National Institute Of Materials Physics - Romania

Annealing effect of the irreversibility fields (H-irr) in (Cu,C)Ba2Ca2Cu3Oy ((Cu,C)-1223) and (Cu,C)Ba2Ca3Cu4Oy ((Cu,C)-1234) have been investigated. The samples prepared under high pressure were post-annealed in nitrogen gas in the temperature range of 400-500degreesC. H-irr of (Cu,C)-1234 was improved after annealing at 450degreesC while T-c remained almost constant. This suggests that carrier density of five-coordinated CuO2 planes or conductivity of charge reservoir layers is changed by annealing while keeping that of four-coordinated CuO2 planes almost constant. Both samples show H-irr similar or equal to 4 T at 90 K by the post-annealing, which is quite attractive for practical application. (C) 2003 Elsevier Science B.V. All rights reserved.

To enhance or improve critical currents density (J(c)) and irreversibility field (H-irr) of (Cu,C)Ba2Ca3Cu4O10.5-delta ((Cu,C)-1234), pinning centers were introduced by heavy-ion irradiation. The polycrystalline samples were irradiated with An(15+) ions (240 MeV energy) at various fluence of 1 x 10(11), 2.5 x 10(11) and 5 x 10(11) ions/cm(2). J(c) and H-irr were determined for the irradiated samples as well as unirradiated samples. J(c) (77 K, 1 T) increase from 3.9 x 10(4) A/cm(2) to 4.1 x 10(6) A/cm(2) for at fluence of 1 x 10(11) ions/cm(2) of heavy-ion irradiated sample and decreases with further increase of fluence. These results indicate the possibility of further enhancement of Jc and of achieving a very high H-irr of (Cu,C)-1234 below a fluence of 1 x 10(11) ions/cm(2). (C) 2003 Elsevier Science B.V. All rights reserved.

Thin films of (Ca1-xBax)CuO2 (x = 0-1) have been grown on (0 0 1) SrTiO3 by metal organic chemical vapor deposition. These films are potential candidates as barrier layer for SIS and/or SNS Josephson junctions. For this purpose, the films should be flat and smooth. In this regard we have investigated the possibility of decreasing the films roughness by the control of the Ba/Ca ratio, the substrate temperature and oxygen partial pressure during the growth of the film. The minimum mean square root roughness and regular morphology are obtained for the x = 0.3-0.5. In the samples with x = 0.5 the highest intensity of the X-ray diffraction peaks for the (Ca0.5Ba0.5)CuO2 phase and the lowest for the impurity phases were attained at a substrate temperature of 750-780 degreesC and oxygen partial pressure of 15 Torr. (C) 2002 Published by Elsevier Science B.V.

Superconducting (0 0 1) thin films of SmBa2Cu3Oy have been grown by MOCVD on (0 0 1) SrTiO3 and (0 0 1) LaAlO3 substrates. The films on both substrates are smooth with RMS of 1.428 and 1.037 nm respectively, but have different morphology (revealed by atomic force microscopy) for the same synthesis conditions. The films grown on SrTiO3 have shown rectangular- or square-type terraces. The height between the neighboring terraces was corresponding to the c-axis length of one unit dell. This type of morphology is due to the two-dimensional nucleation and growth mechanism. In the case of the films on the LaAlO3 the surface morphology consisted of irregular grains (close to spherical). For both situations we have not found any evidence for a spiral growth mechanism. Therefore, our films are suitable for electronics applications of integrate type and not only. The as-grown films were oxygen deficient and due to this exhibited rather poor superconducting properties. Oxygen annealing at temperatures of 500-600 degreesC increased T-c(R=0) up to 87.8 K. (C) 2002 Published by Elsevier Science B.V.

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 our comparative results on preparation by MOCVD and characterization by atomic force microscopy, X-ray diffraction and transport measurements of as-grown (119) Bi-2223 and (103) Y-123 thin films on (110) SrTiO3. The films have shown the same type of surface morphology (grains with mountain-range shape, aligned in parallel chains to the [001] substrate's azimuth). This morphology is the result of the orientation relationship between the (119) Bi-2223 or (10 3) Y-123 and (110) SrTiO3. The transport measurements with temperature revealed strong anisotropy of the films in the normal state region when the measuring current has been applied in-plane, parallel and perpendicular to the edges (from the basal plane) of the mountain-range grains. Higher values for T-c0 have been obtained for the Y-123 films (42 K) than for the Bi-2223 ones (31 K). The both films are as-grown and non-oxygenated ones. (C) 2002 Published by Elsevier Science B.V.

The intermetallic compounds in the Au-Al system were studied by Au-197 Mossbauer spectroscopy. The observed isomer shifts and electric quadrupole splittings are compared with the electron densities and electric field gradients at the gold nuclei obtained by LAPW calculations using the WIEN97 computer code. This provides insights into the electronic structure of Au-Al alloys and yields values for the groundstate quadrupole moment of Au-197 and for the change of the nuclear charge radius for the 77 keV transition of Au-197. (C) 2002 Elsevier Science B.V. All rights reserved.

Applied field Mossbauer spectrometry is used to investigate the magnetic structure in two amorphous Sm-Fe-B ribbons with measured compositions Sm7.2Fe72.8B20 and Sm12Fe68B20. The applied field dependence of the hyperfine field agrees with the ferromagnetic coupling between the Fe and Sm magnetic moments. For the Sm7.2Fe72.8B20 ribbon, the magnetic moments are aligned in the applied field direction for fields higher than 4 T. For the Sm12Fe68B20 ribbon, the magnetic moments are not completely aligned in the direction of the applied field for fields up to 6 T. This spin canting is attributed to the influence of the samarium anisotropy, which probably dominates in the Sm12Fe68B20 ribbon, as the related composition lies in the concentration range where the local order around iron atoms is strongly influenced by the samarium atoms. (C) 2002 Elsevier Science B.V. All rights reserved.

The gamma-Fe2O3 nanoparticles with different surface states are investigated by X-ray Diffraction, Electron Microscopy, Differential Thermal Analysis and magnetic: measurements in low applied field. The influence of the adsorbed species (H2O and sulphate complexes) on the inter-particle magnetic interactions is analysed in relation to both the magnetic states of the surface and the inter-particle distance.

Easy axis distribution in metal particle magnetic tapes for linear digital data storage were studied by Mossbauer spectroscopy and magnetometry, for comparison. Qualitative and quantitative agreement between the distribution parameters deduced at room temperature via the two different techniques was found. The proposed methodology based on the interpretation of the Mossbauer data has the advantage that it can be applied at different temperatures and for very thin films. Moreover, angular spin distribution. at low temperatures were obtained by Mossbauer spectroscopy and discussed with respect to the room temperature results. There was evidenced that the room temperature easy axis angular distribution contains both static orientation and dynamical components.

The electronic mechanism responsible for the optical behavior of the mixed Fe and Sn doped polymers were studied in comparison with the correspondent single doped (Fe or Sn) ones. Optical absorption as well as Fe-57 and Sn-119 Mossbauer measurements were performed on different UV exposed films. The new data provided the role of each element from the pair on the involved electronic mechanism induced by irradiation.