National Institute Of Materials Physics - Romania

Mossbauer spectrometry and magnetic measurements are employed to experimentally investigate the magnetic behavior of nanocrystalline Fe73.5Cu1Nb3Si13.5B9 ribbons obtained by appropriate annealing of the amorphous precursor. A detailed analysis of the correlation between the microstructure of annealed samples and their magnetic properties is provided. Thermomagnetic data allow the Curie temperatures of both amorphous residual matrix and nanocrystalline phase to be estimated. The differences between Curie temperatures of amorphous residual matrix and amorphous precursor are investigated and explained in terms of magnetic polarization of the matrix by exchange fields arising from the nanocrystalline grains. Theoretical systems of spins consisting of a single ferromagnetic nanocrystalline grain immersed in weakly ferromagnetic environment, quite similar to our real samples, are considered and their magnetic behavior is investigated by Monte Carlo simulation of low temperature spin ordering, with emphasize on the matrix-nanocrystalline grain interface which is shown to exhibit peculiar magnetic behavior. The magnetic features of the matrix-nanocrystalline grain interface are studied, as depending on matrix-nanocrystalline grain exchange coupling as well as crystalline fraction of the nanocrystalline systems.

We investigated the superconducting critical temperature, the intra- and intergranular critical current density, and the thermopower properties of Bi1.7Pb0.4Sr1.5Ca2.5Cu3.6Ox/(LiCl)(y) samples. All these properties have been compared with those of Bi1.7Pb0.4Sr1.5Ca2.5Cu3.6Ox/(LiF)(y) specimens. It was found that the critical temperature determined from resistive and AC complex susceptibility measurements show a maximum and the transition width shows a minimum for the intermediate values of y. Powder X-ray diffraction studies and the AC complex susceptibility measurements reveal that in our samples the amount of Bi2Sr2Ca2Cu3O10+delta high-temperature superconducting phase is maximum for y approximate to 0: 02: The amount of LiCl in Bi1.7Pb0.4Sr1.5Ca2.5Cu3.6Ox/(LiCl)(y) changes the superconducting properties of the grains as well as of the intergrain matrix. The splitting of the peak in the temperature dependence of the imaginary part of the complex susceptibility, corresponding to the dissipation inside the grains, was also observed.

Superconducting YBCO c-axis oriented thin films have been grown by MOMBE (metalorganic molecular-beam-epitaxy). Growth of the films on different substrates (100) (Y, Nd)AlO3, (100) SrTiO3 and (100) MgO has been investigated by, in situ monitoring the reflection high energy electron diffraction (RHEED). The paper discusses our results suggesting significant differences in films growth on the three presented substrates. The highest quality (high uniformity, epitaxy and low roughness) has been attained for the films prepared on SrTiO3. All films, regardless substrates, have shown values between 81 and 84 K for zero resistance critical temperature T-c0.

Orientation control of high T-c superconductors is essential for superior device performance, because the coherence length is longer along the non c-axis directions than along c-axis direction. In this study, we report on the first successful preparation of (119) oriented Bi-2223 films by MOCVD using (100) NdGaO3 Substrates. Atomic force microscopy observations of the (119) Bi-2223 films are also reported. Our films have an in-plane aligned mountain-range-shaped surface morphology. This morphology is resulting from the epitaxial relationship between the (119) Bi-2223 film and (100) NdGaO3 substrate. Films exhibited a large in-plane anisotropy as revealed by, resistivity measurements and surface morphology observations.

By very short time rf sputtering in certain deposition conditions we have grown three-dimensional Ag nanodots on the substrate prior to the growth of TI-based superconducting films. These nanodots create pinning centers, leading to an increase in the critical current density about 10 times. From the frequency dependence of the critical current density we estimated also the, actiation energy for the flux jumps, which resulted to. be several times higher. The rate of the thermally-activated flux jumps decreases several orders of magnitude. We suggest that our method can be used for the reduction of thermal noise in high-T-c dc SQUID's.

We report vortex imaging and local magnetometry studies near the edge of a YBCO thin film using scanning Hall probe microscopy (SHPM). Our data show some unusual features, namely local hysteresis inversion and negative remanent fields, which are semi-quantitatively explained in terms of a theoretical model of flux penetration in an infinite-long thin superconducting strip.

Epitaxial Tl2Ba2CaCu2Oy and Tl(Ba,Sr)(2)Ca-2 Cu3Oy thin films have been prepared on CeO2 buffered sapphire substrate by an ex situ process with high reproducibility. Microstructure analysis of the surface of the TI-2212 films showed well connected and smaller sized grains of 1 mum or less. In the case of TI-1223 film, plate-like crystals with pits and pin holes could be observed. The superconducting critical temperatures of both films are around 96 K. Critical current density measured by ac susceptibility technique is as high as 1 MA/cm(2) at 77 K. The microwave surface resistance of these films was measured over a wide range of temperatures by a HTS-sapphire-HTS resonator method at 38 GHz on films with area 10 mm x 10 mm. The temperature dependence of surface resistance of TI-2212 film follows very closely to that of the best YBCO films on MgO substrate, whereas the TI-1223 exhibits slightly higher values resulting from an inferior surface morphology.

From DC magnetization studies in fields up to 14 T and temperatures between 20 and 100 K, we estimated the intra-and inter-grain critical current density J(c) of (Cu,C): 1234 high-T-c superconductors, in the frame of critical state models. The inter-grain J(c) was determined by comparing the magnetization loops of as-grown sample and of the ground sample. Finally, short comments on J(c) resulted from AC susceptibility measurements and of the impressive increase of intra-grain J(c) due to heavy-ion and neutron irradiation are presented. (C) 2003 Elsevier Science B.V. All rights reserved.

Thin films of infinite-layer compound Sr1-xCaxCuO2-delta have been prepared by rf magnetron sputtering. For an optimum level of oxygen vacancy superconductivities were observed (T-c (onset) = 42 and 50 K, T-c(rho=0) = 11 and 20 K). Structural and transport data suggest that the doping mechanism is electron-type. The superconducting transitions were also confirmed by ac susceptibility. Further increase of doping showed to destroy superconductivity. (C) 2003 Elsevier Science B.V. All rights reserved.