National Institute Of Materials Physics - Romania

Publications

892 articles found

781. Exchange coupling effects in nanocrystalline alloys studied by Monte Carlo simulation

Authors: Crisan, O; Labaye, Y; Berger, L; Coey, JMD; Greneche, JM

Published: MAY 15 2002, JOURNAL OF APPLIED PHYSICS, 91, 8729, DOI: 10.1063/1.1456420

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.

782. Monte Carlo simulation of the magnetization of a ferromagnet with antiphase boundaries

Authors: Berger, L; Labaye, Y; Crisan, O; Greneche, JM; Coey, JMD

Published: MAY 15 2002, JOURNAL OF APPLIED PHYSICS, 91, 7636, DOI: 10.1063/1.1456404

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.

783. Surface anisotropy in ferromagnetic nanoparticles

Authors: Labaye, Y; Crisan, O; Berger, L; Greneche, JM; Coey, JMD

Published: MAY 15 2002, JOURNAL OF APPLIED PHYSICS, 91, 8717, DOI: 10.1063/1.1456419

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.

784. Strong reduction of thermally activated flux jump rate in superconducting thin films by nanodot-induced pinning centers

Authors: Crisan, A; Badica, P; Fujiwara, S; Nie, JC; Sundaresan, A; Tanaka, Y; Ihara, H

Published: MAY 13 2002, APPLIED PHYSICS LETTERS, 80, 3568, DOI: 10.1063/1.1478782

From frequency-dependent ac susceptibility studies of (Cu,T1)BaSrCa2Cu3Oy superconducting thin films, with and without nanodot-induced artificial pinning centers, we estimated the activation energy of flux jumps. The result was that, in the film with nanodots, the pinning potential is several times higher, leading to a probability of thermally activated flux jumps several orders of magnitude lower than in the film without artificial pinning centers. We suggest that our no cost straightforward method for creating extended defects can be successfully employed for the reduction of thermal noise in superconducting electronic devices. (C) 2002 American Institute of Physics.

785. Crystallization processes and resulting phase structure of Sm-Fe-B melt-spun ribbons

Authors: Crisan, O; Le Breton, JM; Machizaud, F; Labaye, Y; Filoti, G

Published: APR 2002, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 242, 1299, DOI: 10.1016/S0304-8853(01)01268-9

The crystallization behaviour of amorphous melt-spun SmxFe80-xB20 ribbons is investigated by differential scanning calorimetry, X-ray diffraction and Mossbauer spectrometry. On increasing the Sm content. the crystallization temperature reaches a maximum for a critical Sm content (x(c) approximate to 8) and the main crystallization products are different below and above x,. This is attributed to a change in the short-range local order in the amorphous alloy. (C) 2002 Elsevier Science B.V. All rights reserved.

786. Magnetic properties of nanocrystalline materials: A Monte Carlo simulation

Authors: Crisan, O; Labaye, Y; Berger, L; Greneche, JM

Published: FEB 23 2002, PHYSICA STATUS SOLIDI A-APPLIED RESEARCH, 189, 848, DOI: 10.1002/1521-396X(200202)189:3<845::AID-PSSA845>3.0.CO;2-#

The magnetic properties of interfacial regions between ferromagnetic nanograins and the residual matrix in nanocrystalline materials, are modelled using Monte Carlo simulation. The exchange coupling between matrix and nanograin influences differently the nanograin surface and the interface between nanograin and matrix. This behaviour has been explained in terms of a polarisation mechanism of the interfacial zones leading to magnetic correlation between spins of nanograin surface and core.

787. Decomposition of Ca : Cu=1 : 1 nitrate powder: thermal analysis and structural studies

Authors: Badica, P; Aldica, G; Crisan, A

Published: FEB 1 2002, JOURNAL OF MATERIALS SCIENCE, 37, 594, DOI: 10.1023/A:1013777826319

Thermal decomposition processes in mixtures of Ca : Cu = 1 : 1 nitrate powders produced by a conventional technique and by freeze drying method have been investigated by thermal analysis and X-ray diffraction. Results have been compared with literature values and data obtained by our investigations of individual calcium and copper nitrates. Temperatures at which decomposition processes occur change with by +/-30degreesC and depend also on the level of mixing of the nitrates. An exemption is the temperature for CuO phase formation, which was determined for the Cu-nitrate and for both 1 : 1-nitrate powders to be at 266degreesC. The domains of stability for different phases are shown to be variable. Also, for 1 : 1-powders, alpha-type and beta-type Ca(NO3)(2) . 2H(2)O phases coexist, while in Ca-nitrate only the alpha-type phase was observed. (C) 2002 Kluwer Academic Publishers.

788. Reactive-field-assisted-sintering of freeze-dried powders in the BSCCO system

Authors: Badica, P; Aldica, G; Groza, JR; Bunescu, MC; Mandache, S

Published: JAN 2002, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 15, 42, DOI: 10.1088/0953-2048/15/1/307

The reactive-field-assisted-sintering technique (RFAST) has been applied to freeze-dried decomposed precursor powders with a starting cation composition of Bi:Pb:Sr:Ca:Cu = 1.7:0.3:2.0:2.5:3.5. A short time (< 15 min) RFAST treatment at 800-900degreesC did not lead to the formation of Bi2Sr2Ca2Cu3O10+x, (2223-phase). But RFAST was favourable to Bi2Sr2CaCu2O8+y, (2212-phase) formation particularly in samples with a low content of the 2212-phase in the precursor powder. Heat treatment (HT) after RFAST showed that the 2223 phase could form more rapidly than in freeze-dried pressed and sintered pellets produced by the traditional technique. The resultant ceramics after 70 h of annealing contain 75% 2223-phase and have T-c(R=0) = 100.7 K, T-c = 109.5 K and DeltaT(c) = 22.4 K.

789. (119) Bi-2223 thin films grown by MOCVD on (100) NdGaO3 and (110) SrTiO3

Authors: Endo, K; Badica, P

Published: 2002, SUPERCONDUCTING AND RELATED OXIDES: PHYSICS AND NANOENGINEERING V, 4811, 139, DOI: 10.1117/12.457698

In high temperature superconductors (HTS) the coherence length along non-c axis directions is longer. This feature can be useful when designing electronics devices based on HTS. Therefore growth and characterization of non-c axis oriented thin HTS films is of great interest. In this paper we present a short review of our data regarding (119) Bi-2223 thin films grown by MOCVD on (100) NdGaO3 and (110) SrTiO3. The emphasis is made on improvement and control of the quality of the films by the "two-temperature" technological approach and/or use of the vicinal substrates. Phase and morphology evolution for different processing conditions, substrate's type and off-angle are presented. The highest critical temperatures of T-c0=67.2 K and T-c0=74 K for the "single-" and "two-" temperature routes were obtained on vicinal SrTiO3 with the off-angle of 20degrees. A higher off-angle promoted the formation of a specific step-like morphology with lower roughness. For the films grown on flat substrates the morphology was of mountain-range shape. Surface morphology as a result of two types of growth mechanisms (two-dimensional (2D), assisted by a so-called "twin"-growth and step-flow growth) for the (119) Bi-2223 films are discussed.

790. The effect of pinning centers in Zn-doped CuBa2Ca3Cu4O12-y high-temperature superconductors

Authors: Crisan, A; Agarwal, SK; Koganezawa, T; Kuroda, R; Tokiwa, K; Watanabe, T; Iyo, A; Tanaka, Y; Ihara, H

Published: JUN-AUG 2002, JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS, 63, 1076, DOI: 10.1016/S0022-3697(02)00030-6

CuBa2Ca3Cu4O12-y (Cu:1234) high-temperature superconductors (HTS) doped with up to 2% Zn were grown using the high-pressure synthesis technique. Magnetization loops of the samples were measured at various temperatures between 5 and 77.3 K and magnetic fields up to 14 T. Critical current densities J(c) of the samples were estimated using the critical state model. The results show that Zn-induced pinning centers increase J(c) of Cu:1234 several times, depending on field and temperature. From the experimentally determined field-temperature region in which a higher Zn concentration lead to a higher J(c), we suggest the existence of a cross-over from quite efficient, extended (in the c-axes direction) pinning centers to point-like (inefficient) pinning centers at a certain temperature, depending on field. This effect can be attributed to the fact that, unlike other HTS, in Cu: 1234 there is a second critical temperature T-c2 of about 70-80 K (in zero field, and 50-60 K in 15 T), related to the over-doping of pyramidal basal plane (outer CuO2 plane). (C) 2002 Elsevier Science Ltd. All rights reserved.