National Institute Of Materials Physics - Romania

From frequency-dependent ac susceptibility studies of (Cu,T1)BaSrCa2Cu3Oy superconducting thin films, with and without nanodot-induced artificial pinning centers, we estimated the activation energy of flux jumps. The result was that, in the film with nanodots, the pinning potential is several times higher, leading to a probability of thermally activated flux jumps several orders of magnitude lower than in the film without artificial pinning centers. We suggest that our no cost straightforward method for creating extended defects can be successfully employed for the reduction of thermal noise in superconducting electronic devices. (C) 2002 American Institute of Physics.

The crystallization behaviour of amorphous melt-spun SmxFe80-xB20 ribbons is investigated by differential scanning calorimetry, X-ray diffraction and Mossbauer spectrometry. On increasing the Sm content. the crystallization temperature reaches a maximum for a critical Sm content (x(c) approximate to 8) and the main crystallization products are different below and above x,. This is attributed to a change in the short-range local order in the amorphous alloy. (C) 2002 Elsevier Science B.V. All rights reserved.

The magnetic properties of interfacial regions between ferromagnetic nanograins and the residual matrix in nanocrystalline materials, are modelled using Monte Carlo simulation. The exchange coupling between matrix and nanograin influences differently the nanograin surface and the interface between nanograin and matrix. This behaviour has been explained in terms of a polarisation mechanism of the interfacial zones leading to magnetic correlation between spins of nanograin surface and core.

Thermal decomposition processes in mixtures of Ca : Cu = 1 : 1 nitrate powders produced by a conventional technique and by freeze drying method have been investigated by thermal analysis and X-ray diffraction. Results have been compared with literature values and data obtained by our investigations of individual calcium and copper nitrates. Temperatures at which decomposition processes occur change with by +/-30degreesC and depend also on the level of mixing of the nitrates. An exemption is the temperature for CuO phase formation, which was determined for the Cu-nitrate and for both 1 : 1-nitrate powders to be at 266degreesC. The domains of stability for different phases are shown to be variable. Also, for 1 : 1-powders, alpha-type and beta-type Ca(NO3)(2) . 2H(2)O phases coexist, while in Ca-nitrate only the alpha-type phase was observed. (C) 2002 Kluwer Academic Publishers.

The reactive-field-assisted-sintering technique (RFAST) has been applied to freeze-dried decomposed precursor powders with a starting cation composition of Bi:Pb:Sr:Ca:Cu = 1.7:0.3:2.0:2.5:3.5. A short time (< 15 min) RFAST treatment at 800-900degreesC did not lead to the formation of Bi2Sr2Ca2Cu3O10+x, (2223-phase). But RFAST was favourable to Bi2Sr2CaCu2O8+y, (2212-phase) formation particularly in samples with a low content of the 2212-phase in the precursor powder. Heat treatment (HT) after RFAST showed that the 2223 phase could form more rapidly than in freeze-dried pressed and sintered pellets produced by the traditional technique. The resultant ceramics after 70 h of annealing contain 75% 2223-phase and have T-c(R=0) = 100.7 K, T-c = 109.5 K and DeltaT(c) = 22.4 K.

In high temperature superconductors (HTS) the coherence length along non-c axis directions is longer. This feature can be useful when designing electronics devices based on HTS. Therefore growth and characterization of non-c axis oriented thin HTS films is of great interest. In this paper we present a short review of our data regarding (119) Bi-2223 thin films grown by MOCVD on (100) NdGaO3 and (110) SrTiO3. The emphasis is made on improvement and control of the quality of the films by the "two-temperature" technological approach and/or use of the vicinal substrates. Phase and morphology evolution for different processing conditions, substrate's type and off-angle are presented. The highest critical temperatures of T-c0=67.2 K and T-c0=74 K for the "single-" and "two-" temperature routes were obtained on vicinal SrTiO3 with the off-angle of 20degrees. A higher off-angle promoted the formation of a specific step-like morphology with lower roughness. For the films grown on flat substrates the morphology was of mountain-range shape. Surface morphology as a result of two types of growth mechanisms (two-dimensional (2D), assisted by a so-called "twin"-growth and step-flow growth) for the (119) Bi-2223 films are discussed.

CuBa2Ca3Cu4O12-y (Cu:1234) high-temperature superconductors (HTS) doped with up to 2% Zn were grown using the high-pressure synthesis technique. Magnetization loops of the samples were measured at various temperatures between 5 and 77.3 K and magnetic fields up to 14 T. Critical current densities J(c) of the samples were estimated using the critical state model. The results show that Zn-induced pinning centers increase J(c) of Cu:1234 several times, depending on field and temperature. From the experimentally determined field-temperature region in which a higher Zn concentration lead to a higher J(c), we suggest the existence of a cross-over from quite efficient, extended (in the c-axes direction) pinning centers to point-like (inefficient) pinning centers at a certain temperature, depending on field. This effect can be attributed to the fact that, unlike other HTS, in Cu: 1234 there is a second critical temperature T-c2 of about 70-80 K (in zero field, and 50-60 K in 15 T), related to the over-doping of pyramidal basal plane (outer CuO2 plane). (C) 2002 Elsevier Science Ltd. All rights reserved.

The electron density at the gold nuclei in metallic gold and in the cubic intermetallic compounds CsAu, RbAu, AuMg, AuZn, AuCd, AuSc, AuAl2, AuGa2, AuIn2, AuSn2 and AuSb2 have been calculated using the Wien97 computer code. The electron densities thus obtained and the experimental values of the isomer shifts define a linear correlation spanning nearly the whole range of isomer shifts observed for the 77 keV transition of Au-197. From the slope of this correlation one obtains Delta = (4.6 +/- 0.2).10(-3) fm(2) for the change of the mean square nuclear charge radius accompanying 77 keV transition in Au-197.

A straightforward and cheap method for creating extended defects, strong pinning centers, in superconducting thin films is proposed. Clearly, by very short time (3-5 s) rf sputtering at suitable substrate temperatures, we deposited Ag nanodots on SrTiO3 substrates prior to the growth of superconducting thin films. The nanodots were studied by atomic force microscopy. Due to the lattice mismatch and/or chemical poisoning, on top of the nanodots the superconducting phase does not form, creating in this way extended and effective pinning centers which increase the critical current density of the film. The method was applied to (Cu, Tl)BaSrCa2Cu3Oy films grown by amorphous phase epitaxy. Thin films grown in similar conditions, with and without nanodots, were characterized by x-ray diffraction and ac susceptibility. The results show that the nanodots increased the critical current density more than one order of magnitude. (C) 2001 American Institute of Physics.

We investigated the superconducting critical temperature, transport critical-current density and the thermo-power properties of Bi1.7Pb0.4Sr1.5Ca2.5Cu3.6Ox/(LiF)(y) samples. It was found that the midpoint critical temperature increases with increasing y, attaining approximate to 119.5 K for y = 0.15. The transition width shows a minimum and the critical temperature (zero electrical resistance) exhibits a maximum as a function of y. The observed behavior is correlated with the SEM analysis. Powder X-ray diffraction studies reveal that in our samples the amount of Bi2Sr2Ca2Cu3O10+delta high-temperature superconducting phase is maximal (70-78%) for y = 0.05-0.07.