National Institute Of Materials Physics - Romania

Mg-Ni-Fe magnesium-rich intermetallic compounds were prepared following two distinct routes. A Mg88Ni11Fe1 sample (A) was prepared by melt spinning Mg-Ni-Fe pellets and then by high-energy ball milling for 6 h the obtained ribbons. A (MgH2)(88)Ni11Fe1 sample (B) was obtained by high-energy ball milling for 20 h a mixture of Ni, Fe and MgH2 powders in the due proportions. A SPEX8000 shaker mill with a 10:1 ball to powder ratio was used for milling in argon atmosphere. The samples were submitted to repeated hydrogen absorption/desorption cycles in a Sievert type gas-solid reaction controller at temperatures in the range 520 divided by 590 K and a maximum pressure of 2.5 MPa during absorption. The samples were analysed before and after the hydrogen absorption/desorption cycles by X-ray diffraction and Mossbauer spectroscopy. The results concerning the hydrogen storage properties of the studied compounds are discussed in connection with the micro-structural characteristics found by means of the used analytical techniques. The improved kinetics of hydrogen desorption for sample A, in comparison to sample B, has been ascribed to the different behaviour of iron atoms in the two cases, as proved by Mossbauer spectroscopy. In fact, iron results homogeneously distributed in sample A, partly at the Mg2Ni grain boundaries, with catalytic effect on the gas-solid reaction; in sample B, instead, iron is dispersed inside the hydride powder as metallic iron or superparamagnetic iron.

Thin films of MgB2 on r-cut Al2O3 substrates have been grown by pulsed-laser deposition (PLD) using a Nd-YAG laser (fourth harmonic-266 nm) instead of the popular KrF excimer laser. The growth window to obtain superconducting films is laser energy 350-450 mJ and vacuum pressure with Ar-buffer gas of 1-8/10 Pa (initial background vacuum 0.5-1 x 10(-3) Pa). Films were deposited at room temperature and post-annealed in situ and ex situ at temperatures of 500-780 degrees C and up to 1 h. Films are randomly oriented with maximum critical temperature (offset of resistive transition) of 27 K. SEM/TEM/EDS investigations show that they are mainly composed of small sphere-like particles (<= 20 nm), and contain oxygen and some carbon, uniformly distributed in the flat matrix, but the amount of Mg and/or oxygen is higher in the aggregates-droplets (100-1000 nm) observed on the Surface of the film's matrix. Some aspects of the processing control and dependences on film characteristics are discussed. The technique is promising for future development of coated conductors.

We have studied a portion of H-T phase diagram of the anisotropic Bi2223 bulk system using multi-harmonic ac susceptibility measurements (chi(n)). Based on the behavior of the third harmonic modulus,vertical bar chi(3)vertical bar, the irreversibility line (IL) was obtained from the onset of vertical bar chi(3)vertical bar, and an anomalous peak effect (PE) was observed. It has been shown that the anomalous peak is due to the crossover between a three-dimensional (3D) and a quasi-two dimensional (quasi-2D) peculiarity of vortex dynamics, the crossover magnetic field being H-CR approximate to 0.1T. The obtained portion of B-irr(T) is well described by the melting line of the flux lattice. The 3D flux fluctuation part (low field and high temperature) is described by the nearly parabolic temperature dependence B=B-o(Tc/T - 1)" with n = 1.58 (sample 1) and n = 1.48 (sample 2). The other region (high field and low temperature) is well described by the temperature dependence of the quasi-2D flux fluctuations, T-m(H) = T-m(2D)[1 + b/(In B/B-cr )(1/v)] corresponding to the weak interaction between pancake vortices from adjacent planes. The 2D limit temperature was determined as T-m(2D) = 36.7K (sample 1) and T-m(2D) = 27.3K (sample 2). (c) 2005 Elsevier B.V. All rights reserved.

High quality superconducting thin films of HTS have been grown by MOCVD on substrates with artificial steps of predefined height and width. The surface of the films grown on the steps having width equal to the 'double of the migration length' of the atomic species depositing on the substrate is totally free of precipitates: precipitates are gathered at the step edges where the free energy is lowest. The method has several advantages: it is simple, universal (it is independent of the materials, substrates, deposition technique or application) and allows control of precipitates segregates so that the quality and growth conditions of the films are the same as for the films grown on conventional substrates. The method is expected to result in new opportunities for the device fabrication, design and performance.

This paper presents experimental results concerning the influence of the additives on the physical properties of polymer films. Metal oxides/inorganic salts were used as additives in vinyl-polymers solutions. The physical properties of the metal doped polymer films can be significantly modified by the composition of the doping elements and the curing conditions of the polymer. Morphologic, electronic, magnetic and optic properties of the doped polymers were analysed by AFM. SEM. Mossbauer spectroscopy and optical measurements. The film composition and the deposition processes were optimized to allow a better control of the optical parameters (refractive index. transmission). to reduce the processing temperatures and to improve the chemical sensitivity of the films for sensor applications. These compounds can be easily spin coated onto a variety of directly patterned semiconductor substrates. (C) 2005 Elsevier Ltd. All rights reserved.

In this paper we present a penetration depth study on the newly discovered superconductors Li2Pd3B and Li2Pt3B. Surprisingly, the low-temperature penetration depth lambda(T) demonstrates distinct behavior in these two isostructural compounds. In Li2Pd3B, lambda(T) follows an exponential decay and can be nicely fitted by a two-gap BCS superconducting model with a small gap Delta(1)=3.2K and a large gap Delta(2)=11.5K. However, linear temperature dependence of lambda(T) is observed in Li2Pt3B below 03T(c), giving evidence of line nodes in the energy gap.

Giant magnetoresistive (GMR) coatings on silicon, glass and Kapton were prepared by simultaneous thermionic vacuum are (TVA) discharges in high vacuum conditions. The magnetic metal or alloy (Fe, Co, Ni, Permalloy) together with the noble non magnetic metal were deposited on substrates having different positions relative to the discharges. Local magnetic interactions and Fe-phase composition were obtained by Mossbauer spectroscopy whereas the magnetoresistance effects were measured by a dc method using a four-point configuration with perpendicular to plane magnetic fields up to 0.8 T. The surface morphology of the coatings was characterized by atomic force microscopy (AFM) in contact mode.

Magnetization M(H,T) and M(T,H) curves with magnetic field approximately parallel and perpendicular to the c-axis of Na0.35CoO2.1.3H(2)O superconducting crystals have been measured (T-c = 3.7 K). The shape of the M(H,T) loops resembles Bi-based high-T-c superconductors (HTS), but the superconducting anisotropy is below that of Bi-system; this parameter is as for HTS with middle or moderately high anisotropy. (c) 2006 Elsevier Ltd. All rights reserved.

A scanning Hall probe microscope (SHPM) with an effective spatial resolution of 1 similar to mu m has been used to study the local induction in high quality superconducting Bi2Sr2CaCu2O8+delta single crystals at high temperatures and low magnetic fields. We observed, for the first time to our knowledge, an anomalous splitting of the peak of first full penetration of magnetic field, We discuss the observed splitting, which is connected to the effects of surface and geometrical barriers on the vortex lattice.

Scanning Hall probe microscopy and magnetometry have been used to study pancake vortices (PV's) in highly anisotropic Bi2Sr2CaCu2O8+delta (BSCCO) thin films. For small perpendicular applied fields we observe well-defined PV stacks which are randomly pinned due to a high density of relatively strong pinning centers in the films. When a strong in-plane magnetic field is added images reveal stripelike flux structures which are quite closely aligned to the in-plane field direction. We conclude that this is a manifestation of Josephson vortex (JV) "decoration" by PV's, which was recently demonstrated in highly ordered BSCCO single crystals in the crossing lattices regime, allowing independent estimates of the anisotropy parameter to be made. This interpretation is supported by the observation that the presence of an in-plane field suppresses the irreversible magnetization of thicker films. Estimates of the JV-PV crossing force and the PV pinning force are found to be comparable in magnitude and fully consistent with this scenario. We believe that this is the first confirmation of the existence of interacting crossing vortex lattices in strongly pinning BSCCO thin films and could find potential applications in superconducting devices.