Direct observation of dipolar chains in iron ferrofluids by cryogenic electron microscopy

Abstract

A key issue in research on ferrofluids (dispersions of magnetic colloids) is the effect of dipolar interactions on their structure and phase behaviour, which is not only important for practical applications but gives fundamental insight in dipolar fluids in general. In 1970, de Gennes and Pincus predicted a Van der Waals-like phase diagram and the presence of linear chains of particles in ferrofluids in zero magnetic field. Despite many experimental studies, no direct evidence of the existence of linear chains of dipoles has been reported in the absence of magnetic field, although simulations clearly show the presence of chain-like structures. Here, we show in situ linear dipolar structures in ferrofluids in zero field, visualized on the particle level by electron cryo-microscopy on thin, vitrified films of organic dispersions of monodisperse metallic iron particles. On systematically increasing the particle size, we find an abrupt transition from separate particles to randomly oriented linear aggregates and branched chains or networks. When vitrified in a permanent magnetic field, these chains align and form thick elongated structures, indicating lateral attraction between parallel dipole chains. These findings show that the experimental model used is well suited to study the structural properties of dipolar particle systems.

Influence of dipolar interactions on hyperthermia properties of ferromagnetic particles

“…12,13 At low concentrations, particles are prone to appear in the form of interconnected chainlike structures, thus demonstrating strong dipolar interactions. 14 In order to assess the effects of dipolar interactions on the NP intrinsic magnetic properties, particles were diluted at different concentrations into distilled water or ethanol. Magnetic hyperthermia experiments were carried out under various ac magnetic fields by means of a water cooled induction coil of 23 mm diameter consisting of three turns, and a commercial generator with a power of 4.5 kW.…”

Oriented attachment and exchange coupling ofα−Fe2O3nanoparticles

“…15 In order to control positioning of magnetic nanoparticles, their magnetic dipole moments have been utilized to guide them in applied fields or in stray fields from neighboring particles. [16][17][18][19][20] Another potential method of controlling the position and properties of magnetic nanoparticles is to utilize exchange interaction between particles. Although this kind of interaction is rather unexplored, it holds interesting prospects.…”

Dipolar Structures in Colloidal Dispersions of PbSe and CdSe Quantum Dots

“…The imaged structures strongly resemble the magnetic dipolar structures that have been observed recently in colloidal iron and magnetite dispersions. 13,14 As a first quantification of the imaged dipolar structures, we calculated the radial distribution function g(r), defined as:…”