In SituImaging of Field-Induced Hexagonal Columns in Magnetite Ferrofluids

Abstract

Field-induced structures in a ferrofluid with well-defined magnetite nanoparticles with a permanent magnetic dipole moment are analyzed on a single-particle level by in situ cryogenic transmission electron microscopy (2D). The field-induced columnar phase locally exhibits hexagonal symmetry and confirms the structures observed in simulations for ferromagnetic dipolar fluids in 2D. The columns are distorted by lens-shaped voids, due to the weak interchain attraction relative to field-directed dipole-dipole attraction. Both dipolar coupling and the dipole concentration determine the dimensions and the spatial arrangement of the columns. Their regular spacing manifests long-range end-pole repulsions that eventually dominate the fluctuation-induced attractions between dipole chains that initiate the columnar transition. DOI: 10.1103/PhysRevLett.97.185702 PACS numbers: 64.70.Nd, 75.50.Mm, 82.70.Dd Nanoparticles dispersed in a solvent and with a sufficiently large permanent magnetic dipole moment selfassemble into a variety of magnetic equilibrium structures such as (flux-closure) rings and wormlike, branched dipole chains [1,2]. The morphology of these clusters, formed in absence of an external field, has been examined in detail, together with a determination of pair correlation functions and chain-length distributions [3]. In contrast, much less is known about the structural transitions induced by an external (homogeneous) magnetic field for fluids of permanent dipoles. Interestingly, magnetic colloids in an external field are nevertheless frequently encountered in practical applications [4] and biomedicine [5,6].The structure formation and phase behavior of colloidal systems in reduced dimensions is not necessarily equivalent to that of three-dimensional (3D) systems [7][8][9]. In particular, for permanent dipolar spheres confined to two dimensions (2D) a field-induced transition to a columnar phase with local hexagonal symmetry was predicted [10], although a conclusive experimental real-space analysis is still lacking. Elongated iron-particle clusters have been imaged [1] but the irregular particle shape and the bidisperse size distribution obstruct the wanted single-particle analysis. Parallel structures have also been observed for maghemite ferrofluids dried in the presence of a homogeneous field [11,12]. However, we have shown elsewhere that drying procedures may drastically change structure morphology [2]. Moreover, dipole interactions in conventional ferrofluids are in general too weak for a realistic comparison to the purely dipolar spheres from simulations.In this Letter, we report unequivocal real-space evidence for the predicted columnar phase transition [10] from in situ cryo-TEM images of monodisperse magnetic colloids with dominating dipolar interactions. The particle positions are confined by a 2D film whereas the dipole orientations can thermally fluctuate in 3D. Our imaging results, in addition, allow to quantify positional and angular interparticle correlations showing, among other things, a pr...

Surface analysis of magnetite nanoparticles in cyclohexane solutions of oleic acid and oleylamine

“…Surfactant adsorption apparently strongly depends on how the nanoparticles are prepared, since a plateau is reached at significantly lower concentration by Dubois et al [6], and complete desorption is observed at low surfactant concentrations by Korolev et al [7], in contrast to our findings and those of Dubois et al [6]. The presence of a fractional monolayer is supported by other techniques: both from statistical analysis of the particle-particle contact distance observed by TEM [11] and from small angle neutron scattering (SANS) [30], a surfactant layer of about 2 nm thickness was deduced. This agrees with the layer thickness expected from the size of the surfactant molecules on the basis of monolayer coverage of the as-prepared nanoparticles by surfactant molecules attached perpendicular to the surface.…”

Static and dynamic correlations in magnetic nanomaterials studied by small angle neutron scattering techniques

“…Spontaneous chaining in zero field and coexistence of chains segments with hexagonal ordered domains has been observed when the magnetic field was increased [26]. Similar distorted hexagonal structures have been observed in Magnetite based ferrofluids by applying magnetic fields [43,45].…”

“…Two types of ferrofluids with nearly monodisperse nanoparticles have been investigated, namely concentrated Co-ferrofluid ("MFT3") [26,39,44] with a cobalt core of radius R c = 4.4 nm dispersed in a viscous synthetic hydrocarbon oil "Edwards L9" (monoalkylnaphthalene) and magnetite (Fe 3 O 4 ) particles of R c = 8.5 nm dispersed in decalin [43,45] both stabilised by a surfactant layer of a thickness D of about 2 nm. The maximal dipolar interaction energy E dd (max) for particles with saturation magnetisation M sat in head-to-tail conformation and closest contact = 2(R c + D) is given by…”

“…The latter should give rise to a spontaneous arrangement of particles in chains or rings with magnetic moments parallel to each other [27]. Such spontaneous chaining in zero field has been predicted by of Monte-Carlo simulations [28] and recently observed by high-resolution cryogenic electron microscopy [23,43]. In an external magnetic field these chains are expected to be aligned along the field direction that gives rise to anisotropic structure factors [29].…”

Spatial Distribution of Nanocrystals Imaged at the Liquid-Air Interface

“…Also, in the field of materials science, CEM is becoming an established way to visualize fragile structures at the nanoscale [18]. We have introduced CEM before to image nanoparticle structures in colloidal dispersions, be it only in two dimensions [19][20][21][22]. Here, the sample structure is obtained in three dimensions using tomography [23].…”