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Two-stage kinetics of field-induced aggregation of medium-sized magnetic nanoparticles
Journal of Chemical Physics, 2017
The present paper is focused on theoretical and experimental study of the kinetics of fieldinduced aggregation of magnetic nanoparticles of a size range of 20-100 nm. Our results demonstrate that (a) in polydisperse suspensions, the largest particles could play a role of the centers of nucleation for smaller particles during the earliest heterogeneous nucleation stage; (b) an intermediate stage of the aggregate growth (due to diffusion and migration of individual nanoparticles towards the aggregates) is weakly influenced by the magnetic field strength; (c) the stage of direct coalescence of drop-like aggregates (occurring under magnetic attraction between them) plays a dominant role at the intermediate and late stages of the phase separation, with the timescale decreasing as a square of the aggregate magnetization.
2019
in-flight assembly of magnetic particles into macroscopic chains Lluis Balcells,† Igor Stanković,∗,‡ Zorica Konstantinović,¶ Aanchal Alagh,† Victor Fuentes,† Laura López-Mir,† Judit Oró,† Narcis Mestres,† Carlos Garćıa,§ Alberto Pomar,† and Benjamin Mart́ınez† †Institut de Ciència de Materials de Barcelona, ICMAB-CSIC, Campus de la UAB, 08193 Bellaterra, Catalonia, Spain ‡ Scientific Computing Laboratory, Center for the Study of Complex Systems, Institute of Physics Belgrade, University of Belgrade, 11080 Belgrade, Serbia. ¶Center for Solid State Physics and New Materials, Institute of Physics Belgrade, University of Belgrade, Pregrevica 118, 11080 Belgrade, Serbia §Departamento de F́ısica & Centro Cient́ıfico Tecnológico de Valparáıso-CCTVal, Universidad Técnica Federico Santa Maŕıa, Av. España 1680, Casilla 110-V, Valparáıso, Chile.
Template-assisted self-assembly of individual and clusters of magnetic nanoparticles
Nanotechnology, 2011
The deliberate control over the spatial arrangement of nanostructures is the desired goal for many applications as e.g. in data storage, plasmonics or sensor arrays. Here we present a novel method to assist the self-assembly process of magnetic nanoparticles. The method makes use of nanostructured aluminum templates obtained after anodization of aluminum disks and the subsequent growth and removal of the newly formed alumina layer, resulting in a regular honeycomb type array of hexagonally shaped valleys. The iron oxide nanoparticles, 20 nm in diameter, are spin coated onto the nanostructured templates. Depending on the size, each hexagon site can host up to 30 nanoparticles. These nanoparticles form clusters of different arrangements within the valleys, such as collars, chains, and hexagonally closed islands. Ultimately, it is possible to isolate individual nanoparticles. The strengths of magnetic interaction between particles in a cluster is probed using the memory effect known from the coupled state in superspin glass systems.
Self-assembly of magnetic spheres: a new experimental method and related theory
Journal of Physics Communications
Simple experimental method was developed to examine magnetic self-assembly of macroscopic magnetic spheres of 3mm and 5mm diameters in the lack of external magnetic field. Magnetic force driven aggregation was followed up by video recording and was analysed in detail to identify the processes that lead to the creation of clusters (chains, pairs, circles, etc). Self-aggregation of randomly distributed single spheres, pairs and triplets were examined with this method as well. Applying several liquid media with different viscosity helped to characterize the aggregation processes regarding the kinetic energy of collisions. The results were compared with results of previous experimental works and computer simulations of aggregation of magnetic nanoparticles and magnetic-dipolar systems. Besides to earlier described rings and chains that represent the lowest potential energy of the system of ideal magnetic dipoles in two dimensions, we observed several other structures during the experiments. The most frequently appearing clusters were investigated by direct minimization of the potential energy function of these structures.
Enhanced capability in a gas aggregation source for magnetic nanoparticles
Journal of Applied Physics, 2009
We describe the characterization of a high-temperature ͑2000 K͒ thermal gas aggregation source that is ultrahigh vacuum compatible and can cleanly deposit transition metal clusters with partial pressures of contaminants in the 10 −11 mbar range allowing codeposition with highly reactive matrices. In particular, we investigate the effect of varying ͑i͒ the bath gas pressure and composition on the size distribution and flux of clusters produced and ͑ii͒ the position of the crucible within the source. The mass spectra of Fe clusters produced, recorded using a quadrupole filter, show that changing the operating conditions and configuration of the source allow a wide range of cluster sizes-3000-320 000 amu ͑ϳ50-6000 atoms for Fe or Co͒ to be produced. We demonstrate the cleanliness of the source by producing uncontaminated Fe clusters in rare-earth matrices.
Self-Assembly of Magnetic Nanoparticles in Evaporating Solution
Journal of the American Chemical Society, 2011
When deposited from an evaporating solution onto a substrate, even nondescript nanoparticles can organize into intricate spatial patterns. Here we show that a simple but long-ranged anisotropy in nanoparticles' interactions can greatly enrich this scenario. In experiments with colloidal Co nanocrystals, which bear a substantial magnetic dipole, we observe assemblies quite distinct from those formed by nonmagnetic particles. Reflecting the strongly nonequilibrium nature of this process, nanocrystal aggregates also differ substantially from expected low-energy arrangements. Using coarse-grained computer simulations of dipolar nanoparticles, we have identified several dynamical mechanisms from which such unusual morphologies can arise. For particles with modest dipole moments, transient connections between growing domains frustrate phase separation into sparse and dense regions on the substrate. Characteristic length scales of the resulting cellular networks depend non-monotonically on the depth of quenches we use to mimic the effects of solvent evaporation. For particles with strong dipole moments, chain-like aggregates formed at early times serve as the agents of assembly at larger scales. Their effective interactions drive the formation of layered loop structures similar to those observed in experiments.
Self-organization of magnetic nanoparticles: A Monte Carlo study
Physical Review E, 2008
To understand the self-organization of magnetic nanocrystals in an applied field, we perform Monte Carlo simulations of Stockmayer fluids confined between two parallel walls. The system is examined in the gasliquid coexistence region of its phase diagram and the field is applied perpendicular to the walls. Gibbs ensemble simulations are carried out to determine the phase coexistence curves of the confined Stockmayer fluid. In canonical simulations, different types of organizations appear dependent on particle density: columns, walls, and elongated and spherical holes. The morphology and size of structures are in good agreement with results obtained by free energy minimization and experiments. The influence of a distribution of particle sizes on the particle organization is investigated.
Self-assembly of magnetic nanostructures
1997
We use Monte Carlo and quaternion molecular dynamics simulations to study the self-assembly of intriguing structures which form in colloidal suspensions of small magnetite particles. We show that the only stable isomers with few particles, a ring and a chain, can be e ciently interconverted using a magnetizable tip. We propose to use the oscillating dipole eld of the tip to locally anneal the aggregates to either a ring in zero eld or a chain in nonzero applied eld.
Aggregation of Magnetic Microspheres: Experiments and Simulations
Physical Review Letters, 1988
Uniformly sized microspheres interacting via long-range magnetic dipolar forces are used to study diffusion-limited cluster aggregation in a plane. The results show that it is possible to scale the temporal evolution of the cluster size distribution and that there is a crossover in fractal dimension from D =1.52~0.05 to D =1.16+'0.05 in the limit of weak and strong dipolar coupling. External magnetic fields are shown to produce pronounced chaining with D approaching 1. The results compare favorably with computer simulations of aggregation of the same type of particles.
Universal behavior of dense clusters of magnetic nanoparticles
AIP Advances, 2016
A detailed numerical simulation of quasistatic hysteresis loops of dense clusters of interacting magnetic nanoparticles is carried out. Both clusters of magnetically soft and magnetically hard nanoparticles are considered. The clusters are characterized by an average particle diameter D, the cluster radius R c , the particle saturation magnetization M s , and the uniaxial anisotropy constant K. The number of particles in the cluster varies between N p = 30-120. The particle centers are randomly distributed within the cluster, their easy anisotropy axes being randomly oriented. It is shown that a rare assembly of identical random clusters of magnetic nanoparticles can be characterized by two dimensionless parameters: 1) the relative strength of magneto-dipole interaction, K/M s 2 , and the average particle concentration within the cluster, η = VN p /V c. Here V is the nanoparticle volume, and V c is the volume of the cluster, respectively. In the strong interaction limit, M s η/H a >> 1, where H a = 2K/M s is the anisotropy field, the ultimate hysteresis loops of dilute assemblies of clusters have been constructed. In the variables (M/M s , H/M s) these hysteresis loops depend only on the particle volume fraction η. In the weak interaction limit, M s η/H a << 1, the assembly hysteresis loops in the variables (M/M s , H/H a) are close to the standard Stoner-Wohlfarth hysteresis loop.