| 1. |
- Carstensen, Hauke, et al.
(författare)
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Phase formation in colloidal systems with tunable interaction
- 2015
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Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - 1539-3755 .- 1550-2376. ; 92:1
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Tidskriftsartikel (refereegranskat)abstract
- Self-assembly is one of the most fascinating phenomena in nature and is one key component in the formation of hierarchical structures. The formation of structures depends critically on the interaction between the different constituents, and therefore the link between these interactions and the resulting structure is fundamental for the understanding of materials. We have realized a two-dimensional system of colloidal particles with tunable magnetic dipole forces. The phase formation is studied by transmission optical microscopy and a phase diagram is constructed. We report a phase transition from hexagonal to random and square arrangements when the magnetic interaction between the individual particles is tuned from antiferromagnetic to ferrimagnetic.
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| 2. |
- Carstensen, Hauke, et al.
(författare)
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Self-assembly and percolation in two dimensional binary magnetic colloids
- 2022
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Ingår i: Soft Matter. - : Royal Society of Chemistry (RSC). - 1744-683X .- 1744-6848. ; 18:33, s. 6222-6228
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Tidskriftsartikel (refereegranskat)abstract
- We study the self-assembly of branching-chain networks and crystals in a binary colloidal system with tunable interactions. The particle positions are extracted from microscopy images and order parameters are extracted by image processing and statistical analysis. With these, we construct phase diagrams with respect to particle density, ratio and interaction. In order to draw a more complete picture, we complement the experiments with computer simulations. We establish a region in the phase diagram, where bead ratios and interactions are symmetric, promoting percolated structures.
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| 3. |
- Carstensen, Hauke
(författare)
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Self-assembly of magnetic particles
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Self-assembly is the spontaneous formation of larger structures from small building blocks. This process is driven and determined by the interactions between the constituents. Examples of self assembly are found almost everywhere and, in particular, biological systems in general rely on a hierarchical formation of structures over a range of length scales. Technologically, self-assembly can be used to form mesoscopic structures and artificial crystals. In the case of particles with micrometer size suspended in a liquid phase, it is possible to use optical microscopy for the the investigation of self-assembly.In this thesis, the self-assembly of microbeads with tunable magnetic interactions is studied, based on the statistic analysis of microscope images and computer simulations. Magnetic and non-magnetic microbeads are suspended in a ferrofluid, which is a dispersion of magnetic nanoparticles in water. As a result, the magnetic properties of the microbeads in the ferrofluid are altered and can be described by effective magnetic susceptibilities and magnetic dipole moments, which can be tuned continuously. The liquid is confined between glass slides and effectively the microbeads are studied in a 2D geometry under a magnetic field, applied either in- or out-of-plane. The resulting structures are detected by image analysis algorithms, analyzed and correlated to the dipolar interaction between the beads, as well as to macroscopic quantities, like the particle density and ratio. For the in-plane field a phase transition from square to hexagonal lattice is observed. This phase transition is explained by the change in dipole interaction between the microbeads as the moments change from anti-parallel to parallel alignment. For the out-of-plane field the situation becomes diverse and more phases appear. It turns out that the phase formation in this case is strongly dependent on the bead ratio, density and interactions.We identify regions in the phase diagram, where isolated beads, percolated structures, and crystals dominate. To cover a wide parameter range the experiments are complemented by computer simulations. The tools developed in this thesis enable us to construct phase diagrams extracted from direct imaging and dependence on the extracted relevant parameters.
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| 4. |
- Carstensen, Hauke, et al.
(författare)
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Statistical analysis of phase formation in 2D colloidal systems
- 2018
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Ingår i: The European Physical Journal E Soft matter. - : SPRINGER. - 1292-8941 .- 1292-895X. ; 41:1
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Tidskriftsartikel (refereegranskat)abstract
- Colloidal systems offer unique opportunities for the study of phase formation and structure since their characteristic length scales are accessible to visible light. As a model system the two-dimensional assembly of colloidal magnetic and non-magnetic particles dispersed in a ferrofluid (FF) matrix is studied by transmission optical microscopy. We present a method to statistically evaluate images with thousands of particles and map phases by extraction of local variables. Different lattice structures and long-range connected branching chains are observed, when tuning the effective magnetic interaction and varying particle ratios.
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| 5. |
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| 6. |
- Droulias, Sotirios A., et al.
(författare)
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Crystal perfection by strain engineering : The case of Fe/V (001)
- 2017
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Ingår i: Thin Solid Films. - : Elsevier BV. - 0040-6090 .- 1879-2731. ; 636, s. 608-614
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Tidskriftsartikel (refereegranskat)abstract
- We study the effect of bilayer thickness at fixed volume fraction on the structural quality of Fe/V (001)superlattices. We find that such artificial metallic superlattices can be manufactured with excellent crystalquality and layering up to at least 50 Å in repeat distance (K = LFe +LV). For an intended fixed ratio of theconstituents: LFe/LV= 1/7, out-of-plane coherence lengths comparable to the thicknesses of the sampleswere obtained. We evaluate the strain in- and out-of-plane of both layers as a function of the bilayer thicknessand comment on the growth using the framework of linear elasticity theory. We interpret the stabilityof the superlattice against crystal degradation due to the alternating compressive and tensile strain, yieldingclose to ideal lattice matching to the substrate.
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| 7. |
- Poulopoulos, P., et al.
(författare)
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Induced spin-polarization of EuS at room temperature in Ni/EuS multilayers
- 2014
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 104:11, s. 112411-
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Tidskriftsartikel (refereegranskat)abstract
- Ni/EuS multilayers with excellent multilayer sequencing are deposited via e-beam evaporation on the native oxide of Si(100) wafers at 4 x 10(-9) millibars. The samples have very small surface and interface roughness and show sharp interfaces. Ni layers are nanocrystalline 4-8 nm thick and EuS layers are 2-4 nm thick and are either amorphous or nanocrystalline. Unlike for Co/EuS multilayers, all Eu ions are in divalent (ferromagnetic) state. We show a direct antiferromagnetic coupling between EuS and Ni layers. At room temperature, the EuS layers are spin-polarized due to the proximity of Ni. Therefore, Ni/EuS is a candidate for room-temperature spintronics applications. (C) 2014 AIP Publishing LLC.
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| 8. |
- Saini, Apurve, et al.
(författare)
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Electrical Sensing in a Magnetic Liquid
- 2019
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Ingår i: IEEE Sensors Journal. - : IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC. - 1530-437X .- 1558-1748. ; 19:16, s. 6948-6955
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Tidskriftsartikel (refereegranskat)abstract
- We have investigated the electrical current transmission characteristics of a magnetic field controlled self-assembled percolated structure of conducting colloid particles. The colloidal liquid contains conducting magnetic and non-magnetic microbeads suspended in an oil-based ferrofluid. The applied in-plane magnetic field is able to precisely control the current transmission. Optical transmission microscopy reveals the local ordering in the liquid with the microparticles forming percolating chains through which the current is transmitted.
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| 9. |
- Saini, Apurve, et al.
(författare)
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Magnetically Controlled Electrical Current Through a Colloid
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We demonstrate current transmission through a colloid optimized with both conducting and magnetic properties. The current can be controlled precisely by applying a magnetic field and can be switched. The phenomena depend on a collective and simultaneous control of a microscopic entities. Here, we report on measurements of conductivity of such a colloidal liquid and the viability of developing a working contact. The contact is scalable and might be capable of offering lower loss current conduction (for low or medium voltage applications) compared to contemporary mechanical connections and may solve challenges such as oxidation, mechanical wear, contact welding etc.
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| 10. |
- Saini, Apurve
(författare)
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Magnetically governed self-assembly of soft matter : A look into interfacial layering, crystallization and percolation
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Self-organisation is the key route for assembling colloidal particles into well-defined structures. Decisive for this are the interactions between the constituents, which are amongst others steric, electrostatic or magnetic. A deep knowledge on the underlying physical processes during self-assembly is crucial for the design and fabrication of well-defined hierarchical architectures from a nanometer scale as well as for realizing smart, functional or stimuli responsive synthetic materials. In this dissertation, the self-assembly of colloidal magnetic particles into organized and multi-layered structures is studied. Particular emphasis is given to solid-liquid boundaries and the response to applied magnetic fields. Particle coatings with specific functional molecules stabilize the nanoparticles (NPs) in the solvent and can simultaneously promote their assembly at a substrate. An example in this context is N-hydroxysuccinimide interacting with (3-aminopropyl)triethoxy silane at the substrate. As a result of this chemical affinity, uniform and densely packed particle wetting layers are seeded which then instate the layering process. As an alternative to chemical binding, the magnetic stray field of a ferrimagnetic (Tb15Co85 film) deposited on a substrate induces particle self-assembly with dense layers as well. The application of an external magnetic field further promotes densification, particle layering and leads to variations in the assembly characteristics such as quasi-domain formation of closely packed layers. At an interface with a magnetic field applied in the plane of the interface, Brownian motion and Neel relaxation of the NPs are decisive for the layering and give raise to these domains. For a magnetic field oriented along the surface normal similar structural layering but denser packing is found. The self-assembly is a relatively slow process and evolves over hours and is maximized, most ordered and dense for superparamagnetic NPs which are single domain and having a large remanent moment and reduced thermal mobility. Small quantities of magnetic micelles in a hybrid magnetic polymer nanocomposite, facilitate the crystallization of Pluronic F127 micelles dissolved in water into single crystalline structures via a micro-shear effect under applied magnetic field. Also, a magnetic field applied to a colloidal dispersion of conducting magnetic and non-magnetic polystyrene microbeads suspended in an oil-based ferrofluid can lead to percolated structures. This allows current transmission and switching. A working contact for possible applications in automotive, switchboard and telecommunications is demonstrated.
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