National Institute Of Materials Physics - Romania

Publications

892 articles found

681. Giant magnetoresistance effects in correlation with local magnetic interactions in Fe-Cu and Co-Cu granular thin films prepared by thermionic vacuum arc method

Authors: Lungu, CP; Mustata, I; Lungu. AM; Brinza, O; Zaroschi, V; Kuncser, V; Filoti, G; Ion, L

Published: OCT 2005, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 7, 2512, DOI:

The simultaneous discharge of Fe (or Co) and, respectively, CU metal sources via thermionic vacuum arc (TVA) was used as a new processing method to obtain granular magneto-resistive films. The nano-structured films with thicknesses of 120 +/- 1 nm and 250 +/- 1 nm were obtained in definite conditions and subsequently characterized via electrical measurements at room temperature, in applied magnetic field. The paper reveals the importance of different specific factors (from size and dispersion effects to annealing influence) on the magnetoresistance behavior. A special contribution of the Mossbauer Spectroscopy, concerning the local structure and interactions as well as the magnetic phase characterization, was emphasized.

682. Optical and electronic properties of the aluminophosphate glasses doped with 3d-transition metal ions

Authors: Elisa, M; Grigorescu, CEA; Vasiliu, C; Bulinski, M; Kuncser, V; Predoi, D; Filoti, G; Meghea, A; Iftimie, N; Giurginca, M

Published: OCT 2005, REVIEWS ON ADVANCED MATERIALS SCIENCE, 10, 374, DOI:

Aluminophosphate glasses doped with Fe, Mn, and Cr have been obtained by a wet non-conventional method. Structural information was provided by IR absorption spectra in the range 2000-500 cm(-1). The optical behaviour (transmission and refractive index) of the samples has been studied by UV-VIS-NIR spectroscopy. The Fe valence state and the local coordination were also analysed via Fe-57 Mossbauer spectroscopy, whose data revealed the redox equilibrium in the Fe-doped glasses according to the redox potentials of the transition ions.

683. Fe-coverage-induced out-of-plane spin components of the antiferromagnetic spin structure in exchange-biased Fe/FeSn2 bilayers

Authors: Stromberg, F; Keune, W; Kuncser, VE; Westerholt, K

Published: AUG 2005, PHYSICAL REVIEW B, 72, DOI: 10.1103/PhysRevB.72.064440

Exchange-coupled Fe/FeSn2(001) bilayer systems consisting of a polycrystalline ferromagnetic Fe layer grown on an epitaxial antiferromagnetic FeSn2 layer have been prepared by molecular beam epitaxy and investigated by Fe-57 conversion electron Mossbauer spectroscopy and superconducting quantum interference device magnetometry. The systems show a significant exchange bias effect at low temperatures. Tracer layers of Fe-57 (in the Fe layer) and (FeSn2)-Fe-57 (in the FeSn2 layer) have been placed in the samples in order to probe the spontaneous spin orientation at different distances from the Fe/FeSn2 interface. The Fe spins in the ferromagnetic layer are preferentially oriented in the interfacial plane. In as-prepared samples the presence of the Fe top layer induces a striking out-of-plane component of the interfacial Fe spins in the antiferromagnetic FeSn2 film. This perpendicular component decreases in magnitude at a larger distance from the interface. A reorientation transition from out-of-plane toward in-plane spin orientation was observed in the interfacial FeSn2 layer with increasing age of the sample. This effect is correlated with an increased magnitude of the exchange bias field for the aged samples.

684. The influence of neutron irradiation on (B0.65C0.35)Ba1.4Sr0.6Ca2Cu3Oz superconducting phase: The role of the grain edge

Authors: Mihalache, V; Totovana, A; Sandu, V; Popa, S; Aldica, G; Iyo, A

Published: AUG 2005, JOURNAL OF SUPERCONDUCTIVITY, 18, 467, DOI: 10.1007/s10948-005-0029-7

Using the transport and magnetization measurements, the influence of neutron irradiation at a fluence of 5 x 10(17) n cm(-2) on (B0.65C0.35)Ba1.4Sr0.6Ca2Cu3Oz has been investigated. The neutron irradiation was found to decrease critical temperature and transport critical current density, increase the residual and normal state resistivity, and improve the intragranular critical current density with 1.6 x 10(5) A/cm(2) (at 77.3 K and in the applied field up to 160 kA m) and Delta M-irr/Delta M-nonirr ratio (up to factor of 3) at highest field used for investigation. The field dependence of this ratio, which is below the unity at very low field but higher than I at high fields, correlated with the shape of the hysteretic loops as well as with the change of the transport parameters after irradiation suggests the role of the irradiation-induced effects on the grain edges. We discuss these effects in the framework of the Bean-Livingstone surface barriers and geometrical barriers.

685. Preparation of stacked structures based on HTS by MOCVD: growth problems of c-axis and non c-axis BiSrCaCuO/Bi4Ti3O12 heterostructures

Authors: Endo, K; Badica, P

Published: JUN 2005, IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, 15, 3069, DOI: 10.1109/TASC.2005.848741

Several articles report two-layer c-axis heterostructures of Bi4Ti3O12 on BiSrCaCuO. However in some applications, or for integration purposes, multi-layer geometries composed of alternating c-axis or non c-axis thin films might be useful. From this perspective we investigated growth of c-axis and non c-axis BiSrCaCuO on Bi4Ti3O12 structures. The paper mainly focuses on evaluation of the structures from the growth, stability/interdiffusion, uniformity/morphology points of view based on structural and microstructural data.

686. Magnetically modulated microwave absorption investigation of Bi1.8Pb0.4Sr1.8Ba0.2Ca2Cu3Ox/(LiF)(y) superconducting system

Authors: Velter-Stefanescu, M; Duliu, OG; Mihalache, V

Published: JUN 2005, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 7, 1561, DOI:

The effect of LiF content on the 2223 phase of the high temperature superconducting system Bi1.8Pb0.4Sr1.8Ba0.2Ca2Cu3Ox/(LiF)(y) (y = 0.02 to 0.15) was investigated by means of magnetically modulated microwave absorption (MAMMA). The experimental results confirmed a two fold increase of the superconducting phase content together with a slight rise (around 2 %) of the critical temperature for y=0.07. At the same time, for y between 0.05 and 0.12 the -Delta T-c(mw)/Delta B ratio was practically constant and equal to (18.5 +/- 1.5) KT-1, a typical value for 2223 superconducting phase.

687. Micro-structure and magnetic properties of Fe-Cu nanocomposites for anisotropic permanent magnets

Authors: Kuncser, V; Valeanu, M; Lifei, F; Predoi, D; Jianu, A; Kappel, W; Codescu, M; Patroi, E; Pasuk, I; Bulinski, M; Filoti, G

Published: MAY 31 2005, JOURNAL OF ALLOYS AND COMPOUNDS, 395, 6, DOI: 10.1016/j.jallcom.2004.11.059

Melt spun ribbons of FexCu100-x were obtained by different procedures, in order to maximize the dispersion of the Fe atoms in the Cu matrix. Subsequent thermal and mechanical treatments were used for a controlled crystallization process of the soft magnetic phase. A detailed analysis of structural aspects and crystallization dynamics and their relation with the magnetic behavior of the Fe-Cu ribbons has been obtained via Mossbauer spectroscopy, X-ray diffraction (XRD) and magnetic measurements. In order to obtain composite permanent magnets with shape anisotropy, the optimal conditions and the suitable processing were established. © 2004 Elsevier B.V. All rights reserved.

688. Magnetic and Mossbauer spectral studies of R3Fe29-x,MOx compounds (R = Y, Nd, Sm, Gd, Tb, and Dy)

Authors: Lazar, DP; Valeanu, M; Galatanu, A; Leonovici, MR; Dafinei, A; Ion, L

Published: APR 19 2005, JOURNAL OF ALLOYS AND COMPOUNDS, 392, 39, DOI: 10.1016/j.jallcom.2004.09.012

The investigation of the structure, magnetic and Mossbauer properties for the series of R-3(Fe,MO)(29) compounds has been performed, where R = Y, Nd, Sm, Gd, Tb, and Dy. The crystallographic structure of the ternary phase compounds has been investigated by Rietveld refinement of the X-ray diffraction patterns obtained at room temperature. The quality of the single-phase compounds was also checked by thermomagnetic measurements, from room temperature to above the Curie temperatures. From the magnetic isotherms for the free powder samples, measured at 4.2 K, the saturation magnetizations and the iron average magnetic moments have been derived. Fe-57 Mossbauer spectra of the R3 (Fe,MO)29 compounds have been measured at 15 K. The analysis of spectra, in a model which takes into account both the Fe atom nearest neighbor numbers and the Fe-Fe nearest neighbor bond lengths, indicates that the transferred contribution to the hyperfine field at the iron sites, due to rare earth moments, can be correlated with the rare earth effective spin. (c) 2004 Elsevier B.V. All rights reserved.

689. Reply to comment on 'magnetic relaxation phenomena and inter-particle interactions in nanosized gamma-Fe2O3 systems'

Authors: Predoi, D; Kuncser, V; Filoti, G

Published: APR 6 2005, JOURNAL OF PHYSICS-CONDENSED MATTER, 17, 2254, DOI: 10.1088/0953-8984/17/13/N02

690. Anisotropic vortex channeling in YBa2Cu3O7-delta thin films with ordered antidot arrays

Authors: Crisan, A; Pross, A; Cole, D; Bending, SJ; Wordenweber, R; Lahl, P; Brandt, EH

Published: APR 2005, PHYSICAL REVIEW B, 71, DOI: 10.1103/PhysRevB.71.144504

Scanning Hall probe microscopy has been used to make a microscopic study of flux structures and dynamics in yttrium barium copper oxide thin-film disks containing a regular 10-mu m-period square array of 2.5-mu m-sized holes (antidots). Images obtained after field cooling the sample to 77 K in very low fields reveal that the holes can trap two flux quanta at this temperature. Scans obtained after zero-field cooling (ZFC) to 77 K and a subsequent applied field cycle clearly display preferential flux channeling along chains of antidots in the direction of maximum induction gradient. Remarkably, upon reversal of field sweep direction, we observe flux "streaming" out of the holes towards the sample edges with almost uniform density flux "stripes" bridging the holes in the exit direction. We estimate that the antidots can preferentially trap about 15 flux quanta in these ZFC experiments. Classical electrodynamics simulations of our samples appear to be in good qualitative agreement with our results, indicating that many of the observed phenomena may be geometrical effects that depend primarily on the shape and topology of the sample, and potential applications are discussed.