Vincent Dupuis

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.

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.

Growth of a dynamical correlation length in an aging superspin glass

We report on zero-field–cooled magnetization relaxation experiments on a concentrated frozen ferrofluid exhibiting a low-temperature superspin glass transition. With a method initially developed for spin glasses, we investigate the field dependence of the relaxations that take place after different aging times. We extract the typical number of correlated spins involved in the aging dynamics. This brings important insights into the dynamical correlation length and its time growth. Our results, consistent with expressions obtained for spin glasses, extend the generality of these behaviours to the class of superspin glasses. Since the typical flipping time is much larger for superspins than for atomic spins, our experiments probe a time regime much closer to that of numerical simulations.

Experimental Evidence of Fluctuation-Dissipation Theorem Violation in a Superspin Glass

We present the experimental observation of the fluctuation-dissipation theorem violation in an assembly of interacting magnetic nanoparticles in the low temperature superspin-glass phase. The magnetic noise is measured with a two-dimension electron gas Hall probe and compared to the out of phase ac susceptibility of the same ferrofluid. For intermediate aging times of the order of 1 h, the ratio of the effective temperature T(eff) to the bath temperature T grows from 1 to 6.5 when T is lowered from T(g) to 0.3 T(g), regardless of the noise frequency. These values are comparable to those measured in an atomic spin glass as well as those calculated for a Heisenberg spin glass.

In-field Mossbauer study of disordered surface spins in core/shell ferrite nanoparticles

Magnetization and Mossbauer spectroscopy measurements are performed at low temperature under high field, on nanoparticles with a nickel ferrite core and a maghemite shell. These nanoparticles present finite size and surface effects, together with exchange anisotropy. High field magnetization brings the evidences of a monodomain ordered core and surface spins freezing in disorder at low temperature. Mossbauer spectra at 4.2 K present an extra contribution from the disordered surface which is field dependent. Field and size dependences of this latter show a progressive spin alignment along the ferrite core which is size dependent. The weak surface pinning condition of the nanoparticles confirms that the spin disorder is localized in the external shell. The underfield decrease in the mean canting angle in the superficial shell is then directly related to the unidirectional exchange anisotropy through the interface between the ordered core and the disordered shell. The obtained anisotropy field HEa scales as ...

Heterogeneous dynamics and ageing in a dense ferro-glass.

Repulsive magnetic fluids show a dynamical freezing above a volume fraction Φ(*), which depends on the physico-chemistry of the system. Φ(*) is here determined by a magneto-optical technique. The out-of-equilibrium dynamics of a glass-forming magnetic fluid (Φ = 1.2Φ(*)) is studied by x-ray photon correlation spectroscopy and analyzed in terms of intensity auto-correlation functions. The relaxation is age dependent and follows a compressed exponential law with a characteristic time scaling as the inverse of the scattering vector Q. The dynamical susceptibility χ is then deduced from a time resolved correlation analysis at an intermediate Q and for ages larger than 10(4)xa0s.

Anisotropy-axis orientation effect on the magnetization of -Fe2O3 frozen ferrofluid

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 (?-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.

Magnetic noise of a frozen ferrofluid

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.

Heteropolytungstate-decorated core-shell magnetic nanoparticles: A covalent strategy for polyoxometalate-based hybrid nanomaterials

Amino-functionalized core-shell magnetic nanoparticles have been covalently grafted with Polyoxometalates (POMs). These multifunctional nanocomposites have been obtained through the coupling of heteropolytungstate-based hybrids bearing carboxylic acid functions with aminopropyl functions that decorate the core-shell nanoparticles. The physical properties of the resulting materials have been studied by a large set of techniques. The very good nanostructuration of the POMs at the surface of the obtained nanoparticles have thus been directly observed by high-resolution transmission electronic microscopy (HR-TEM). Furthermore, the hyperthermia properties of these nanocomposites have been also considered as a function of the size of the magnetic core. Finally, the stability of these suspensions in organic media makes them particularly interesting in the frame of their processing or their potential use as nanocatalysts.

Noise Measurement of Interacting Ferromagnetic Particles with High Resolution Hall Microprobes

We present our first experimental determination of the magnetic noise of a superspin‐glass made of <1u2009pico‐liter<1u2009pico‐liter frozen ferrofluid. The measurements were performed with a local magnetic field sensor based on Hall microprobes operated with the spinning current technique. The results obtained, though preliminary, qualitatively agree with the theoretical predictions of Fluctuation‐Dissipation theorem (FDT) violation [1].