Showing papers by "Marc Respaud published in 1996"
TL;DR: The magnetic behavior of the colloids was studied by SQUID techniques as discussed by the authors, which showed that Coll. 1−3 display a typical superparamagnetic behavior with blocking temperatures in the range 9−10 K.
Abstract: THF solutions of the new organometallic precursor Co(η3-C8H13)(η4-C8H12) react with 3 bar dihydrogen in the presence of polyvinylpyrrolidone (PVP) at three different temperatures, namely, 0, 20, and 60 °C to give colloidal solutions containing fcc cobalt particles of respectively ≤1 nm (Coll. 1) and ca. 1.5 nm (Coll. 2 and Coll. 3) mean sizes, as evidenced by HREM analysis. All colloids react with carbon monoxide to give new colloids (Coll. 1‘−3‘) displaying broad CO stretches near 2000 and 1890 cm-1 on their infrared spectra recorded in KBr disks. The magnetic behavior of the colloids was studied by SQUID techniques. Coll. 1−3 display a typical superparamagnetic behavior with blocking temperatures in the range 9−10 K. From magnetization studies above and below the blocking temperature, it was possible to determine that Coll. 3 shows a mean size of 1.6 nm and a very narrow size distribution. Because of a magnetic surface effect, the magnetic moment per atom (μCo = 1.94 ± 0.04 μB) appears significantly lar...
195 citations
TL;DR: In this article, HREM analysis of the particles demonstrate that the smaller particles (≤25 A) adopt the α-Fe (bcc) structure, whereas the larger ones adopt the γ-Fe structure.
Abstract: Sonolysis of a solution of Fe(CO)5 in anisol in the presence of poly(dimethylphenylene oxide) (PPO) leads to the formation of small nonagglomerated iron particles. HREM analysis shows that the size of the particles is centered around 30 A with a medium dispersity. HREM analysis of the particles demonstrate that the smaller particles (≤25 A) adopt the α-Fe (bcc) structure, whereas the larger ones (≥25 A) adopt the γ-Fe (fcc) structure. Magnetic measurements confirm the presence of small superparamagnetic particles (α-Fe) and of mostly antiferromagnetic or paramagnetic particles (γ-Fe).
64 citations
TL;DR: In this paper, the results of magnetization and magnetic susceptibility measurements on $n$- and $p$-type $\ensuremath{\beta}$-Fe${\mathrm{Si}}_{2}$ single crystals are presented.
Abstract: The results of magnetization and magnetic susceptibility measurements on $n$- and $p$-type $\ensuremath{\beta}$-Fe${\mathrm{Si}}_{2}$ single crystals are presented. The magnetic susceptibility in the studied crystals are determined by the temperature-independent contribution from the lattice, lattice defects and/or neutral impurities, and by the temperature-dependent parts due to paramagnetic centers as well as due to the carriers excited thermally at high temperature. The temperature variation of the paramagnetic terms are in agreement with the Curie-Weiss law. The values of the Curie-Weiss temperatures, activation energy of the donor and acceptor levels, and the upper (lower) limit of the density of state electron (hole) effective masses were estimated.
21 citations
TL;DR: The temperature dependence of the conductivity, σ(T), of FeSi at T ≈ 100-170 K follows the Arrhenius law with the value of the band gap Eg ≈ 50 ± 5 meV as discussed by the authors.
Abstract: The temperature dependence of the conductivity, σ(T), of FeSi at T ≈ 100–170 K follows the Arrhenius law with the value of the band gap Eg ≈ 50 ± 5 meV. Between Tν ≈ 15–25 K and Tν∗ ≈ 3–4 K it agrees with the Mott-type varriable-range hopping. No tendency of saturation in σ(T) is observed down to 1.4 K. The relative magnetoresistance, Δϱ/ϱ, is negative at T ⩽ 4.2 K and H < 1 T with the amplitude value ≈0.2%. In higher fields, up to 35 T, it is positive and demonstrates a complex magnetic field and temperature dependence suggesting both positive and negative contributions. At T ⩾ 77 K, Δϱ/ϱ is negligible. The positive component of Δϱ/ϱ agrees with that of the Osaka model which takes into account both correlation and spin-flip effects. The negative contribution is attributed to the Zeeman splitting of the mobility threshold predicted by Kamimura and Kurobe. The temperature dependence of the hopping frequency with the pre-exponent ν0 ≈ 2.8 × 1012s−1, the value of the internal field, HI ≈ 60 kOe, and the maximum value of the correlation energy, Um ≈ 17 K, are obtained.
12 citations
TL;DR: In this paper, the authors measured the magnetization up to 36 T in a large range of temperature (0.06-4.2K) and observed steps due to the crossing of the spin states.
Abstract: The compound under study is a magnetic metal oxyde cluster of Mn(II) encapsulated in between two polyoxymetalate clusters PW9O34, i.e. (Mn4(H2O)2(PW9O34)2)K1020H2O. We measured the magnetization up to 36 T in a large range of temperature (0.06–4.2K) and we observed steps due to the crossing of the spin states. We compared these results with the energy spectrum predicted from inelastic neutron scattering and magnetic measurements. Such magnetization measurements under high pulsed field and at very low temperature are performed for the first time.