S. Nakamae

Observation of superspin glass state in magnetically textured ferrofluid (γ-Fe2O3)

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.

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.

Thermoelectric energy recovery at ionic-liquid/electrode interface

A thermally chargeable capacitor containing a binary solution of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)-imide in acetonitrile is electrically charged by applying a temperature gradient to two ideally polarisable electrodes. The corresponding thermoelectric coefficient is -1.7 mV/K for platinum foil electrodes and -0.3 mV/K for nanoporous carbon electrodes. Stored electrical energy is extracted by discharging the capacitor through a resistor. The measured capacitance of the electrode/ionic-liquid interface is 5 μF for each platinum electrode while it becomes four orders of magnitude larger, ≈36 mF, for a single nanoporous carbon electrode. Reproducibility of the effect through repeated charging-discharging cycles under a steady-state temperature gradient demonstrates the robustness of the electrical charging process at the liquid/electrode interface. The acceleration of the charging by convective flows is also observed. This offers the possibility to convert waste-heat into electric energy without exchanging electrons between ions and electrodes, in contrast to what occurs in most thermogalvanic cells.

Thermoelectricity and thermodiffusion in charged colloids.

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 nanoparticles entropy of transfer as high as 82 meV K(-1). Such a property may be used to improve the thermoelectric coefficients in liquid thermocells.

Magnetoresistivity of Ag tape co-processed with Bi1.4Pb0.6Sr2Ca2Cu3Ox superconductor

Abstract The magnetoresistivity of Ag plays an important role in determining the stability and quench propagation behaviour of Ag-clad Bi-Pb-Sr-Ca-Cu-O composite conductors. The magnetoresistivity of a silver tape that was co-processed with Bi 4 Pb 0.6 Sr 2 Ca 2 Cu 3 O x has been measured from 4.2 K to 150 K in magnetic fields up to 20 T. The resistivity dependence on the magnetic field orientation was also studied at 4.2 K. It has been found that impurities introduced from the oxide core during heat treatment cause a significant increase in the resistivity over the entire temperature range.

Superspin glass aging behavior in textured and nontextured frozen ferrofluid

The effect of magnetic anisotropy-axis alignment of individual nanoparticles on the collective aging behavior in the superspin glass state of a frozen ferrofluid has been investigated. The ferrofluid studied here consists of maghemite nanoparticles (γ-Fe2O3, mean diameter=8.6 nm) dispersed in glycerin at a volume fraction of ∼15%. The low temperature aging behavior has been explored through “zero-field cooled magnetization” (ZFCM) relaxation measurements using superconducting quantum interference device magnetometry. The ZFCM response functions were found to scale with effective age of the system in both textured and nontextured superspin glass states, but with markedly different scaling exponents, μ. The value of μ was found to shift from ∼0.9 in nontextured case to ∼0.6 in the textured case, despite the identical cooling protocol used in both experiments.

Effect of Premelting on Conductivity of DNA-Lipid Films

We have measured temperature-dependent (between 20 and 80∘C) electrical conductivity and molecular structure (Raman spectroscopy) of DNA-lipid cast film. Our findings show that the conductivity is strongly influenced by premelting effects in the molecular structure starting near physiological temperatures (∼40∘C), prior to the global DNA denaturation.

Anisotropic thermal conductivity of c-axis aligned Bi2Sr2CaCu2Ox superconductor in high magnetic fields

The anomalous behavior of the thermal conductivity, κ, of high temperature superconductors (HTS) has been attributed to two conflicting theories (phonon domination and electron domination) since the discovery of HTS. Change in κ due to a magnetic field is a direct consequence of additional scattering processes between the phonons, the charge carriers, and the flux lines. The existing theories predict different scattering effects for the phonons and the electrons. Therefore, knowing magnetothermal conductivity can provide useful information for the nature of heat carriers in HTS and the understanding of the vortex dynamics. We have measured the in-plane thermal conductivity of a c-axis aligned Bi2Sr 2CaCu2Ox superconductor in magnetic fields up to 17 T. The magnetic field was applied both parallel and perpendicular to the c axis and always perpendicular to the direction of heat flow. The results are compared to existing electronic and phononic models. It has been discovered that the electronic thermal cond...

Thermal conductivity of Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub x/ superconductors in high magnetic fields

The thermal conductivity of high temperature superconductors provides an understanding of the nature of the charge carriers, phonons, vortices, and the scattering processes between them. Recently, the magnetothermal conductivity has been considered to be a key factor in the understanding of the vortex dynamics. Here we report thermal conductivity measurements of bulk Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub x/ superconductors in an applied magnetic field up to 20 T. The measurements were taken for a wide range of temperatures around T/sub c/. The superconducting parameters were recovered from the obtained data using theoretical model developed by Ausloos et al.

Analysis of superconducting magnet (SCM)-ground coil interactions for EDS Maglev coil configurations

The performance of electrodynamic magnetic levitation systems is dominated primarily by the electromagnetic interactions between the onboard superconducting magnets (SCMs) and the normal levitation and propulsion coils situated on the guideway. Alternative designs are examined for coil configurations in terms of the resulting SCM-ground coil interactions. In particular, the modified null flux configuration and the ladder-on-box-beam approach are compared. Magnetic drag, levitation, and guidance forces are considered on an absolute and per (kg/m) of ground coil basis.<<ETX>>