| 1. |
- Lagerqvist, Ulrika, et al.
(författare)
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Morphology effects on exchange anisotropy in Co-CoO nanocomposite films
- 2015
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Ingår i: Thin Solid Films. - : Elsevier BV. - 0040-6090 .- 1879-2731. ; 576, s. 11-18
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Tidskriftsartikel (refereegranskat)abstract
- Co-CoO composite films were prepared by solution chemical technique using amine-modified nitrates and acetates in methanol. We study how particle size and porosity can be tuned through the synthesis parameters and how this influences the magnetic properties. Phase content and microstructure were characterised with grazing incidence X-ray diffraction and electron microscopy, and the magnetic properties were studied by magnetometry and magnetic force microscopy. Composite films were obtained by heating spin-coated films in Ar followed by oxidation in air at room temperature, and the porosity and particle size of the films were controlled by gas flow and heating rate. The synthesis yielded dense films with a random distribution of metal and oxide nanoparticles, and layered films with porosity and sintered primary particles. Exchange anisotropy, revealed as a shift towards negative fields of the magnetic hysteresis curve, was found in all films. The films with a random distribution of metal and oxide nanoparticles displayed a significantly larger coercivity and exchange anisotropy field compared to the films with a layered structure, whereas the layered films displayed a larger nominal saturation magnetisation. The magnitude of the coercivity decreased with increasing Co grain size, whereas increased porosity caused an increased tilt of the magnetic hysteresis curve. (C) 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license
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| 2. |
- Lagerqvist, Ulrika, 1980-
(författare)
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Solution-Chemical Synthesis of Cobalt and Iron:Zinc Oxide Nanocomposite Films
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The potentially most important challenges today are related to energy and the environment. New materials and methods are needed in order to, in a sustainable way, convert and store energy, reduce pollution, and clean the air and water from contaminations. In this, nanomaterials and nanocomposites play a key role, and hence knowledge about the relation between synthesis, structure, and properties of nanosystems is paramount.This thesis demonstrates that solution-chemical synthesis, using amine-modified acetates and nitrates, can be used to prepare widely different nanostructured films. By adjusting the synthesis parameters, metals, oxides, and metal–oxide or oxide–oxide nanocomposites were prepared for two systems based on Co and Zn:Fe, respectively, and the films were characterised using diffraction, spectroscopy, and microscopy techniques, and SQUID magnetometry.A variety of crystalline cobalt films—Co metal, CoO, Co3O4, and composites with different metal:oxide ratios—were synthesised. Heat-treatment parameters and control of the film thickness enabled tuning of the phase ratios. Random and layered Co–CoO composites were prepared by utilising different heating rates and gas flow rates together with a morphology effect associated with the furnace tube. The Co–CoO films exhibited exchange bias due to the ferromagnetic–antiferromagnetic interaction between the Co and CoO, whereas variations in e.g. coercivity and exchange bias field were attributed to differences in the structure and phase distribution.Ordered structures of wurtzite ZnO surrounded by amorphous ZnxFeyO were prepared through controlled phase segregation during the heating, which after multiple coating and heating cycles yielded ZnO–ZnxFeyO superlattices. The amorphous ZnxFeyO was a prerequisite for superlattice formation, and it profoundly affected the ZnO phase, inhibiting grain growth and texture, already from 1% Fe. In addition, ZnO–ZnxFeyO exhibited a photocatalytic activity for the oxidation of water that was higher than results reported for pure ZnO, and comparable to recent results reported for graphene-modified ZnO.
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| 3. |
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| 4. |
- Wikberg, J Magnus, et al.
(författare)
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Annealing effects on structural and magnetic properties of Co implanted ZnO single crystals
- 2011
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 109:8, s. 083918-
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 5. |
- Wikberg, J. M., et al.
(författare)
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Evolving Magnetization Dynamics in Mn3-xGa
- 2015
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Ingår i: Springer Proceedings in Physics. - Cham : Springer International Publishing. - 0930-8989. ; 159, s. 23-25, s. 23-25
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Konferensbidrag (refereegranskat)abstract
- Magnetic materials with high magnetic coercivity (Hc), magnetic anisotropy (Ku) and low Gilbert damping (α) are of great importance for future spintronics devices. For instance, for spin-transfer-torque (STT) memory devices low α and high Ku are desired. Such a combination of material properties includes a built in contradiction since both Ku and α are dependent on the spin-orbit interaction (SO); a high Ku material is expected to have a high α. However, recent experimental investigations of Mn3-xGa, a material exhibiting high Curie temperature (Tc) > 700 K and (out-of-plane) perpendicular anisotropy, have shown that such a contradictorily low α and high Ku material exists.
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| 6. |
- Wikberg, J Magnus, et al.
(författare)
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Magnetocrystalline anisotropy and uniaxiality of MnAs/GaAs(100) films
- 2011
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 83:2, s. 024417-
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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