Direct calorimetric measurements of isothermal entropy change on single crystal W-type hexaferrites at the spin reorientation transition

We report on the magnetic field induced isothermal entropy change, \Delta s(Ha, T), of W-type ferrite with CoZn substitution. Entropy measurements are performed by direct calorimetry. Single crystals of the composition BaCo$_0.62$Zn$_1.38$Fe$_16$O$_27$, prepared by the flux method, are measured at different fixed temperatures under an applied field perpendicular and parallel to the c axis. At 296 K one deduces a value of K$_1$ = 8.7 \times 10^{4} J m$^-3$ for the first anisotropy constant, which is in good agreement with the literature. The spin reorientation transition temperature is estimated to take place between 200 and 220 K

Synthesis and characterization of core-shell structure silica-coated Fe29.5Ni70.5 nanoparticles

In view of potential applications of magnetic particles in biomedicine and electromagnetic devices, we made use of the classical Stober method base-catalysed hydrolysis and condensation of tetraethoxysilane (TEOS) to encapsulate FeNi nanoparticles within a silica shell. An original stirring system under high power ultrasounds made possible to disperse the otherwise agglomerated particles. Sonication guaranteed particles to remain dispersed during the Stober synthesis and also improved the efficiency of the method. The coated particles are characterized by electron microscopy (TEM) and spectroscopy (EDX) showing a core-shell structure with a uniform layer of silica. Silica-coating does not affect the core magnetic properties. Indeed, all samples are ferromagnetic at 77 K and room temperature and the Curie point remains unchanged. Only the coercive force shows an unexpected non-monotonous dependence on silica layer thickness.Comment: Regular paper submited to international peer-reveiwed journa

Optimum and Rapid Annealing of Nanocrystalline Fe73.5_{73.5}Cu1_{1}Nb3_{3}Six_{x}B(22.5−x)_{(22.5-x)} Ferromagnets-Correlation Between Magnetic, Mechanic and Thermal Analysis

In this paper we discuss the optimal and rapid treatment of Fe$_{73.5}$Cu$_{1}$Nb$_{3}$Si$_{x}$B$_{22.5-x}$ nanocrystalline alloy. In order to define an optimisation parameter of the coupled temperature/annealing time, several treatments of two alloys ($x=13.5$% and $x=15.5$%) have been studied. A rapid annealing (630 $^{\circ}$C with no step) which gives rise to better magnetic properties is proposed. It is shown that the alloy containing 15.5% of silicon is mechanically stronger when rapidly treated. Finally, it is attempted to correlate magnetic, mechanical and thermal analysis showing also how this can help to gain insight into magneto-mechanical coupling through amorphous and crystalline phase.Dans ce travail on étudie les traitements thermiques optimaux et rapides de l'alliage Fe$_{73.5}$Cu$_{1}$Nb$_{3}$Si$_{x}$B$_{22.5-x}$ nanocristallin. Nous avons essayé différents traitements avec deux alliages ($x=13,5$ % and $x=15,5$ %) et nous avons défini un paramètre d'optimisation du couple température/durée de recuit. Nous proposons un recuit rapide (630 $^{\circ}$C sans palier) qui permet d'améliorer les performances magnétiques et nous montrons que l'alliage contenant 15,5 % de silicium est mécaniquement plus résistant quand il est recuit de cette manière. Enfin, nous essaierons de corréler les analyses magnétiques, mécaniques et thermiques

Synthesis of terminally protected 9-amino-4,5-diazafluorene-9-carboxylic acid, the first rigid, transition-metal receptor, C-alpha,alpha-disubstituted glycine

N-tert-Butyloxycarbonyl-9-amino-4,5-diazafluorene-9-carboxylic acid methyl ester, Boc-Daf-OMe, the first C-alpha,C-alpha-disubstituted glycine containing a rigid bipyridine ligand in a totally controlled spatial situation relative to the C-alpha atom of the amino acid, and a potential building block for the synthesis of peptide supramolecular devices, has been synthesized by acylation of the anion of N-benzyl-4,5-diazafluorene-9-methylenamine, followed by N-protection. Hydrazinolysis of the ester function afforded the hydrazide Boc-Daf-NHNH2, a key precursor for the acylazide coupling method

Synthesis, conformational study, and spectroscopic characterization of the cyclic C-alpha,alpha-disubstituted glycine 9-amino-9-fluorenecarboxylic acid

A series of terminally blocked peptides (to the pentamer level) from L-Ala and the cyclic C-alpha,C-alpha-disubstituted Gly residue Afc and one Gly/Afc dipeptide have been synthesized by solution method and fully characterized. The molecular structure of the amino acid derivative Boc-Afc-OMe and the dipeptide Boc-Afc-Gly-OMe were determined in the crystal state by X-ray diffraction. In addition, the preferred conformation of all of the model peptides was assessed in deuterochloroform solution by FT-IR absorption and H-1-NMR. The experimental data favour the conclusion that the Afc residue tends to adopt either the fully-extended (C-5) or a folded/helical structure. In particular, the former conformation is highly populated in solution and is also that found in the crystal state in the two compounds investigated. A comparison with the structural propensities of the strictly related C-alpha,C-alpha-disubstituted Gly residues Ac(5)c and D Phi g is made and the implications for the use of the Afc residue in conformationally constrained analogues of bioactive peptides are briefly examined. A spectroscopic (UV absorption, fluorescence, CD) characterization of this novel aromatic C-alpha,C-alpha-disubstituted Gly residue is also reported

beta-Homo-peptides built from beta-2,2-HBip, a biphenyl-substituted 3-amino-2,2-dimethylpropanoic acid

A novel beta(2,2)-gem-disubstituted amino acid, beta(2,2)-HBip, has been synthesized by alpha,alpha-bis-alkylation of alkyl cyanoacetates with 2,2'-bis-(bromomethyl)-1,1'-diphenyl, followed by NaBH4/CoCl2 reduction of the cyano group. Both its C- and N-protected derivatives have been obtained. A slow interconversion at the NMR time scale is generally observed between the two enantiomers of the conformationally labile beta(2,2)-HBip residue. The homo-peptides Boc-(beta(2,2)-HBip)(n)-OMe have been prepared in solution by the EDC/HOBt coupling method to the hexamer level and a preliminary conformational analysis has been performed by H-1 NMR and FT-IR absorption techniques