-01-27 Last updated: 2012-09-18Bibliographically approved

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Knut, RonnySanyal, BiplabKaris, OlofEriksson, OlleWikberg, J. MagnusSvedlindh, Peter

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