J.M. González-Calbet

Thin films of magnesium oxide by modified CVD : a buffer layer for HTCS films

Abstract Magnesium oxide thin films, as buffer layers for depositing high-T c superconducting thin films, were prepared by a modified CVD process. This process uses pyrolysis of an aerosol produced by ultra-high frequency spraying of a solution. The raw material was magnesium acetate dissolved in ethanol or butanol. The deposition was carried out in air between 450 and 600°C and thickness varies between 100 and 500nm. A highly 〈100〉 oriented polycrystalline film can be obtained on oxydized silicon wafers. The influence of deposition parameters on the film growth was examined by X-ray, SEM. Grain size varies between 60 and 150 nm. Electrical properties were examined by complex impedance spectroscopy and I(V) characteristic: resistivity (>10 12 Ω.cm) and dielectric constant.

Specific heat, magnetic susceptibility and electrical resistivity measurements on LaNiO3

Abstract Measurements of specific heat, magnetic susceptibility and electrical resistivity on LaNiO 3 are reported. No evidence of magnetic ordering or metal-insulator transitions is found. From the measured electronic specific heat parameter γ and the Pauli susceptibility χ P , a Stoner enhancement parameter is derived. Comparison with band structure calculations performed by other workers is also made.

The oxycarbonate Ba{sub 3}Co{sub 2}O{sub 6}(CO{sub 3}){sub 0.60} with a 2H-ABO{sub 3}-related structure

The crystal structure of Ba{sub 3}Co{sub 2}O{sub 6}(CO{sub 3}){sub 0.60} has been determined by means of X-ray, neutron and electron diffraction, and high-resolution electron microscopy. This compound has a hexagonal symmetry (space group P{bar 6}) and lattice parameters a = 0.9683(1) and c = 0.95180(8) nm related to the 2H-ABO{sub 3} by means of the expression a = a{sub 2H}(3){sup 1/2} and c = 2c{sub 2H}. It can be described from the ordered intergrowth of two blocks, [Ba{sub 2}Co{sub 2}O{sub 6}]{sup {delta}{minus}} and [Ba(CO{sub 3}){sub 0.60}]{sup {delta}+}, alternating along the a-axis. The first block corresponds to two 2H-like rods and the second one can be related to the BaCO{sub 3} aragonite-type structure. Neutron diffraction data elucidate the relative orientation of carbonate groups along c-axis in the aragonite-like blocks.

Magnetic irreversibility in granular superconductors: AC susceptibility study

Abstract Ac susceptibility measurements of a ceramic weak-coupled superconductor in very low ac fields (2mG, 111 Hz) are reported. We present evidence for the observation of the magnetic irreversibility following a ZFC-FC thermal cycling by means of ac susceptibility measurements. It is shown that this technique also reflect local magnetic field effects in granular superconductors, as previously suggested in microwave surface resistance and I-V characteristics.

Phase identification and superconducting transitions in Sr-doped Pr{sub 1.85}Ce{sub 0.15}CuO{sub 4+{delta}}

Sr-doped Pr{sub 1.85}Ce{sub 0.15}CuO{sub 4+{delta}} samples have been prepared with accurate control of the oxygen content. The stability of both T{sup {prime}} and T{sup {asterisk}} phases is strongly dependent on Sr and oxygen content. An electron diffraction study indicates that, in some cases, anionic vacancies are ordered leading to a pseudo-tetragonal superlattice with unit cell parameters 2{radical}2{bold a}{sub t}{times}{bold c}{sub t}. Structural transitions and superconducting phases created by hole doping in such a system are also reported. {copyright} {ital 1997 Materials Research Society.}

Electron microscopy, electrical resistivity and magnetic properties of the new tubular phase Bi4Sr8Cu5O19+x

Abstract We report the study of the structural and physical properties of the tubular oxide Bi 4 Sr 8 Cu 5 O 19+x by means of electron microscopy, electrical resistivity and magnetic susceptibility. Electrical resistivity measurements in the 60K≤T≤300K temperature range indicate a semiconducting behaviour. The observed magnetic susceptibility is consistent with the existence of paramagnetic defects within a 2D strongly correlated antiferromagnetism.

Chapter 44:Nano and Mesoporous Materials: A Study by HREM

Host–guest materials comprise a wide group of great interest due to their intrinsic properties that allow the development of classic and new applications. Among them, silica ordered porous materials constitute a very interesting example. There are materials with different pore sizes that can be used...

Temperature dependence of the magnetic properties in LaMnO/sub 3.02/

The temperature dependence of the hysteretic and relaxational behaviours of a high coercivity La manganite was measured. The results were discussed in terms of their magnetic structure and they indicated that the vacancy-induced ferromagnetic Mn/sup 4+/-Mn/sup 3+/ clusters are coupled to the antiferromagnetic matrix in a way that involves decompensation of the antiferromagnetic lattice. It therefore resulted on the formation of net moments at the interfaces.