W.A. Ortiz

Ferromagnetism induced by oxygen and cerium vacancies above the percolation limit in CeO2

We studied the structural, chemical and magnetic properties of non-doped ceria (CeO(2)) thin films electrodeposited on silicon substrates. Experimental results confirm that the observed room temperature ferromagnetism is driven by both cerium and oxygen vacancies. We investigated ceria films presenting vacancy concentrations well above the percolation limit. Irradiation experiments with neon ions were employed to generate highly oxygen defective CeO(2-δ) structures. X-ray photoelectron spectroscopy and x-ray absorption near-edge structure spectroscopy were used to estimate the concentration of Ce(3+) sites in the films, which can reach up to 50% of Ce(3+) replacing Ce(4+), compared to a stoichiometric CeO(2) structure. Despite the increment of structural disorder, we observe that the saturation magnetization continuously increases with Ce(3+) concentration. Our experiments demonstrate that the ferromagnetism observed in ceria thin films, highly disordered and oxygen-deficient, preserving the fluorite-type structure only in a nanometer scale, remains intrinsically stable at room temperature.

Enhanced pinning in superconducting thin films with graded pinning landscapes

A graded distribution of antidots in superconducting a-Mo79Ge21 thin films has been investigated by magnetization and magneto-optical imaging measurements. The pinning landscape has maximum density at the sample border, decreasing linearly towards the center. Its overall performance is noticeably superior than that for a sample with uniformly distributed antidots: For high temperatures and low fields, the critical current is enhanced, whereas the region of thermomagnetic instabilities in the field-temperature diagram is significantly suppressed. These findings confirm the relevance of graded landscapes on the enhancement of pinning efficiency, as recently predicted by Misko and Nori.

Synthesis and characterization of Li2ZnTi3O8 spinel using the modified polymeric precursor method

In this work we report the synthesis procedure, crystallographic, structural and magnetic properties of the Li 2ZnTi3O8 spinel obtained using a modified polymeric precursor method. This synthesis method generates very reactive and property-controlled nanoparticles. The samples were characterized using X-ray powder diffraction (XRD) associated to the Rietveld refinement method, thermogravimetric analysis (TG), specific surface area, scanning electron microscopy (SEM) and magnetic susceptibility measurements. The phase formation temperature of the lithium zinc titanate spinel was observed to decrease due to the homogeneity and highly controlled nanometric particle size.

A new structure in the REBaCuFeO5+δ series: LaBaCuFeO5+δ. Structure and magnetic properties in the La1−xPrxBaCuFeO5+δ system

Abstract The crystal structure of LaBaCuFeO 5+ δ is determined by neutron powder diffraction at 15 K and the structure and magnetic properties of the La 1− x Pr x BaCuFeO 5+ δ system are studied by AC-susceptibility measurements and RT 57 Fe Mossbauer spectroscopy. The best fit using the Rietveld method is obtained for an orthorhombic model in space group I mmm with unit cell parameters a =5.5586(8) A, b =5.5550(9) A and c =7.8155(2) A. The main feature of this model is a disordered structure where the oxygen atom vacancies play an important role. The magnetic susceptibility in the La 1− x Pr x BaCuFeO 5+ δ system is studied and T f , μ eff and Θ are determined. Mossbauer spectra provide information relative to the iron electronic state and environment. They support the hypothesis of the key role that the oxygen atom positions play in the magnetic properties of the system by affecting the superexchange interactions in the network.

Influence of processing conditions on the crystal structure and magnetic behavior of La0.7Ca0.3MnO3±δ samples

Abstract In this work we report crystallographic structure variations and the related modifications on the magnetic behavior of La 0.7 Ca 0.3 MnO 3± δ introduced by heat-treatments in different synthesis atmospheric conditions. We have prepared polycrystalline ceramic samples using a modified polymeric precursors method, which produces highly homogeneous specimens. The use of argon atmosphere enlarges the crystalline c -axis as detected by Rietveld refinements. As a consequence, an improvement in the magnetic transition temperature T C of the samples was observed. Our results also indicate that different heat-treatment conditions change the magnetic interactions between the ferromagnetic (F) and antiferromagnetic (AF) structures of these systems. Our conclusions rely on the use of AC magnetic susceptibility measurements as the experimental tool for measuring these variations.

Granularity in superconductors: intrinsic properties and processing-dependent effects

Abstract This contribution presents a selected set of results, obtained as part of a systematic investigation, evidencing that many effects exhibited by superconductors are distinct manifestations of granularity which, in turn, is envisaged as a break of symmetry. The Wohlleben effect, the “fishtail anomaly”, the magnetic remanence exhibited by Josephson junction arrays, and the jumps on the magnetic moment of superconducting samples of mesoscopic dimensions, are examples which we briefly review and discuss taking granularity as the basic ingredient. The emphasis of the present approach is to recognize the importance of granularity in every scenario intended to explain the magnetic properties of superconducting systems.

Crystallographic and magnetic structure of polycrystalline Zn7 xNixSb2O12 spinels

The crystallographic and magnetic structure of sintered, polycrystalline samples of zinc‐antimony spinel, Zn 7 x Nix Sb2O12, have been investigated. The samples were prepared by the modified polymeric precursors method. The magnetic contributions of the Ni 2C ions distributed in three non-equivalent crystallographic sites were investigated, revealing the occurrence of different magnetic regimes. A hysteresis response in the magnetic susceptibility indicates a spin-glasslike behavior at low temperatures.

Anisotropy of Magnetization and Nanocrystalline Texture in Electrodeposited CeO2 Films

Anisotropy of Magnetization and Nanocrystalline Texture in Electrodeposited CeO2 Films V. Fernandes, P. Schio, R. J. O. Mossanek, A. J. A. de Oliveira, W. A. Ortiz, D. Demaille, F. Vidal, Y. Zheng, P. Fichtner,* L. Amaral, M. Abbate, J. Varalda, W. H. Schreiner, and D. H. Mosca Departamento de Fisica, Universidade Federal do Parana, 81531-990 Curitiba PR, Brazil Departamento de Fisica, Universidade Federal de Sao Carlos, 13565-905, S. Carlos SP, Brazil Institut des NanoSciences de Paris, UPMC-Paris 6, CNRS UMR 7588, 75252 Paris Cedex 05, France Escola de Engenharia and Instituto de Fisica, Universidade Federal do Rio Grande do Sul, 91501-970 Porto Alegre, Brazil

Suppression of flux avalanches in superconducting films by electromagnetic braking

Magnetic fields perpendicular to superconducting films often trigger vortex avalanches, which always are very harmful for electronic devices and other applications. Such avalanches can be suppressed by a metal layer placed in contact with the superconductor surface, an effect that up to now has been thought to be a consequence of improved heat conduction. Here we show experimentally that the role of the metal layer is not that of a heat-sink, but rather that of an electromagnetic drag due to eddy currents induced in the metal layer during the abrupt onset of the flux avalanches. The effect is demonstrated for films of MgB2 and Nb.

Superconductivity in sintered-polycrystalline PrBa2Cu3O7−δ

Abstract Superconductivity in thin films and powders, and in single crystals of PrBa 2 Cu 3 O 7− δ has been found by Blackstead et al. and Zou et al., respectively. Nevertheless, up to now it has never been reported in sintered-polycrystalline samples. We have prepared high-quality samples of this material by following a sol–gel method. We characterized the structure of all samples by using XRD and SEM-EDS techniques. Magnetic characterization was performed by measuring the magnetization as a function of temperature T , and the applied magnetic field H . Measurements were taken in the ranges 2 K K and 0 T . In this work we report for the first time superconductivity in PrBa 2 Cu 3 O 7− δ sintered-polycristalline samples, with T c around 90 K, and H c1 around 870 Oe.