Anisotropic Superparamagnetism of Monodispersive Cobalt-Platinum Nanocrystals

Based on the high-temperature organometallic route (Sun et al. Science 287, 1989 (2000)), we have synthesized powders containing CoPt_3 single crystals with mean diameters of 3.3(2) nm and 6.0(2) nm and small log-normal widths sigma=0.15(1). In the entire temperature range from 5 K to 400 K, the zero-field cooled susceptibility chi(T) displays significant deviations from ideal superparamagnetism. Approaching the Curie temperature of 450(10) K, the deviations arise from the (mean-field) type reduction of the ferromagnetic moments, while below the blocking temperature T_b, chi(T) is suppressed by the presence of energy barriers, the distributions of which scale with the particle volumes obtained from transmission electron microscopy (TEM). This indication for volume anisotropy is supported by scaling analyses of the shape of the magnetic absorption chi''(T,omega) which reveal distribution functions for the barriers being also consistent with the volume distributions observed by TEM. Above 200 K, the magnetization isotherms M(H,T) display Langevin behavior providing 2.5(1) mu_B per CoPt_3 in agreement with reports on bulk and thin film CoPt_3. The non-Langevin shape of the magnetization curves at lower temperatures is for the first time interpreted as anisotropic superparamagnetism by taking into account an anisotropy energy of the nanoparticles E_A(T). Using the magnitude and temperature variation of E_A(T), the mean energy barriers and 'unphysical' small switching times of the particles obtained from the analyses of chi''(T,omega) are explained. Below T_b hysteresis loops appear and are quantitatively described by a blocking model, which also ignores particle interactions, but takes the size distributions from TEM and the conventional field dependence of E_A into account.Comment: 12 pages with 10 figures and 1 table. Version accepted for publication in Phys. Rev. B . Two-column layou

Hydrocraquage sous pression d'une huile de soja : procédé d'étude et allure générale de la transformation

Un procédé d'étude de l'hydrocraquage des huiles végétales en autoclave est décrit. Les principaux objectifs de cette étude sont relatifs à la transformation de l'huile de soja (composé modèle) en essence et gazole, en limitant la formation de polymères et de coke. L'analyse des gaz et la chromatographie haute résolution de deux fractions résultant de la distillation sous vide de la phase liquide ont permis de mettre en évidence les principales étapes de l'hydrocraquage. Pour cette première approche du processus général, trois types d'hydrocraquages ont été utilisés : thermique et en présence d'un oxyde ou d'un catalyseur bifonctionnel performant (hydrogénant/craquant). Dans ce dernier cas, l'hydrogénation des doubles liaisons des chaînes latérales des triglycérides a été mise en évidence pendant le chauffage du réacteur jusqu'à environ 673 K, température à laquelle s'initie le véritable hydrocraquage. A cette température sont observées la décarbonylation/décarboxylation des acides gras (précédemment issus de la coupure des liaisons ester des triglycérides) et une hydrogénolyse marquée en présence d'un catalyseur métallique. Il en résulte principalement des alcanes dont la structure linéaire est celle des acides gras initiaux. Le rendement global brut peut atteindre 83 % dans les conditions de température et de pression choisies, y inclus le pourcentage en fraction gazeuse (CO, CO2, C1-C4

Electrochemical reduction of goethite in the solid state : a topotactic process yielding an oriented-texture iron powder

X-ray diffraction, transmission electron microscopy examinations and selected area electron diffraction patterns were used to investigate the crystallographic parameters and morphologies of the starting Goethite and of the iron resulting from its electrochemical reduction, in a 50 % NaOH-H2O mixture, at 363 K. It was shown that the iron crystallites exhibit essentially the same morphology and particle size as α-FeOOH. Furthermore, the metal crystals stay in a definite orientation in relation to the Goethite ones. This electrochemical process leads to a metal powder presenting a well-defined and reproducible texture

SUPERPARAMAGNETIC-PARAMAGNETIC TRANSITION IN SMALL PARTICLES

The paramagnetic state and the superparamagnetic regime of small particles of the granular system Fe64Cr16B20-Al2O3 have been studied by mean of magnetization and susceptibility and compared to the properties of the corresponding bulk metal. The evolution of the magnetic structure near and above Tc is the same for both compounds. Deductions upon the particle size have been attempted