One-dimensional assemblies of silica-coated cobalt nanoparticles: Magnetic pearl necklaces (original) (raw)
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Self-Organization of Magnetic Nanosized Cobalt Particles
Advanced Materials, 1998
The synthesis of nanoparticles, characterized by a low size distribution, is a new challenge in solid-state chemistry. Due to their small size, nanoparticles exhibit novel materials properties that differ considerably from those of the bulk solid state. [1] In this emerging field, finely divided magnetic nanoparticles are desirable owing to their broad range of applications, especially in data storage devices and sensors. A great deal of work on large magnetic nanoparticles has been carried out, [2] but, although the magnetic properties of isolated atoms are well understood, there are still questions about the development of magnetic order on a macroscopic scale. [3,4] The creation of perfect nanometer-scale magnetic crystallites identically replicated with long-range order in a state that can be manipulated and understood in terms of a pure macromolecular substance is an ultimate challenge in present materials research and could help us to understand the formation of ferromagnetism. To develop this application, it is crucial to be able to control the spatial arrangement of these nanoparticles in 2D or 3D arrays. Recently, in our laboratory spontaneous arrangement of particles either into monolayer organized hexagonal networks or into 3D face-centered cubic (fcc) arrangements was observed with silver sulfide and silver nanosized clusters. [5±8] Similar arrangements with metal particles such as gold or silver [9±11] and CdSe semiconductors [12] have been reported elsewhere. Here, we report, for the first time, self-organization of magnetic cobalt nanoparticles into 2D superlattices. The magnetic properties of isolated and organized particles are compared. Reverse micelles are water in oil droplets stabilized by a monolayer of surfactant (e.g., sodium bis(2-ethylhexyl)sulfosuccinate, usually called Na(AOT)
Dipolar chains formed by chemically synthesized cobalt nanocubes
Journal of Magnetism and Magnetic Materials, 2009
Cobalt nanocubes with an average edge length of 50 nm and epsilon crystalline structure were synthesized via thermo-decomposition in 1,2-dichlorobenzene at 120 1C in the presence of surfactants. These nanocubes form dipolar chains upon drying in zero applied field and bundles of chains along the direction of an applied magnetic field. The magnetic measurements reveal strong interparticle couplings among the nanocubes in their dried magnetic-field-induced assemblies. The constricted hysteresis loops and near-zero coercivity indicate the existence of vortex states in the assemblies. Exposure to electron beam heats up the nanocubes and turns the dipolar chains into nanowires.
Effect of magnetic field on self-assembling of colloidal Co magnetic nanoparticles
Applied Surface Science, 2006
In this paper the formation of 3-D structures composed of Co nanoparticles (NPs) is reported. Structures were obtained by drying a droplet of a colloidal solution of NPs in a magnetic field perpendicular to the substrate. The Co nanoparticles were prepared by thermolysis of Co 2 (CO) 8 . The 3-D NP structures were characterized by scanning electron microscopy (SEM) and atomic and magnetic force microscopy (AFM/MFM). It has been found that at the border of the droplet, NPs assemble into hexagonally ordered 3-D columns or they form a labyrinthine structure. The formation of the 3-D structures can be explained by the outflow of NPs to the border of the droplet during the drying process. Within this model the pattern formation is dependent on the concentration of the NPs and the degree of alignment of the magnetic moments of NPs in the 3-D columns. #
Solvothermal synthesis of magnetic chains self-assembled by flowerlike cobalt submicrospheres
Crystal Growth and …, 2008
Cobalt chains with lengths of up to 4-20 µm, self-assembled by flowerlike cobalt submicrospheres, have been synthesized at 200°C for 4 h by a solvothermal method with the surfactant poly(vinyl pyrrolidone) (PVP). The average diameter of individual flowerlike submicrospheres is 700-900 nm, which are composed of compact nanosheets with an average thickness of about 50 nm. The products were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDS). The effects of synthetic conditions, such as reaction temperature and the amount of reducing agent, on the morphology and size of the chains were investigated. The growth mechanism of the chains was proposed, based on the evolution of the structure and the morphology with increasing the reaction time. The magnetic hysteresis loops at 5 and 295 K of the chains show ferromagnetic characteristics with coercivities of 347 and 90 Oe, respectively. Our work may shed light on the design fabrication of one-dimensional chainlike structures self-assembled by complex three-dimensional architectures of materials.
Nature materials, 2004
The use of magnetic nanoparticles in the development of ultra-high-density recording media is the subject of intense research. Much of the attention of this research is devoted to the stability of magnetic moments, often neglecting the influence of dipolar interactions. Here, we explore the magnetic microstructure of different assemblies of monodisperse cobalt single-domain nanoparticles by magnetic force microscopy and magnetometric measurements. We observe that when the density of particles per unit area is higher than a determined threshold, the two-dimensional self-assemblies behave as a continuous ferromagnetic thin film. Correlated areas (similar to domains) of parallel magnetization roughly ten particles in diameter appear. As this magnetic percolation is mediated by dipolar interactions, the magnetic microstructure, its distribution and stability, is strongly dependent on the topological distribution of the dipoles. Thus, the magnetic structures of three-dimensional assembli...
SAXS study of chain-like structures formed by magnetic nanoparticles
Materials Science and Engineering: C, 2007
Nanoparticles consisting of a magnetic core (γ-Fe 2 O 3 ) and an organic shell were prepared by thermal decomposition of an iron carbonyl complex in the presence of oleic acid. The nanoparticles were then dispersed in cyclohexane to form a stable ferrofluid. These dispersions were investigated by means of Small Angle Scattering of X-rays (SAXS) and the data were interpreted according to a "pearl necklace" model, opportunely modified to account for the core-shell structure of the magnetic nanoparticles. The results of the fittings show that nearly monodisperse spherical particles with a radius of the magnetic core of about 3.9 nm and a shell of about 2 nm were obtained. These nanoparticles self-assemble in chain-like structures, even in the absence of an applied magnetic field.
Unravelling Finite Size Effects on Magnetic Properties of Cobalt Nanoparticles
The Journal of Physical Chemistry C, 2019
Using first principle calculations, the problem of scaling magnetic properties in nanoparticles is addressed. To this aim, the local electronic structure is characterized in cobalt quasi-spherical magnetic nanoparticles, in a large size range, from 0.5 to 2 nm of diameter. First, specific patterns of the magnitude of local spin magnetic moments are evidenced depending on the shape and the size of the nanoparticles. Then, effects of local structural environment (atomic coordination, structural deformations, finite size effects, shape changes) are unravelled. In small icosahedral nanoparticles, the local spin magnetic moment is found to decrease from the surface to the center. General rules driving charge transfers are observed whereby donor atomic sites are exclusively subsurface atoms and more unexpected vertex surface atomic sites. The variation of the magnetic moment is driven by the coupling between cluster microstructure and complex hybridization effects. In larger truncated octahedral clusters, whereas some properties are still found (quasi-zero inter-atomic site charge transfer, the reversal from anti-ferromagnetic to ferromagnetic coupling of electronic sp states with d ones at vertex atomic sites), other vary such as the behavior of the local spin magnetic moment which now presents weak oscillations.
Microstructure and magnetic properties of colloidal cobalt nano-clusters
Journal of Magnetism and Magnetic Materials, 2010
The magnetic response of nanometer sized Co nanoparticles (NP) prepared using reverse micelle solutions are presented. The use of complementary structural and morphological probes (like transmission electron microscopy, high resolution electron microscopy, X-ray absorption spectroscopy) allowed to relate the magnetic properties to the size, morphology, composition and atomic structure of the nanoparticles. All data agree on the presence of a core-shell structure of NPs made of a metallic Co core surrounded by a thin Co-oxide layer. The core-shell microstructure of NPs affects its magnetic response mainly raising the anisotropy constant.
Calixarene-stabilised cobalt nanoparticle rings: Self-assembly and collective magnetic properties
2009
Calixarenes can be used to promote the self-assembly of thermoremanent cobalt nanoparticles into bracelet-like rings below 100 nm in diameter. These kinetically stable assemblies are regulated by the equilibrium between enthalpic gain (dipole–dipole and long-range van der Waals interactions) and entropic loss, analogous to the thermodynamic balance of forces governing supramolecular self-assembly.
On the road from single, nanosized magnetic clusters to multi-dimensional nanostructures
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2002
Nanosized colloidal cobalt (Co) with primary particles in the size range 8 -12 nm have been successfully obtained using a combination of oleic acid and oleyl amine to stabilise the colloids. Using a magnetophoretic technique, it was possible to create two-and three-dimensional structures. The nanostructures were characterised by transmission electron microscopy and magnetic measurements.