National Institute Of Materials Physics - Romania

The paper presents a review of our data and the current status on synthesis and processing of Bi-2223 superconducting ceramic, by spray frozen, freeze drying method (SF-FD). By this method powders of 200-300 nm were obtained and processed by several different routes to the final superconducting product. Optimization process and comparative analysis with the literature data revealed intrinsic problems associated to the specific nature (i.e high degree of mixing and large surface area relative to the volume) of the SF-FD material. The paper focuses on the effects induced by the specific features of the SF-FD material, sometimes opposite to those observed in the conventional materials produced by solid-state method. It resulted that in order to take full advantage of the specific features for a nano SF-FD material, it is necessary to consider a special, non-conventional, processing approach and some examples in this regard will be presented.

The subject of magnetic anisotropy in nanostructured materials is of considerable interest from both experimental and theoretical points of view. Systems that consist of either isolated or interacting nanoparticles are shown to exhibit different magnetic properties of the nanoparticle surface than those of the bulk. This altering of magnetic behavior is mainly related to surface spins structure cot-related with finite size effects, providing that the nanoparticles are sufficiently small so that the surface-to-volume atomic ratio would be high enough. This work reports on both the simulation and experimental approach on the magnetic behavior of ferromagnetic nanoparticles. We investigate the competition between surface and bulk magnetocrystalline anisotropy in small magnetic particles. The experimentally obtained magnetic data for ferromagnetic core-shell-type AgCo nanoparticles are interpreted consistently with the results of Monte Carlo simulation of the magnetic behavior of single domain nanoparticles.

From ac susceptibility measurements of the [Bi(Pb)](2)Ca2Sr2Cu3Oz (Bi-2223) samples doped with LiCl, we have found and report for the first time anomalously suppressed superconductivity (T, is anomalously decreasing) in the slightly under-doped region. For our BPSCCO/(LiCl)(y) samples this region is situated around y = 0.07. Hole concentration (p) per number of Cu-O planes of the Bi-2223 unit cell, determined from room temperature thermopower (S-300 K) measurements, is approximately 0.15 when y is within anomalous region. Literature data shows that for the La-based cuprates, similar suppression of superconductivity was observed at p similar to 0.12 (so-called "1/8 problem"). This discrepancy would appear because charge ordering phenomenon (probably induced by Li known as a pair breaker as e.g. Zn) may be accompanied by changes in oxygen content or oxygen ordering effects. (C) 2003 Elsevier B.V. All rights reserved.

Those nanocrystalline materials which consist of ferromagnetic nanograins embedded in a ferromagnetic matrix were modelled as a cubic lattice composed of a central sphere with strongly coupled spins surrounded by weakly coupled spins. The magnetic behaviour was studied by Monte Carlo simulation, especially the low temperature spin ordering and features exhibited in the temperature range between the Curie temperatures of the two phases. The magnetization and magnetic susceptibility are calculated as a function of temperature for different values of the interfacial exchange interactions. It is shown that the exchange coupling between matrix and nanograin influences the magnetic properties of the nanograin, the matrix and the interfacial regions differently. The magnetic behaviour of different regions has been explained in terms of a polarization mechanism acting on the surface and in the core, leading to magnetic correlation between the spins. Such features are quite consistent with the experimental results obtained on nanocrystalline alloys.

The nanocrystallization of two soft magnetic amorphous Finemet-type ribbons of the nominal composition Fe73.5Cu1Nb3Si13.5B9 and Fe73.5Cu1Gd1Nb2Si13.5B9 synthesized by a melt spinning technique is reported. The crystallization process was monitored by differential scanning calorimetry (DSC) that allows identification of the primary crystallization temperature as well as the other exothermic events occurring during crystallization process. The resulting phase structure of the properly annealed samples is investigated by means of X-ray diffraction (XRD) and transmission electron microscopy (TEM) and is shown to consist of size-distributed body-centred cubic (bcc) alpha-Fe(Si) (rather than Fe3Si, DO3-type), nanograins, embedded into the amorphous Nb-rich residual matrix. The degree of dispersion of the nanograins in the amorphous matrix is estimated from the nanocrystalline volume fraction and the mean size of the nanograins obtained both from XRD data and TEM images. (C) 2003 Elsevier B.V. All rights reserved.

Magnetic behavior of the isolated ferromagnetic bimetallic nanoparticles is investigated in relation with the competition between exchange interaction and surface and bulk anisotropies. Experimental results on AgCo bimetallic nanoparticles are presented and correlated with Monte Carlo simulation of low temperature spin ordering. The nature of spin disorder at the surface, responsible for the reduction of the overall saturation magnetization is briefly discussed. (C) 2003 Elsevier B.V. All rights reserved.

Antiferromagnetic FeSn2(001) thin films with different thicknesses and relatively low Neel temperatures were grown on InSb(001)(4x2) surfaces by molecular-beam epitaxy. The Neel temperature could be increased to above room temperature by subsequent thermal annealing. In situ structural characterization was performed by high- and low-energy electron diffraction. The degree of the structural (001) texture as a function of the preparation and annealing conditions was investigated by x-ray diffractometry. The local magnetic properties and the spin structure were studied using Fe-57 conversion electron Mossbauer spectroscopy (CEMS) at different temperatures. The epitaxial FeSn2(001) thin films exhibit in-plane Fe spin orientation and appear to be suitable antiferromagnets for studying the interfacial spin structure in exchange-biased bilayers by CEMS. (C) 2003 American Institute of Physics.

The melt spun nanocrystalline alloys such as Fe73.5Cu1Nb3Si13.5B9 (FINEMET) have attracted great deal of interest in recent years due to their excellent soft magnetic properties mostly related to the exchange coupling between nanograins, through the amorphous matrix. The microstructural evolution of both nanocrystalline and amorphous residual phases during annealing of the ribbons give rise to crystallization products that determine the expected magnetic properties for specific applications. Upon addition of the rare earth (RE), the evolution of the crystalline phases which emerge from metastable precursors during annealing is investigated. It is expected that the RE presence should strongly modify both the phase structure via new ternary metastable precursors and the magnetic properties of the ribbons by inducing enhanced exchange correlation due to novel ternary crystalline phases.

The fundamental and third-harmonic susceptibility response of YBa2Cu3O7-delta (YBCO) and TlBa2Ca2Cu3O10-y (Tl:1223) high-critical temperature (T-c) superconductors with preferentially oriented crystallites has been measured in various magnetic fields up to 14 T, and with very low ac field amplitude, to probe the melting transition of the vortex matter. This method does not require large single crystals, being therefore suitable for high-T-c superconductors with small crystallites, prepared with high-pressure synthesis techniques. We determine experimentally the melting line of vortices and the anisotropy factor in high-pressure-grown Tl:1223 with a high T-c of 133.5 K. Results from similar measurements on YBCO with preferentially oriented grains are consistent with both commonly accepted models of the melting line and values of the anisotropy factor, supporting the validity of our approach. (C) 2003 American Institute of Physics.

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.