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Magnetic and electronic characterization of highly Co-doped ZnO: An annealing study at the solubility limit
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Surface and Interface Science.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry, Inorganic Chemistry.
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2010 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 82, no 9, 094438- p.Article in journal (Refereed) Published
Abstract [en]

We report on investigations of the electronic structure and magnetic properties of ZnO doped with 15 at. % Co and postgrowth annealed at temperatures ranging between 250 and 800°C. In particular, we demonstrate how the presence of Co3+, indicative of secondary phases, is manifested in spectroscopy. Through resonant photoemmision spectroscopy we have found that x-ray diffraction in some cases underestimates or does not reveal the presence of secondary phases, possibly due to unrelaxed structures or structural arrangements with sizes below the detection limit. The magnetic properties are in most cases understood by assuming small antiferromagnetic clusters but can also show a behavior indicative of ferromagnetic interactions.

Place, publisher, year, edition, pages
2010. Vol. 82, no 9, 094438- p.
National Category
Physical Sciences Engineering and Technology Inorganic Chemistry
Research subject
Chemistry with specialization in Inorganic Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-133314DOI: 10.1103/PhysRevB.82.094438ISI: 000282097200003OAI: oai:DiVA.org:uu-133314DiVA: diva2:361005
Available from: 2010-11-06 Created: 2010-11-06 Last updated: 2017-12-12
In thesis
1. 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
2. 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, RonnyWikberg, J. MagnusWestin, GunnarSvedlindh, PeterKaris, Olof

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