National Institute Of Materials Physics - Romania

Superconducting samples with a starting composition Cu0.5TlxBa2Ca3Cu4O12 (x = 1-0.35) were prepared in Au tubes from oxides by using different heating rates. The optimum heating rate was 0.11 degreesC min(-1) applied between 860 and 880 degreesC. These samples show the highest content of 1234 phase and the lowest of Ba-Cu-O phases as well as the highest critical temperatures. The highest values were T-c = 119.2 K and T-c0 = 115.1 K. All samples show J(c) values around 6 x 10(5) A cm(-2), at 60 K and 0.5 T. In the rest of the samples, heated by using lower or higher heating rates, a high concentration of 1223 and/or 1245 phases is detected. The content of the 1234 phase and of Ba(Ca.Tl)-Cu-O residual liquid phase, in the samples synthesized for the optimum heating rate, depends on the Tl content in the starting mixture. The influence of the oxygen content in the starting mixture on phase composition is not as strong as of Tl, but it has a major role in establishing the superconducting behaviour of the 1234 grains. Some arguments that suggest the dependence of the transport properties of the non-superconducting matrix on oxygen content will be discussed. In our technological arrangement the determined optimum heating rate can be applied to the synthesis of other single-layered superconducting phases, e.g. 1212 phase from a Cu0.25Tl0.75Ba2CaCu2O8.115 starting composition.

After a presentation of structural aspects of iron-based nanocrystalline alloys, the magnetic behaviours of those two-phase magnetic structures are conceptually discussed at high temperatures as a function of the crystalline volumetric fraction, on the basis of the magnetic correlation length compared to the distances between crystalline grains. In addition, some results predicted by an approach based on Monte-Carlo simulation are then briefly presented: they show a qualitative agreement with experimental results.

Temperature and time dependence of the phase formation in the (Cu-0.5,Tl-0.5)-1234 composition have been studied. The liquid-vapour deposition mechanism for the formation of 12(n - 1)n superconducting phases is presented. XRD, SEM, EDS and weight loss data are discussed. Samples were superconducting with maximum T-c and J(c) (at 60 K and 0.5 T), determined from M(H) loops of 118 K and 4 x 10(5) A cm(-2), respectively.

The size-broadened profile given by the lognormal and gamma size distributions of spherical crystallites is considered. An analytical approximation for the size-broadened profile is derived that can be analytically convolved with the strain-broadened and instrumental-broadened profiles. The method is tested on two CeO2 powders; one shows 'super-Lorentzian' profiles that were successfully modelled under the assumption of a broad lognormal size distribution. It is shown that the Voigt function, as a common model for a size-broadened profile, fails for both very narrow and broad size distributions. It is argued that the size-broadened line profile is not very sensitive to variations in size distribution and that an apparent domain size or even column-length distribution function can correspond to significantly different size distributions.

The dispersion curves of the phonons propagating in polycrystalline suspensions of hard-sphere colloids are measured by Brillouin scattering and calculated by band structure techniques. Acoustic-like and optical-like experimental phonon dispersion relations are attributed to the interplay of the solvent matrix mode with the single-sphere vibration eigen-modes in these lattice structures.

Thin films of Tl(Ba, Sr)(2)Ca2Cu3Oy superconductor have been grown on CeO2 buffered r-cut sapphire substrate with the use of a template TlSr2CaCu2Oy by the amorphous phase epitaxy method. By using this template containing Sr, we have overcome the problem of chemical reactions between Ba and Cc present in the superconducting and buffer layers, respectively, at the normal processing temperatures of 840-890 degreesC. Films grown by this method have an excellent c-axis orientation as well as in-plane alignment with the TlSr2CaCu2Oy template, which in turn is aligned with the CeO2 layers. These films have superconducting critical temperature values up to 104 K and critical current density values as high as 1 MA cm(-2) at 77.3 K and 0 T.

The magnetic behavior of nanocrystalline alloys has been modeled using atomic Monte Carlo simulation. The model consists of a cubic lattice composed of a ferromagnetic nanograin in a ferromagnetic matrix. The magnetizations of nanograin core, surface and interface regions and matrix were studied as a function of the exchange coupling between the nanograin and the matrix, as well as of the nanograin/matrix volume ratio, equivalent to the crystalline fraction in the nanocrystalline alloys. The mechanism of polarization of the matrix by fields penetrating from the nanograin is discussed and correlated with the matrix-nanograin exchange coupling. Competition between interface anisotropy and magnetocrystalline anisotropy produces spin-glass-like magnetic order of the interfacial regions. (C) 2002 American Institute of Physics.

Opposite to the well-known statistical limit, we study a semiconductor quantum system with only one relevant resonance which is in contact with a probe acting as a particle reservoir. We find that the quasibound state that exists in the nearly closed system develops at a transition to the open system into a separate type of resonance. In contrast to the quasibound state (i) it is localized in the space between the probe and the isolated quantum system, (ii) its energy lies in the classically allowed regime, and (iii) its line shape is strongly asymmetric. Excellent quantitative agreement shows that this transition is seen in capacitance experiments on MIS (metal-insulator-semiconductor)-type semiconductor heterostructures in which a field-induced two-dimensional electron gas is formed.

The magnetic properties of a ferromagnetic cylinder containing an antiphase boundary with antiferromagnetic interactions are studied using atomic Monte Carlo simulation. The approach to saturation shows a reduced magnetization compared with the completely ferromagnetic sample, even in huge applied fields. The magnetization profiles are studied as a function of the number of ferromagnetic planes in the cylinder with 80 atoms per plane and also as function of the ratio of the bulk-to-antiphase exchange coupling. Hysteresis appears after removing the applied field for relatively low antiphase boundary densities. It is shown that the antiferromagnetic coupling across the antiphase boundary leads to a progressive rotation of spins as the two half "domain walls" at the boundary with opposite chirality narrow with increasing applied field. (C) 2002 American Institute of Physics.

The effect of surface anisotropy on the magnetic ground state of a ferromagnetic nanoparticle is investigated using atomic Monte Carlo simulation for spheres of radius R=6a and R=15a, where a is the interatomic spacing. It is found that the competition between surface and bulk magnetocrystalline anisotropy imposes a "throttled" spin structure where the spins of outer shells tend to orient normal to the surface while the core spins remain parallel to each other. For large values of surface anisotropy, the spins in sufficiently small particles become radially oriented either inward or outward in a "hedgehog" configuration with no net magnetization. Implications for FePt nanoparticles are discussed. (C) 2002 American Institute of Physics.