E. Dubois

Preparation and properties of monodisperse magnetic fluids

Abstract A ‘size sorting’ method performed on ionic magnetic fluids, constituted by polydisperse anionic particles dispersed in water at pH 7 allows ‘monodisperse’ samples to be obtained. Here results on phase transitions observed in these samples with decreasing temperature, application of a magnetic field or increasing ionic strength are presented.

Nanooxidation using a scanning probe microscope: An analytical model based on field induced oxidation

The formation of a nanometer-size oxide pattern on silicon using a scanning probe microscope (SPM) has been widely reported in the literature. No analytical model has been proposed, however, to explain the variation of the oxide height with both polarization and speed of the SPM tip. In this letter, we explain quantitatively the variation of the oxide height with the polarization and the speed of the tip with a model based on field induced oxidation. Data analysis also allows us to estimate the thermal activation energy of the oxidation process, (∼0.15 eV). This low value is compared with activation energies measured for thermal and plasma oxidation of silicon.

Characterization of scanning tunneling microscopy and atomic force microscopy-based techniques for nanolithography on hydrogen-passivated silicon

A comparison between scanning tunneling microscope (STM) and atomic force microscope (AFM) nanolithography techniques based on local oxidation of silicon is proposed. This work deals with the three different near-field microscopy techniques, namely, STM, AFM in contact mode (CM-AFM), and tapping mode (TM-AFM), all of them operated in air. The thickness and width of oxide stripes are studied as a function of the applied probe–sample voltage, the speed of the probe and the setpoint (current, applied force, and vibration amplitude for STM, AFM contact, and tapping, respectively). The advantages and drawbacks of each technique are analyzed, establishing TM-AFM as the best candidate for scanning probe microscope nanolithography.

MONODISPERSE MAGNETIC NANOPARTICLES : PREPARATION AND DISPERSION IN WATER AND OILS

Nanometric maghemite and cobalt ferrite particles are chemically synthesized. The process produces particles polydisperse in size. The positive charges of their surface allow one to disperse them in aqueous acidic solutions and to obtain dispersions stabilized through electrostatic repulsions. Increasing acid concentration (in the range 0.1 to 0.5 mol.L −1 ), interparticles repulsions are screened and phase transitions are induced. Using this phenomenon, we describe a two-step size sorting process, in order to get significant amounts of nanometric monosized particles (with diameters monitored between typically 6 and 13 nm). As the surface of the latter is not modified by the size sorting process, usual procedures are used to disperse them in several aqueous or oily media.

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.

Water dynamics in hectorite clays: influence of temperature studied by coupling neutron spin echo and molecular dynamics.

Within the wider context of water behavior in soils, and with a particular emphasis on clays surrounding underground radioactive waste packages, we present here the translational dynamics of water in clays in low hydrated states as studied by coupling molecular dynamics (MD) simulations and quasielastic neutron scattering experiments by neutron spin echo (NSE). A natural montmorillonite clay of interest is modeled by a synthetic clay which allows us to understand the determining parameters from MD simulations by comparison with the experimental values. We focus on temperatures between 300 and 350 K, i.e., the range relevant to the highlighted application. The activation energy Ea experimentally determined is 6.6 kJ/mol higher than that for bulk water. Simulations are in good agreement with experiments for the relevant set of conditions, and they give more insight into the origin of the observed dynamics.

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.

Static and dynamic structural probing of swollen polyacrylamide ferrogels

The local structure of hybrid ferrogels resulting from the incorporation of γ-Fe2O3 nanoparticles into a polyacrylamide polymeric network is probed from a static and a dynamic point of view, under various synthesis conditions and at swelling equilibrium. Relaxation of magneto-optical birefringence and small-angle neutron scattering measurements show adsorption of the nanoparticles onto the polymer, which is ascribed to H-bonding. These measurements allow one to identify the conditions leading either to a homogeneous scaffold reinforced by the nanoparticles, or to a nano-structured composite with 2D nanoparticle decoration on the walls of percolating pockets, initially filled by the ferrofluid during the synthesis and leaking out during the swelling. Two parameters rule the structure and the properties of these ferrogels – the osmotic pressure ratio RΠ (ratio of the osmotic pressure of the initial ferrofluid to that of the hydrogel) and the steric parameter RS (ratio of the nanoparticle volume to the mesh volume of the polymeric hydrogel).

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.