R. Perzynski

Structural analogy between aqueous and oily magnetic fluids

In this paper, we compare the structure and the phase behavior of two kinds of magnetic fluids, also called ferrofluids. They are constituted of the same maghemite particles, the diameters of which lie around 8 nm, dispersed either in water or in cyclohexane. Both systems are constructed to get the same interparticle interactions and differ only through the nature of the repulsion. Repulsion is either electrostatic, due to the charges of citrate molecules adsorbed on the particles surface in water, or steric, due to the alkyl chains of adsorbed surfactants in cyclohexane. Small angle neutron scattering (SANS) experiments show that both systems are highly repulsive and that the structure factors are very similar. This is confirmed by stability measurements: the samples are stable if temperature is decreased and if a magnetic field is applied. If the repulsion is decreased by the addition of electrolyte in water or bad solvent in cyclohexane, a gas–liquid-like transition is observed in both systems. However...

Spin glass behavior in an interacting gamma-Fe2O3 nanoparticle system

In this paper we investigate the superspin glass behavior of a concentrated assembly of interacting maghemite nanoparticles and compare it to that of canonical atomic spin glass systems. ac versus temperature and frequency measurements show evidence of a superspin glass transition taking place at low temperature. In order to fully characterize the superspin glass phase, the aging behavior of both the thermo-remanent magnetization (TRM) and ac susceptibility has been investigated. It is shown that the scaling laws obeyed by superspin glasses and atomic spin glasses are essentially the same, after subtraction of a superparamagnetic contribution from the superspin glass response functions. Finally, we discuss a possible origin of this superparamagnetic contribution in terms of dilute spin glass models.

Ferromagnetic resonance in ferrite nanoparticles with uniaxial surface anisotropy

Magnetization oscillations in a single-domain spherical ferromagnetic particle with uniaxial surface and bulk anisotropies are studied. In a linear approximation we consider a weakly nonuniform precession mode and derive the dispersion relationships for the cases of the applied field either parallel or normal to the particle easy axis. It is shown that in both situations the surface can produce a considerable shift of the precession frequency as observed with the conventional ferromagnetic resonance (FMR) technique. The data obtained for fine particle assemblies (frozen magnetic fluids based on γ-Fe2O3 nanosize grains) support the conclusion. Moreover, in the systems under study the surface anisotropy contribution to the FMR resonance field appears to entirely dominate the bulk one.

NiFe2O4 nanoparticles in ferrofluids: evidence of spin disorder in the surface layer

We show that surface magnetic properties of NiFe2O4 nanoparticles constituting ionic ferrofluids can be investigated in macroscopic experiments. Cross-analysis of static magnetization and field-induced birefringence prove that the particles consist of a uniformly magnetized core and a spin-disordered surface layer of comparable thickness. r 2002 Elsevier Science B.V. All rights reserved.

Optical properties of nickel ferrite ferrofluids

We investigate magneto-optical properties of chemically synthesized ionic ferrofluids based on nickel ferrite nanoparticles. These new ferrofluids with potential biological applications become birefringent under low magnetic fields. Both a static and a dynamic probing are here presented.

Static and quasi-elastic small angle neutron scattering on biocompatible ionic ferrofluids: magnetic and hydrodynamic interactions

We investigate the structure and dynamics of ionic magnetic fluids (MFs), based on ferrite nanoparticles, dispersed at pH ≈ 7 either in H2O or in D2 O. Polarized and non-polarized static small angle neutron scattering (SANS) experiments in zero magnetic field allow us to study both the magnetic and the nuclear contributions to the neutron scattering. The magnetic interparticle attraction is probed separately from the global thermodynamic repulsion and compares well to direct magnetic susceptibility measurements. The magnetic interparticle correlation is in these fluid samples independent of the probed spatial scale. In contrast, a spatial dependence of the interparticle correlation is evidenced at large Φ by the nuclear structure factor. A model of magnetic interaction quantitatively explains the under-field anisotropy of the SANS nuclear contribution. In a quasi-elastic neutron spin-echo experiment, we probe the Brownian dynamics of translation of the nanoparticles in the range 1.3 ≤ qRgN ≤ 10 (q, scattering vector; RgN, nuclear radius of gyration of the nanoparticles). For the first time in an MF, we determine the hydrodynamic function at large q vectors.

Experimental investigation of superspin glass dynamics

Magnetic nanoparticle assemblies in high concentrations can exhibit spin glass like properties in the low temperature phase due to the influence of strong dipolar interactions. To address the question of the extent to which the properties of these “superspin” glasses mimic those of an atomic spin glass, several homogeneously dispersed γ‐Fe2O3 nanoparticle assemblies of varying volume fraction have been studied. In a concentrated sample, the main features of atomic spin glasses have been observed including aging phenomena for which we have proposed a rescaling.

Ionic ferrofluid: Optical properties

Abstract The alignment of magnetic colloidal particles along a magnetic field induces a briefringence of the solution. Static measurements of this birefringence with the field allow a determination of the size distribution of the grains. As the magnetic field is switched off, the birefringence relaxes to zero with a characteristic time proportional to the viscosity of the solution: we built up a viscometer working over 7 decades and with a 5% accuracy.

Acoustic Study of Suspension Sedimentation

By means of an acoustic technique, we follow the time and space dependence of the concentration of a settling suspension composed of spherical glass beads in water. The concentration profile consists of two shock fronts, one at the top, one at the bottom of the sedimentation column. The velocity determination of these two fronts allows a precise analysis of how the settling velocity for suspension concentrations close to the packing vanishes.

Liquid–liquid phase-transfer of magnetic nanoparticles in organic solvents

We report a novel route for the preparation of well-defined colloidal dispersions of magnetic nanoparticles stabilized by steric repulsion in organic solvents. The usual methods standardly lead to the surfaction of multiparticle aggregates, incompatible with our long-term aim of studying and modeling the influence of magnetic dipolar interactions in colloidal dispersions which are free of aggregates, all other interactions being perfectly defined. A new and reproducible method based on a surfactant-mediated liquid-liquid phase transfer of individually dispersed gamma-Fe(2)O(3) nanoparticles from an aqueous colloidal dispersion to an organic phase is developed. The choice of the reagent and the preparation techniques is discussed. Among several solvent/surfactant pairs, the cyclohexane/dimethyldidodecylammonium bromide (DDAB) system is found to fulfill the colloidal stability criterion: aggregation does not appear, even upon aging. A complete transfer of isolated particles is observed above a threshold in DDAB concentration. The nanoparticle surface is then fully covered with adsorbed DDAB molecules, each surfactant head occupying a surface of 0.57+/-0.05 nm(2). The volume fraction of the cyclohexane-based organosols is easily tunable up to a volume fraction of 12% by modifying the volume ratio of the organic and of the aqueous phases during the liquid-liquid phase transfer.