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The Seebeck and Soret coefficients of ionically stabilized suspension of maghemite nanoparticles in dimethyl sulfoxide are experimentally studied as a function of nanoparticle volume fraction. In the presence of a temperature gradient, the charged colloidal nanoparticles experience both thermal drift due to their interactions with the solvent and electric forces proportional to the internal thermoelectric field. The resulting thermodiffusion of nanoparticles is observed through forced Rayleigh scattering measurements, while the thermoelectric field is accessed through voltage measurements in a thermocell. Both techniques provide independent estimates of nanoparticle's entropy of transfer as high as 82 meV K-1. Such a property may be used to improve the thermoelectric coefficients in liquid thermocells. [ABSTRACT FROM AUTHOR]

Compares the structure and phase behavior of two kinds of magnetic fluids or ferrofluids. Same maghemite particles; Construction of both systems to get the same interparticle interactions; Nature of repulsions; Stability measurements; Finite size effects of the trivalent ions; Presence of the adsorbed surface species which ensure the repulsion between particles.

The magnetic noise of a frozen ferrofluid made of maghemite nanoparticles dispersed in glycerin is measured using a two-dimensional electron gas based quantum well Hall sensor (QWHS) with the spinning current technique. The frozen ferrofluid shows a superspin glass transition at 67 K. Below this glass transition temperature, the relation between the imaginary part of the ac susceptibility of a bulk ferrofluid sample and the magnetic noise measured with the QWHS gives an indication of a violation of the fluctuation dissipation theorem. [ABSTRACT FROM AUTHOR]

Magnetic properties in a magnetically textured ferrofluid made out of interacting maghemite (γ-Fe2O3) nanoparticles suspended in glycerin have been investigated. Despite the loss of uniform distribution of anisotropy axes, a superspin glass state exists at low temperature in a concentrated textured ferrofluid as in the case of its nontextured counterpart. The onset of superspin glass state was verified from the sample’s ac susceptibility. The influence of the anisotropy axis orientation on the aging behavior in the glassy states is also discussed. [ABSTRACT FROM AUTHOR]

Low-temperature magnetic properties of assemblies of 3.3 nm sized nanoparticles (NPs) based on a MnFe2O4 core protected by a maghemite shell are investigated. These NPs are obtained by a chemical core/shell method developed for the synthesis of the electrostatically stabilized ferrofluid colloidal dispersions that we probe here. They are model systems where the interparticle interaction is tuned by the NP volume fractions, ranging here between 0.4% and 13.9%. It has been shown that these NPs consist of a well-ordered ferrimagnetic core surrounded by a disordered spin glass-like surface layer and that they display uniaxial magnetic anisotropy. We compare the magnetic hysteresis loops of non-textured frozen dispersions (with magnetic anisotropy axis of NPs distributed at random) with those of a powder based on the same NPs. After cooling under field the hysteresis loops shift along the H axis, expressing the coupling between the spin-ordered cores and the disordered surface layers. The negative H-shift provides an evaluation for the exchange bias (EB) field. The EB field is optimum for a cooling field of the order of the anisotropy field. A comparison between frozen dispersions and disordered powder allows us to distinguish the influence of intra- and interparticle interactions on the EB. Interparticle collective effects dominate in the powder while an intraparticle EB, eventually hindered by dipolar interactions at large volume fraction, is observed in frozen dispersions. [ABSTRACT FROM AUTHOR]

Due to a finely tuned interparticle interaction, the microstructure of home-made magnetic fluids based on maghemite (γ - Fe2O3) nanoparticles is monitored from attractive on average towards repulsive on average, at low and large volume fractions. The microstructure is probed in zero field and under an applied magnetic field by Small Angle Neutron Scattering and Small Angle X-ray Scattering. [ABSTRACT FROM AUTHOR]

The effect of magnetic anisotropy-axis alignment on the superparamagnetic (SPM) and superspin glass (SSG) states in a frozen ferrofluid has been investigated. The ferrofluid studied here consists of maghemite nanoparticles (g-Fe2O3, mean diameter = 8.6 nm) dispersed in glycerine at a volume fraction of [?]15%. In the high temperature SPM state, the magnetization of aligned ferrofluid increased by a factor varying between 2 and 4 with respect to that in the randomly oriented state. The negative interaction energy obtained from the Curie-Weiss fit to the high temperature susceptibility in the SPM states as well as the SSG phase onset temperature determined from the linear magnetization curves were found to be rather insensitive to the anisotropy-axis alignment. The low temperature ageing behaviour, explored via 'zero-field cooled magnetization' relaxation measurements, however, shows a distinct difference in the ageing dynamics in the anisotropy-axis aligned and randomly oriented SSG states. [ABSTRACT FROM AUTHOR]

Magnetic properties in a magnetically textured ferrofluid made out of interacting maghemite (γ-Fe2O3) nanoparticles suspended in glycerin have been investigated. Despite the loss of uniform distribution of anisotropy axes, a superspin glass state exists at low temperature in a concentrated textured ferrofluid as in the case of its nontextured counterpart. The onset of superspin glass state was verified from the sample’s ac susceptibility. The influence of the anisotropy axis orientation on the aging behavior in the glassy states is also discussed. [ABSTRACT FROM AUTHOR]

Abstract The mixture of choline chloride and ethyleneglycol ((1:3) on the molar scale) is first characterized using density, viscosimetry and refractive index measurements, in the temperature range 20–45°C, with controlled amounts of water. Thermodynamic parameters such as the activation entropy, enthalpy and Gibbs energy for the viscous flow are deduced from the data and show that the solvent is highly structured. A protocol to disperse in this solvent 10 nm maghemite nanoparticles previously synthesized in water is described. The good quality of the dispersion is assessed by visual observations, optical microscopy, small angle X-ray scattering (SAXS), magnetization measurements, viscosimetry, and light scattering. The results lead to the description of a local viscosity around the particles, lower than the macroscopic one, that could be the result of the solvent segregation with ethyleneglycol being mostly present in the close vicinity of the particles. Highlights • The complex solvent made of a mixture of ethylene glycol-choline chlorine is characterized (viscosity and density). • The influence of water is examined. • The Eyring model for the molecular flow shows high associativity (H-bonds). • Stable dispersions of magnetic nanoparticles (MNPs) were dispersed in this solvent. • The local viscosity around the NPs is lower than the bulk one. [ABSTRACT FROM AUTHOR]

We compare here exchange bias (EB) properties of chemically synthesized core-shell nanoparticles (NPs), based either on a core of soft ferrite (MnFe2O4) or hard ferrite (CoFe2O4) protected by a maghemite shell (α-Fe2O3). These NPs dispersed in acidic solutions are electrostatically stabilized, yielding to stable colloidal dispersions with a strong interparticle repulsion and negligible dipolar interactions in the probed range of temperatures. Field cooled (FC) magnetic hysteresis loops of non-textured frozen dispersions (with magnetic anisotropy axis of NPs distributed at random) and those of a powder based on the same NPs present a shift along the H-axis, expressing the coupling between the spin-ordered cores and the disordered surface layer of the NPs. The bias field is found to present a maximum, larger for NPs based on harder ferrite core. It is obtained for a cooling field of the order of one half of the anisotropy field, which is much larger for the CoFe2O4 cores than for MnFe2O4 ones. In powders, particles are in contact leading to an interparticle exchange which is not present in the dilute solutions where exchange bias properties are only due to an intraparticle exchange between core and surface. The thermal dependence of the bias field is well described by a reduced exponential behavior with a characteristic freezing temperature of about 8 K. [ABSTRACT FROM AUTHOR]

Abstract: The dynamical rotational behavior of magnetic nanostructured rods based on the auto-association of maghemite nanoparticles and block-copolymers is probed by optical microscopy under rotating fields in a simple liquid. The re-orientation of the rods by a field rotated by 90° is first studied. The measured relaxation is characteristic of paramagnetic objects. Under a stationary rotating field, a synchronous rotational regime is observed at low field frequency. Above a frequency threshold which scales as H 2, the dynamics becomes asynchronous with back-and-forth rotations. These behaviors are well predicted by the presented model. [Copyright &y& Elsevier]