A. M. Testa

Double perovskite Sr2FeMoO6 films : Growth, structure, and magnetic behavior

The structural and magnetic properties of Sr2FeMoO6 thin films prepared by pulsed laser deposition were studied as a function of the laser energy density used to ablate a noncommercial stoichiometric target. Films deposited at a laser fluence as low as 1.6J∕cm2 exhibit a high degree of double perovskite lattice ordering, a ferromagnetic-paramagnetic transition around 355K, and a saturation magnetization of ∼3.4μB∕f.u. On the contrary, films deposited at φ values >3J∕cm2 show a vertically elongated lattice unit cell and the lack of long range ferromagnetic order with a severe decrease of the saturation magnetization (Ms∼1μB∕f.u.). The structural and magnetic properties observed in the latter samples are attributed to lattice disorder and to secondary phases resulting from the ablation process performed at high laser energy density. In these samples the presence of Fe ions not arranged in the Sr2FeMoO6 is confirmed by x-ray absorption measurement at the Fe L2,3 edge.

Frequency dependence of susceptibility higher harmonics in YBCO samples by numerical solutions of the non linear magnetic flux diffusion equation

Abstract We present a numerical analysis of the diffusion equation of the magnetic flux in YBCO in presence of ac magnetic field taking into account flux creep and flux flow. Despite critical state models, the results show the dependence of harmonic susceptibilities on the field frequency in agreement with the experimental behavior.

Hysteretic and ac dynamic losses of high T/sub c/ superconductors by numerical solution of the nonlinear magnetic diffusion equation

The complex ac susceptibility (/spl chi/) of high T/sub c/ superconducting materials has been described in terms of hysteretic and dynamic losses. By including both flux creep and flux flow resistivities in the expression of the flux flow resistivities in the expression of the flux diffusivity, induction profiles have been numerically calculated from the nonlinear flux diffusion equation. The imaginary part of /spl chi/ has been evaluated as function of temperature, frequency and amplitude of the applied magnetic field. The role of pinning and dynamic losses occuring in high T/sub c/ materials is discussed.

Numerical study of the frequency and temperature dependence of the ac magnetic susceptibility in presence of a static magnetic field in HTS

The temperature dependence of the harmonics (/spl chi//sub n/) of the ac magnetic susceptibility in HTS have been investigated by numerical solutions of the non linear diffusion equation for the magnetic flux. Within the framework of the collective pinning model, we show that the transition between different regimes for the flux dynamics (taff, creep, flow) is determined by the field frequency for a fixed dc field, or by the dc field for a fixed frequency. As a consequence, a non universal behavior arises for the temperature dependence of /spl chi//sub n/. In particular, in this approach the frequency dependencies of both amplitude and temperature of the peak of the imaginary part of the first harmonic and of the modulus of the third harmonic are discussed.

AC susceptibility study of YBCO sintered samples near the transition temperature

Abstract Y 1 Ba 2 Cu 3 O 7−x sintered samples have been investigated near the transition temperature by means of AC susceptibility technique. In particular, the intragranular contribution to the imaginary part of susceptibility (χ″) has been studied as a function of the field frequency (f = 17 Hz + 1070 Hz) in a narrow temperature range (few degrees) around T c (≅92.0 K). Different frequency dependences of χ″ have been found as the temperature varies. For temperatures T ln f, whereas for T≥ 91.9 K a linear frequency dependence is found. Moreover, for T 92.3K such limit is zero.

Magnetic and X-ray absorption investigations of Co-doped ZnO films

We present an investigation of magnetic and structural properties of Co-doped ZnO (ZCO) film grown by pulsed laser deposition at different dopant concentrations (cCo). X-ray diffraction patterns show that the films are single phase and exhibit ferromagnetism (FM) above room temperature (RT) with coercive fields up to 700 Oe. X-ray absorption fine-structure spectroscopy (XAFS) at the Co edge suggests that in films grown below 600°C dopant clustering involve less than 10% of the Co atoms in the alloy. In samples grown at higher temperature a larger fraction of Co atoms is involved in the formation of small metallic clusters. The experimental work has been accompanied by preliminary first-principles Density Functional Theory calculations.

Third-harmonic susceptibility: frequency dependence and pinning potential

Abstract We have analyzed the temperature dependence of the third harmonic of the AC susceptibility in HTSC for different frequencies in presence of large DC magnetic field. We have numerically solved the nonlinear magnetic diffusion equation for a slab in a parallel magnetic field taking into account the creep and flux flow resistivities. A logarithmic frequency dependence of the peak temperature of the third-harmonic module is found. We discuss the large difference between the numerical slope and the analytical result derived by assuming a frequency-dependent critical current density.

AC magnetic susceptibility measurements near the superconducting transition temperature of YBCO sintered samples

Abstract The temperature dependence of the ac susceptibility of sintered YBCO pellets is experimentally analysed in detail few Kelvin below the transition temperature. The quite complex dependence of χ ′ and χ ″ on the frequency and the amplitude of the ac magnetic field cannot be explained by an ohmic Flux Flow or Critical State models, so that intermediate regimes or different non linear phenomena are to be considered.

Seeded directional growth of superconducting bulk Y123

Abstract We report a fabrication process of textured YBCO grown by seeded directional solidification using a MgO crystal as a seed, which alignes YBCO grains along c-axis with a texturing degree reduction as the distance from the seed increases. Measurements of magnetic hysteresis cycles were performed by means of vibrating sample magnetometry and the critical current density was estimated using Beans critical state model.

Dissipative effects near the transition temperature of high Tc superconductor

The complex magnetic susceptibility of oxide superconductors near the transition temperature has been computed as a function of temperature and frequency by numerical solution of the non linear, flux diffusion equation. For field values much lower than the upper critical field, a description of dissipative effects in terms of the conventional flux creep and flux flow pictures is not able to reproduce experimental data.