National Institute Of Materials Physics - Romania

Publications

892 articles found

851. Variable rate spin freezing and long range antiferromagnetic order in Bi2FeRhO6

Authors: Filoti, G; Kuncser, V; Rosenberg, M; Schinzer, C; KemmlerSack, S

Published: JUL 5 1997, JOURNAL OF ALLOYS AND COMPOUNDS, 256, 91, DOI: 10.1016/S0925-8388(96)03110-6

A magnetic susceptibility and Mossbauer spectroscopy study of the perovskite Bi2FeRhO6 was undertaken. A freezing start temperature around 270 K was evidenced by both methods. Unlike homogeneous magnetic systems the freezing occurs by involving continuously more and more Fe spins with decreasing temperature over the whole interval down to 4.2 K. At low temperature some evidence for long range antiferromagnetic interactions was found. The Mossbauer data allowed a satisfactory description of the peculiar temperature dependence of the magnetic susceptibility as a superposition of a slain glass antiferromagnetic component and a paramagnetic one. The gradual spin freezing and the absence of percolation derived from the strong influence of the short range interactions, their strengths reflecting the random distribution of the Fe and Rh ions over the B sites of the ABO(3) perovskite. (C) 1997 Elsevier Science S.A.

852. Site occupation and Fe-57 Mossbauer spectra of RE6Fe14-xMx with RE=Nd, Pr and M=Ga, Al

Authors: Kuncser, V; Rosenberg, M; Buschow, KHJ; Filoti, G

Published: JUN 20 1997, JOURNAL OF ALLOYS AND COMPOUNDS, 255, 66, DOI: 10.1016/S0925-8388(96)02851-4

A Mossbauer spectroscopy study of the intermetallics RE6Fe14-xMx where RE=Nd or Pr, M=Ga or Al and x=1 or 2 was undertaken. A comparative analysis of the Mossbauer spectra showed that the second Al or Ga atom substitutes preferentially for the Fe atoms located at the 161 sites. The reduction of the hyperfine fields in Nd6Fe12M2, as compared with Nd6Fe13M, mainly at the 16k, 161, and 161(2) and the corresponding decrease of the average Fe moment gives rise to a total magnetic moment of 41 mu(B) per formula unit in excellent agreement with previous saturation magnetisation measurements.

853. Metastable phase in the Ag-Ge rapidly quenched alloys

Authors: Manaila, R; Zavaliche, F; Popescu, R; Macovei, D; Devenyi, A; Bunescu, C; Vasile, E; Jianu, A

Published: JUN 15 1997, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, 226, 295, DOI: 10.1016/S0921-5093(96)10632-8

Metastable phases can appear in metallic alloys if their surface response function (liquidus temperature versus cooling rate) is less steep than that of the stable ones. in this case, rapid solidification procedures can result in their formation. On the other hand, metastable alloys are expected to yield special microstructures when the equilibrium phase composition is restored by their transformation or decomposition. These microstructures depend on the thermal history of the sample (e.g., the heating rate and temperature). Metastable phase was obtained by melt-quenching in the Ag1-xGex (x = 0.2, 0.35). X-ray diffraction revealed in Ag-Ge samples a metastable h.c.p. The h.c.p. Ag-Ge phase is the Hume-Rothery type. Besides, an alpha-Ag-Ge solid solution was present. Amounts of non-reacted Ge were also found, with a lattice constant close to the ideal value. Differential scanning calorimetry (DSC) runs revealed stability of the Ag-Ge metastable phase up to temperatures of 521 K (x = 2.0) and 626 K (x = 0.35), where exothermic peaks appeared with relatively high heat release (46.6 and 48.8 J g(-1), respectively). Work is in progress on the inhomogeneous lattice distortions in the metastable phases, which could be induced in the rapid solidification process and are expected to affect their stability. (C) 1997 Elsevier Science S.A.

854. Field and temperature dependence of the current-voltage characteristics of Bi2Sr2Ca2Cu3O10/Ag multifilamentary tapes

Authors: Crisan, A; Miu, L; Popa, S; Beduz, C

Published: JUN 1997, JOURNAL OF SUPERCONDUCTIVITY, 10, 219, DOI: 10.1007/BF02770554

The current-voltage characteristics of Bi2Sr2Ca2Cu3O10/Ag multifilamentary tapes were measured at different temperatures close to the critical temperature and in different applied magnetic fields up to 1000 Oe. The data were interpreted in terms of thermally activated flux creep by using an intermediate phenomenological model that takes into account the collective pinning. Temperature and field dependences of the pinning potential and of the collective pinning exponent were determined through numerical analysis.

855. Structural properties of conducting and semiconducting polymers

Authors: Galatanu, A; Chipara, MI; Chipara, MD; Toacsen, M

Published: JUN 1997, PHYSICA B, 234, 244, DOI: 10.1016/S0921-4526(96)00925-8

Electron spin resonance investigations on polyethylene-polyaniline blends are reported. The temperature dependence of resonance line parameters, in the temperature range 270-400 K is investigated in detail.

856. Magnetic ac susceptibility in Bi2Sr2CaCu2O8 single crystals

Authors: Mandache, S; Crisan, A; Aldica, G; Popa, S

Published: JUN 1997, JOURNAL OF SUPERCONDUCTIVITY, 10, 214, DOI: 10.1007/BF02770553

We measured the temperature dependence of the ac susceptibility in Bi2Sr2CaCu2O8 single crystals. The penetration field H* was determined, at different temperatures, from the field dependence of chi '' in the framework of the critical state model. Using a very simple formula, we estimated the values of the critical current density near T-c, which are consistent with the usually reported data for this material.

857. Current-voltage characteristics of Bi2Sr2Ca2Cu3O10+x/Ag multifilamentary tapes in zero applied magnetic field

Authors: Crisan, A; Miu, L; Popa, S; Yang, Y; Beduz, C

Published: MAY 1997, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 10, 303, DOI: 10.1088/0953-2048/10/5/006

Current-voltage characteristics of multifilamentary Bi2Sr2Ca2Cu3O10/Ag tapes (short samples) produced by the 'powder in tube' technique were measured at different temperatures close to the mean-field critical temperature, and in zero applied magnetic field. After performing the required corrections due to the current flowing in the silver matrix, the I-V curves were interpreted in terms of current-induced unbinding of the thermally created vortex-antivortex pairs. Two possible mechanisms for appearance of a finite critical current in zero applied magnetic field are discussed: the Jensen-Minnhagen quasi-two-dimensional (2D) approach, that takes into account the interlayer Josephson coupling, and a model of size limitation of vortex fluctuations. From our analysis, it seems that the latter model is more suitable for this kind of superconducting material, due probably to an accentuated intrinsic anisotropy.

858. The influence of the structure on the electrical and magnetic properties of Bi2Sr2CaCu2O8+delta single crystals

Authors: Aldica, G; Crisan, A; VelterStefanescu, M; Mandache, S; Bunescu, MC

Published: MAR 1 1997, JOURNAL OF MATERIALS SCIENCE, 32, 1199, DOI: 10.1023/A:1018579817471

The correlation between the structure, chemical composition, electrical and magnetic properties of Bi2Sr2CaCu2O8+delta single crystals is presented. As revealed by SEM measurements, the small samples (approximately 1 mm x 1 mm x 0.2 mm) are stacks of thin plates (1-2 mu m). Microcompositional measurements (energy dispersive spectroscopy) show the Bi2Sr2CaCu2O8+delta (2212)-phase and a small quantity of residual phases enriched in bismuth and strontium and calcium-deficient. Magnetically modulated microwave absorption measurements (MAMMA) lead to a value for the anisotropy of the upper critical field of about 7, which is more than twice the reported value for YBa2Cu3O7-delta single crystals (also determined from MAMMA responses). The plot of the resistance versus temperature suggests that only the first 10-20 plates from the surface of the sample have a contribution to the sample conductivity. The imaginary part, chi'', of the magnetic susceptibility of our samples shows three peaks.

859. The influence of sintering environment and intermediate grounding on the scattering of the superconducting characteristics in BSCCO ceramic produced by spray-frozen, freeze drying method

Authors: Badica, P; Aldica, G; Morozov, VV; Popa, S; Mandache, S

Published: DEC 1996, APPLIED SUPERCONDUCTIVITY, 4, 589, DOI:

The effect of intermediate grounding and sintering environment (air or sintered powder) on the superconducting characteristics of Bi(Pb)-Sr-Ca-Cu-O hulk ceramics obtained by spray-frozen freeze drying technique was investigated by X-ray diffraction, electric and magnetic measurements. Intermediate grounding leads to higher scattering coefficients of the superconducting characteristics. This result does not depend on sintering environment. Improved results are obtained when sintering is performed by immersing pellets "in powder" rather than placing them "on powder". The powder used as sintering environment was pyrolized precursor powder calcinated in air for 100 h. (C) 1998 Elsevier Science Ltd. All rights reserved.

860. Structural modifications of the superconducting phases on Bi system by electron beam irradiation

Authors: Aldica, G; Geru, II; Puscasu, BM; Constantinescu, F; Badica, P

Published: JUN 1996, JOURNAL OF SUPERCONDUCTIVITY, 9, 276, DOI: 10.1007/BF00727547

Superconducting samples of the Bi(Pb)-Sr-Ca-Cu-O system have been irradiated in transmission electron microscopy (TEM) to investigate the effect of a relative high fluence of electrons with 75 and 100 keV energy on the microstructure of the material. The diffraction pattern images show a dramatic change from very uniform lattice spots at ab crystalline planes to a circular pattern corresponding to damage and breaking of the materials in very small crystallites.