M. Vallet

Raman scattering in the high Tc superconductors MBa2Cu3O7-x

Abstract We report Raman scattering measurements of MBa 2 Cu 3 O 7 ( M = Y , Sm , Eu ) polycrystalline superconductors in the region of Cu-O stretching vibrations. Four peaks are seen. On the basis of the factor group analysis given here they are assigned to two Raman allowed and two ir-allowed LO modes. The latter are probably induced by disorder (e.g. O-vacancies).

A new high temperature superconductor: Ba2SmCu3O9−x

Abstract A new high temperature superconducting oxide Ba2SmCu3O9−x (x≈-2.0) has been synthesized and characterized by means of X-ray and electron diffraction as a single phase with a perovskite related structure (acx acx 3ac, ac=perovskite unit cell parameter). The superconducting D.C. transition occurs at Tc=96.5 K with a transition width (10–90%) of 1.7 K. X-ray photoemission experiments reveal a surface concentration of Cu3+ cations of 36%.

Cationic distribution, magnetization and magnetic anisotropy of Co2+ doped M-type barium ferrite

Abstract The cation distribution and the high field magnetization of the Co-Ti M-type barium ferrites have been investigated by means of neutron powder diffraction and high field isothermal magnetic measurements. The composition dependence of the experimental magnetization is compared to that obtained from the cationic distribution assuming a collinear magnetic structure. A considerable amount of Co ions occupy tetrahedral sites being ineffective in the reduction of the uniaxial magnetic anisotropy.

Magnetic properties of Ba2SmCu3O9−x high Tc superconductor

Abstract The magnetic properties of the new high T c superconductor Ba 2 SmCu 3 O 9−x have been investigated from DC magnetization measurements performed in a SQUID magnetometer. The amount of Meissner effect at low temperature is 35% of the full diamagnetic flux expulsion thus indicating bulk superconductivity. The lower critical field at 4.8 K is H c1 = 170 ± 20 Oe and the upper critical field is much higher than 55 KOe. Paramagnetic susceptibility measurements give an effective magnetic moment for Sm 3+ slightly lower than the free ion value.

Magnetic energy absorption in sintered YBa2Cu3O7−δ samples

A.c. magnetic susceptibility measurements of eight microscopically characterized sintered YBa2Cu3O7−δ samples are reported. The samples show losses 0.2 > 4πχ″ > 0.002.χ′vsχ′ plots derived for all samples agree with models and results where flux pinning plays an important role.

Synthesis of BaFe12O19 small particles: A neutron thermodiffractometry study

Abstract “In situ” neutron powder diffraction study of the synthesis reaction of small BaFe12O19 particles, is reported. We show that the synthesis of BaFe12O19 proceeds through the formation of γ-Fe2O3, BaFe2O4 and very small particles or amorphous BaCO3. The growth rate and shape of the BaFe12O19 crystallites is determined from the evolution of peak intensities and broadening.

Diamagnetism and critical currents of BiCaSrCuO samples

Abstract Three different samples of BiCaSrCuO have been obtained. The solid state reaction ceramic method, followed by heat treatments at different temperatures, has been used. The samples have been microscopically characterized by X-ray, neutron diffraction and electron microscopy. From electrical and magnetic measurements the existence of a majority 85 K granular superconducting phase is determined. In comparison to the YBa 2 Cu 3 O 7−δ compound, the inductive intragrain critical current is one order of magnitude smaller, whereas the intergrain current is of the same order of magnitude.

Diamagnetism and electrical connectivity in an inhomogeneous Ba2YCu3O7−x superconductor

Abstract The electrical resistivity and low field ac and dc magnetic suspectibility of an inhomogeneous or composite Ba 2 YCu 3 O 7−x superconductor have been measured. The observation of a rather large resistive transition (T≅3.6 K) allows to get a clear separation among diamagnetism setting of isolated Superconducting grains and a phase transition involving grain boundaries Josephson juntions and leading to electrical connectivity and thus to diamagnetic shielding.

μ+SR study of magnetic order in La 2NiO4+δ

Abstract Increasing attention has been devoted lately to the La 2 NiO 4 -type oxides due to the similarities with the superconducting copper oxides. In this work we report a microscopical study of the magnetic ordering in La 2 NiO 4+δ by using μ + SR. The coherent spin procession of implanted muons due to internal fields in the ordered state, has been detected in three La 2 NiO 4+δ polycrystalline samples. Long range order was apparent only in stoichiometric La 2 NiO 4 , for which T N ≈320K and the first order orthorhombic-to-tetragonal structural phase transition was detected as a gradual increase with decreasing temperature of the precession frequency of the implanted muons bound to apical oxygens. A strongly damped oscillating signal could be detected only at the lowest temperature (5 K) for the non-stoichiometric (oxidized) samples which evidences a highly disordered magnetic state.

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.