| 1. |
- Agnarsson, Björn, 1977, et al.
(författare)
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Rutile TiO 2 thin films grown by reactive high power impulse magnetron sputtering
- 2013
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Ingår i: Thin Solid Films. - : Elsevier BV. - 0040-6090 .- 1879-2731. ; 545, s. 445-450
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Tidskriftsartikel (refereegranskat)abstract
- Thin TiO 2 films were grown on Si(001) substrates by reactive dc magnetron sputtering (dcMS) and high power impulse magnetron sputtering (HiPIMS) at temperatures ranging from 300 to 700 C.Optical and structural properties of films were compared both before and after post-annealing using scanning electron microscopy, low angle X-ray reflection (XRR), grazing inc idence X-ray diffractometry and spectroscopic ellipsometry.Both dcMS- and HiPIMS-grown films reveal polycrystalline rutile TiO 2 , even prior to post-annealing.The HiPIMS-grown films exhibit significantly larger grains compared to that of dcMC-grown films, approaching 100% of the film thickness for films grown at 700 C.In addition, the XRR surface roughness of HiPIMS-grown films was significantly lower than that of dcMS-grown films over the whole temperature range 300-700 C.Dispersion curves could only be obtained for the HiPIMS-grown films, which were shown to have a refractive index in the range of 2.7-2.85 at 500 nm.The results show that thin, rutile TiO 2 films, with high refractive index, can be obtained by HiPIMS at relatively low growth temperatures, without post-annealing.Furthermore, these films are smoother and show better optical characteristics than their dcMS-grown counterparts.© 2013 Elsevier B.V.All rights reserved.
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| 2. |
- Dahlqvist, Martin, et al.
(författare)
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Magnetically driven anisotropic structural changes in the atomic laminate Mn2GaC
- 2016
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Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 93:1
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Tidskriftsartikel (refereegranskat)abstract
- Inherently layered magnetic materials, such as magnetic M(n+1)AX(n) (MAX) phases, offer an intriguing perspective for use in spintronics applications and as ideal model systems for fundamental studies of complex magnetic phenomena. The MAX phase composition M(n+1)AX(n) consists of M(n+1)AX(n) blocks separated by atomically thin A-layers where M is a transition metal, A an A-group element, X refers to carbon and/or nitrogen, and n is typically 1, 2, or 3. Here, we show that the recently discovered magnetic Mn2GaC MAX phase displays structural changes linked to the magnetic anisotropy, and a rich magnetic phase diagram which can be manipulated through temperature and magnetic field. Using first-principles calculations and Monte Carlo simulations, an essentially one-dimensional (1D) interlayer plethora of two-dimensioanl (2D) Mn-C-Mn trilayers with robust intralayer ferromagnetic spin coupling was revealed. The complex transitions between them were observed to induce magnetically driven anisotropic structural changes. The magnetic behavior as well as structural changes dependent on the temperature and applied magnetic field are explained by the large number of low energy, i.e., close to degenerate, collinear and noncollinear spin configurations that become accessible to the system with a change in volume. These results indicate that the magnetic state can be directly controlled by an applied pressure or through the introduction of stress and show promise for the use of Mn2GaC MAX phases in future magnetoelectric and magnetocaloric applications.
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| 3. |
- Frisk, Andreas, 1983-, et al.
(författare)
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Tailoring anisotropy and domain structure in amorphous TbCo thin films through combinatorial methods
- 2016
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Ingår i: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 49:3
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Tidskriftsartikel (refereegranskat)abstract
- We apply an in-plane external magnetic field during growth of amorphous TbCo thin films and examine the effects on the magnetic anisotropy and domain structure. A combinatorial approach is employed throughout the deposition and analysis to study a continuous range of compositions between 7–95 at.% Tb. Magnetometry measurements show that all samples have a strong out-of-plane anisotropy, much larger than any in-plane components, regardless of the presence of a growth field. However, magnetic force microscopy demonstrates that the growth field does indeed have a large effect on the magnetic domain structure, resulting in elongated domains aligned along the imprinting field direction. The results show that the anisotropy can be tuned in intricate ways in amorphous TbCo films giving rise to unusual domain structures. Furthermore the results reveal that a combinatorial approach is highly effective for mapping out these material properties.
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| 4. |
- George, Sebastian, 1990-
(författare)
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Amorphous Magnetic Materials : A Versatile Foundation for Tomorrow’s Applications
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Amorphous magnetic materials exhibit a number of key differentiating properties with respect to crystalline magnets. In some cases, the differences may simply be in the values of macroscopic properties such as saturation magnetization, coercivity, Curie temperature, and electrical conductivity. Other cases are more fundamental, such as the possibility for many amorphous alloys to be produced with nearly arbitrary composition, something that is not always possible in crystal structures that may only be stable for certain specific compositions.Fundamentally, these properties arise due to the disordered arrangement of atoms in amorphous materials. However, this structure is challenging to probe and characterize, either experimentally or theoretically. A significant contribution of this thesis is the development of a new approach for studying the local atomic structure of amorphous materials, specifically amorphous SmCo and FeZr alloys. The strategy combines extended x-ray absorption spectroscopy (EXAFS) measurements with stochastic quenching (SQ) simulations in a way that provides more information than either method can offer alone. Additionally, this approach offers the potential for identifying any shortcomings in the theoretical models obtained via SQ.Having an accurate model of the atomic arrangement is not, however, a prerequisite for developing technical applications of amorphous magnetic materials. For that, it is sufficient to quantify those macroscopic properties that are relevant for a given application. Such is the value of the magnetic characterization of amorphous TbCo and CoFeZr alloy thin films presented here. Both investigations used methods such as vibrating sample magnetometry (VSM) and magneto-optic Kerr effect (MOKE) measurements to highlight the high tunability of the magnetic properties in these materials, which can be achieved simply by changing the chemical composition.The final portion of this thesis examines what can be achieved by combining amorphous SmCo and TbCo alloys together in bilayer structures. This is a step away from the alloy characterization studies, as it focuses on how new properties can be realized when multiple materials are brought together. MOKE measurements were used to identify the conditions under which the bilayers spontaneously become magnetized parallel to the film plane versus when the TbCo magnetization begins to tilt out of the plane. Further investigation combining x-ray circular magnetic dichroism (XMCD) measurements and micromagnetic simulations provided a depth-resolved model of the magnetization throughout the bilayers in the presence of a broad range of external field strengths and directions. These models also showed that the local magnetization just above and just below the SmCo/TbCo interface can be aligned either parallel or antiparallel to one another simply by varying the TbCo composition. This discovery offers a novel method for controlling the magnetic behavior in these materials, and may well be useful for all-optical switching or spintronics applications where amorphous TbCo alloys have already drawn attention.
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| 5. |
- George, Sebastian, 1990-, et al.
(författare)
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Local structure in amorphous SmxCo1-x : a combined experimental and theoretical study
- 2020
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Ingår i: Journal of Materials Science. - : Springer Science and Business Media LLC. - 0022-2461 .- 1573-4803. ; 55, s. 12488-12498
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Tidskriftsartikel (refereegranskat)abstract
- Using a combination of extended X-ray absorption fine structure measurements, stochastic quenching (SQ) calculations and Voronoi tessellation analysis, the local atomic environments in thin films of amorphous SmxCo1−x (x= 0.10, 0.22 and 0.35) are investigated and also compared with crystalline stoichiometric Sm–Co alloys of similar compositions. It is found that the variations in local environment around Co atoms in the amorphous films increase with increasing x and that none of the films exhibit any pronounced short-range order around the Sm atoms. There are, however, signs of clustering of Sm atoms in the SQ-generated simulated amorphous materials. Furthermore, good agreement is observed between experimentally obtained parameters, e.g., interatomic distances and coordination numbers, and those extracted from the simulated alloys. This is a strong indication that SQ provides a powerful route to reliable local structure information for amorphous rare earth–transition metal alloys and that it could be used for designing materials with properties that meet the demands of specific applications.
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| 6. |
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| 7. |
- Hanifpour, Fatemeh, et al.
(författare)
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Investigation into the mechanism of electrochemical nitrogen reduction reaction to ammonia using niobium oxynitride thin-film catalysts
- 2022
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Ingår i: Electrochimica Acta. - : Elsevier. - 0013-4686 .- 1873-3859. ; 403
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Tidskriftsartikel (refereegranskat)abstract
- Niobium oxynitride (NbOxNy) thin films with varying combined non-metal vs. metal stoichiometries ( x + y ) and N/O stoichiometric ratios (y/x) are investigated for their ability to catalyze the nitrogen re-duction reaction and ammonia synthesis at ambient conditions. Electrochemical impedance spectroscopy and ammonia measurements show stark differences both in nitrogen vs. argon media on each surface and on the surfaces in the series when the combined stoichiometry of N + O vs. Nb increases. Surface stability checks at fixed intervals during the experiments and surface characterization after the experiments us -ing X-ray diffraction reveal the least changes occurred to the surface with the highest N + O stoichiometry. Based on these observations, an ammonia synthesis mechanism is proposed. Isotope labeling experiments on the most promising surface of the series, however, show no sign of catalytically produced ammonia, possibly due to the lack of stability of the surface to endure through the ammonia production cycle.
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| 8. |
- Hanifpour, Fatemeh, et al.
(författare)
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Operando quantification of ammonia produced from computationally-derived transition metal nitride electro-catalysts
- 2022
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Ingår i: Journal of Catalysis. - : Elsevier. - 0021-9517 .- 1090-2694. ; 413, s. 956-967
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Tidskriftsartikel (refereegranskat)abstract
- Electrochemical reduction of dinitrogen to ammonia is investigated in a micro-reactor flow-cell using thin films of VN, CrN, NbN and ZrN. Chronoamperometry loops are used for ammonia production analysis. Operando ammonia quantification is accomplished in a flow injection analyzer. Results show the effect of presence/absence of N-2(g) within both the electrochemical characterization and ammonia production for ZrN. However, no ammonia is detected from studies on CrN. VN and NbN are inactivated upon reacting their N atoms of the surface top layer(s). Results obtained from ammonia measurements, electrochemical impedance spectroscopy analysis, surface stability checks, and surface characterization using X-ray reflectivity, reveal certain trends indicating catalytic behavior for ZrN. However, the concentration of produced ammonia is below the detection limit of the methods devised to analyze the samples from isotope labeling experiments. The onset of ammonia production on ZrN appears to be in close agreement with that predicted previously by computational studies.
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| 9. |
- Ingason, Arni Sigurdur, et al.
(författare)
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Magnetic Self-Organized Atomic Laminate from First Principles and Thin Film Synthesis
- 2013
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 110
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Tidskriftsartikel (refereegranskat)abstract
- he first experimental realization of a magnetic Mn+1AXn (MAX) phase, (Cr0.75Mn0.25)2GeC, is presented, synthesized as a heteroepitaxial single crystal thin film, exhibiting excellent structural quality. This self-organized atomic laminate is based on the well-known Cr2GeC, with Mn, a new element in MAX phase research, substituting Cr. The compound was predicted using first-principles calculations, from which a variety of magnetic behavior is envisaged, depending on the Mn concentration and Cr/Mn atomic configuration within the sublattice. The analyzed thin films display a magnetic signal at room temperature.
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| 10. |
- Leosson, K., et al.
(författare)
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Comparing resonant photon tunneling via cavity modes and Tamm plasmon polariton modes in metal-coated Bragg mirrors
- 2012
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Ingår i: Optics Letters. - 0146-9592 .- 1539-4794. ; 37:19, s. 4026-4028
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Tidskriftsartikel (refereegranskat)abstract
- Resonant photon tunneling was investigated experimentally in multilayer structures containing a high-contrast (TiO2/SiO2) Bragg mirror capped with a semitransparent gold film. Transmission via a fundamental cavity resonance was compared with transmission via the Tamm plasmon polariton resonance that appears at the interface between a metal film and a one-dimensional photonic bandgap structure. The Tamm-plasmon-mediated transmission exhibits a smaller dependence on the angle and polarization of the incident light for similar values of peak transmission, resonance wavelength, and finesse. Implications for transparent electrical contacts based on resonant tunneling structures are discussed.
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