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New Materials for Spintronics: Electronic structure and magnetism
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Surface and Interface Science.
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 [en]
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: urn:nbn:se:uu:diva-167415ISBN: 978-91-554-8275-6 (print)OAI: oai:DiVA.org:uu-167415DiVA: diva2:499353
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
List of papers
1. Magnetic and electronic characterization of highly Co-doped ZnO: An annealing study at the solubility limit
Open this publication in new window or tab >>Magnetic and electronic characterization of highly Co-doped ZnO: An annealing study at the solubility limit
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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.

National Category
Physical Sciences Engineering and Technology Inorganic Chemistry
Research subject
Chemistry with specialization in Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-133314 (URN)10.1103/PhysRevB.82.094438 (DOI)000282097200003 ()
Available from: 2010-11-06 Created: 2010-11-06 Last updated: 2017-12-12
2. Electronic structure and chemical and magnetic interactions in ZnO doped with Co and Al: Experiments and ab initio density-functional calculations
Open this publication in new window or tab >>Electronic structure and chemical and magnetic interactions in ZnO doped with Co and Al: Experiments and ab initio density-functional calculations
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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.

National Category
Physical Sciences Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-107891 (URN)10.1103/PhysRevB.78.085319 (DOI)000259406900078 ()
Available from: 2009-09-01 Created: 2009-08-31 Last updated: 2017-12-13
3. Annealing effects on structural and magnetic properties of Co implanted ZnO single crystals
Open this publication in new window or tab >>Annealing effects on structural and magnetic properties of Co implanted ZnO single crystals
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2011 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 109, no 8, 083918- p.Article in journal (Refereed) Published
Abstract [en]

Single crystals of ZnO were implanted with 100 keV-Co ions at room temperature with a fluence of 4.8 x 10(16) cm(-2) and subsequently annealed at different temperatures up to 800 degrees C. The samples were analyzed by Rutherford backscattering spectrometry, secondary ion mass spectrometry, X-ray diffraction, X-ray photoemission spectroscopy and magnetometry. The as-implanted Co:ZnO crystal shows a homogeneous distribution of Co in the near surface region of the crystal. Upon annealing, clear evidence of secondary phases is found. At the highest annealing temperature (800 degrees C) a ferromagnetic behavior is observed at room temperature with a coercive field of 120 Oe assigned mainly to metallic fcc Co nano-crystallites. We find that for the annealed samples, the temperature dependent magnetization cannot be explained within a model containing only a paramagnetic contribution due to well dispersed Co ions and a ferromagnetic contribution due to Co nano-crystallites, at least one more ferromagnetic contribution is needed for a consistent explanation of the experimental results.

National Category
Inorganic Chemistry Engineering and Technology
Research subject
Chemistry with specialization in Inorganic Chemistry; Engineering Science with specialization in Solid State Physics
Identifiers
urn:nbn:se:uu:diva-133317 (URN)10.1063/1.3559263 (DOI)000290047000110 ()
Available from: 2010-11-06 Created: 2010-11-06 Last updated: 2017-12-12
4. Photoinduced reduction of surface states in the visible light sensitized photocatalyst Fe doped ZnO
Open this publication in new window or tab >>Photoinduced reduction of surface states in the visible light sensitized photocatalyst Fe doped ZnO
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(English)Manuscript (preprint) (Other academic)
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-167885 (URN)
Available from: 2012-02-02 Created: 2012-02-02 Last updated: 2012-03-01
5. Monte Carlo simulation as a sensitive characterization tool for exchange interactions and magnetic ion distributions
Open this publication in new window or tab >>Monte Carlo simulation as a sensitive characterization tool for exchange interactions and magnetic ion distributions
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(English)Article in journal (Other academic) Submitted
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-167883 (URN)
Available from: 2012-02-02 Created: 2012-02-02 Last updated: 2012-03-01
6. Magnetocrystalline anisotropy and uniaxiality of MnAs/GaAs(100) films
Open this publication in new window or tab >>Magnetocrystalline anisotropy and uniaxiality of MnAs/GaAs(100) films
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2011 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 83, no 2, 024417- p.Article in journal (Refereed) Published
Abstract [en]

We present an investigation of the magnetic behavior of epitaxial MnAs films grown on GaAs(100). We address the dependence of the magnetic moment, ferromagnetic transition temperature (Tc), and magnetocrystalline anisotropy constants on epitaxial conditions. From thorough structural and magnetic investigations, our findings indicate a more complex relationship between strain and magnetic properties in MnAs films than a simple stretch/compression of the unit cell axes. While a small increase is seen in the anisotropy constants, the enhancement of the magnetic moment at saturation is significant. Results of x-ray magnetic circular dichroism show a behavior of the spin and orbital moment that is consistent with a structural transition at Tc. In particular, we find that the ratio of the orbital to the spin moment shows a marked increase in the coexistence region of the ferromagnetic α- and paramagnetic β-phases a result that is well in accord with the observed increase in the c/a ratio in the same temperature region. The ab initio density functional calculations reveal that the magnetic properties are more sensitive towards change in the ab-plane compared to change in the c-axis which is explained by the analysis of band structures. The effects of electron correlation in MnAs using ab initio dynamical mean field theory are also presented.

National Category
Inorganic Chemistry Engineering and Technology
Research subject
Chemistry with specialization in Inorganic Chemistry; Engineering Science with specialization in Solid State Physics
Identifiers
urn:nbn:se:uu:diva-133316 (URN)10.1103/PhysRevB.83.024417 (DOI)000286755100007 ()
Available from: 2010-11-06 Created: 2010-11-06 Last updated: 2017-12-12
7. XMCD study of PMA in CFB/MgO
Open this publication in new window or tab >>XMCD study of PMA in CFB/MgO
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(English)Manuscript (preprint) (Other academic)
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-167896 (URN)
Available from: 2012-02-02 Created: 2012-02-02 Last updated: 2012-03-01
8. Interface characterization of Co2MnGe/Rh2CuSn Heusler multilayers
Open this publication in new window or tab >>Interface characterization of Co2MnGe/Rh2CuSn Heusler multilayers
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(English)Manuscript (preprint) (Other academic)
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-167887 (URN)
Available from: 2012-02-02 Created: 2012-02-02 Last updated: 2012-03-01
9. Investigation of interface properties of Ni/Cu multilayers by high kinetic energy photoelectron spectroscopy
Open this publication in new window or tab >>Investigation of interface properties of Ni/Cu multilayers by high kinetic energy photoelectron spectroscopy
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2009 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 80, no 9, 094104Article in journal (Refereed) Published
Abstract [en]

High kinetic-energy photoelectron spectroscopy (HIKE) or hard x-ray   photoelectron spectroscopy has been used to investigate the alloying of   Ni/Cu (100) multilayers. Relative intensities of the corelevels and   their chemical shifts derived from binding energy changes are shown to   give precise information on physicochemical properties and quality of   the buried layers. Interface roughening, including kinetic properties   such as the rate of alloying, and temperature effects on the processes   can be analyzed quantitatively. Using HIKE, we have been able to   precisely follow the deterioration of the multilayer structure at the   atomic scale and observe the diffusion of the capping layer into the   multilayer structure which in turn is found to lead to a segregation in   the ternary system. This is of great importance for future research on   multilayered systems of this kind. Our experimental data are   supplemented by first-principles theoretical calculations of the   core-level shifts for a ternary alloy to allow for modeling of the   influence of capping materials on the chemical shifts.

Keyword
ab initio calculations, alloying, chemical shift; copper, diffusion, interface structure, magnetic multilayers, nickel, X-ray photoelectron spectra
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-108327 (URN)10.1103/PhysRevB.80.094104 (DOI)000270383000027 ()
Available from: 2009-09-15 Created: 2009-09-15 Last updated: 2017-12-13Bibliographically approved
10. Understanding interface properties from high kinetic energy photoelectron spectroscopy and first principles theory
Open this publication in new window or tab >>Understanding interface properties from high kinetic energy photoelectron spectroscopy and first principles theory
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2011 (English)In: Journal of Electron Spectroscopy and Related Phenomena, ISSN 0368-2048, E-ISSN 1873-2526, Vol. 183, no 1-3, 80-93 p.Article, review/survey (Refereed) Published
Abstract [en]

Advances in instrumentation regarding 3rd generation synchrotron light sources and electron spectrometers has enabled the field of high kinetic energy photoelectron spectroscopy (HIKE) (also often denoted hard X-ray photoelectron spectroscopy (HX-PES or HAXPES)). Over the last years, the amount of investigations that relies on the HIKE method has increased dramatically and can arguably be said to have given a rebirth of the interest in photoelectron spectroscopy in many areas. It is in particular the much increased mean free path at higher kinetic energies in combination with the elemental selectivity of the core level spectroscopies in general that has lead to this fact, as it makes it possible to investigate the electronic structure of materials with a substantially reduced surface sensitivity. In this review we demonstrate how HIKE can be used to investigate the interface properties in multilayer systems. Relative intensities of the core level photoelectron peaks and their chemical shifts derived from binding energy changes are found to give precise information on physico-chemical properties and quality of the buried layers. Interface roughening, including kinetic properties such as the rate of alloying, and temperature effects on the processes can be analyzed quantitatively. We will also provide an outline of the theoretical framework that is used to support the interpretation of data. We provide examples from our own investigations of multilayer systems which comprises both systems of more model character and a multilayer system very close to real applications in devices that are considered to be viable alternative to the present read head technology. The experimental data presented in this review is exclusively recorded at the BESSY-II synchrotron at the Helmholtz-Zentrum Berlin fur-Materialien und Energie. This HIKE facility is placed at the bending magnet beamline KMC-1, which makes it different from several other facilities which relies on undulators as the source. We will therefore also briefly describe some of the salient design features of this facility.

Keyword
High kinetic energy photoemission, Multilayers, Interface properties, Alloying, Electronic structure, Ab initio theory
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-151667 (URN)10.1016/j.elspec.2010.08.004 (DOI)000288831300012 ()
Available from: 2011-04-15 Created: 2011-04-15 Last updated: 2017-12-11
11. Exploring the accessible frequency range of x-ray detected and phase-resolved ferromagnetic resonance
Open this publication in new window or tab >>Exploring the accessible frequency range of x-ray detected and phase-resolved ferromagnetic resonance
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(English)Manuscript (preprint) (Other academic)
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-167897 (URN)
Available from: 2012-02-02 Created: 2012-02-02 Last updated: 2012-03-01
12. Inhomogeneity in Co Doped ZnO Diluted Magnetic Semiconductor
Open this publication in new window or tab >>Inhomogeneity in Co Doped ZnO Diluted Magnetic Semiconductor
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2008 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 103, no 7, 07D131- p.Article in journal (Refereed) Published
Abstract [en]

Here, we have studied the chemical and magnetic interactions in Co doped ZnO diluted magnetic semiconductor by ab initio density functional calculations. The calculated chemical pair interaction parameters suggest a strong tendency of clustering between Co atoms. Both chemical and magnetic pair interaction parameters are short ranged with a large first neighbor interaction. Monte Carlo simulations show that the Curie temperature for a homogeneous sample is low, whereas allowing for an inhomogeneous growth results in very high Curie temperatures. It is argued that these inhomogeneities may be the reason for the observation of high Curie temperatures in Co doped ZnO. It is also demonstrated that the electronic structure is modified significantly in presence of Co clusters.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-105282 (URN)10.1063/1.2836328 (DOI)000255043200411 ()
Available from: 2009-06-03 Created: 2009-06-03 Last updated: 2017-12-13
13. Ordering in diluted magnetic semiconductors: A magnetic percolation phenomenon (invited)
Open this publication in new window or tab >>Ordering in diluted magnetic semiconductors: A magnetic percolation phenomenon (invited)
2007 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 101, no 9, 09H114- p.Article in journal (Refereed) Published
Abstract [en]

We report on first principles calculations that describe the electronic structure, magnetic exchange interactions, and ordering temperatures of diluted magnetic semiconductors. The calculated interatomic exchange couplings are used in a Heisenberg Hamiltonian, and ordering temperatures are calculated with the use of Monte Carlo simulations. The accuracy of the method is analyzed by comparing observed and calculated ordering temperatures of several Mn and Cr doped III-V and II-VI semiconductors. The effect of magnetic percolation is discussed along with clustering phenomena and the effect of strong electron-electron interaction.

Keyword
magnetic percolation, ordering temperatures, first principles calculations, electron-electron interaction
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-16324 (URN)10.1063/1.2713220 (DOI)000246567900321 ()
Available from: 2008-05-16 Created: 2008-05-16 Last updated: 2017-12-08Bibliographically approved
14. Electronic structure of Co doped ZnO: Theory and experiment
Open this publication in new window or tab >>Electronic structure of Co doped ZnO: Theory and experiment
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2008 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 103, no 7, 07D130- p.Article in journal (Refereed) Published
Abstract [en]

We have studied the electronic structure of Co doped ZnO by theory and experiment. Photoelectron and x-ray absorption spectroscopy experiments were performed on 5% Co doped ZnO thin films. The results show that Co-3d states lie in the valence band and hybridize considerably with O-2p states. These results compare qualitatively with our density functional calculations combined with a Hubbard approach to include strong electron correlations within the Co-3d state.

National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-17447 (URN)10.1063/1.2835695 (DOI)000255043200410 ()
Available from: 2008-06-24 Created: 2008-06-24 Last updated: 2017-12-08

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