National Institute Of Materials Physics - Romania

Publications

903 articles found

291. Structural, morphological, ferromagnetic and photoluminescence properties of Fe-doped ZnO, prepared by hydrothermal route

Authors: Cernea, M; Mihalache, V; Secu, EC; Trusca, R; Bercu, V; Diamandescu, L

Published: APR 2017, SUPERLATTICES AND MICROSTRUCTURES, 104, 373, DOI: 10.1016/j.spmi.2017.02.048

Fe doped ZnO particles of flower-like shape, hexagonal prisms and hybrid structures flower-prisms have been synthesized by hydrothermal technique, and their luminescence and magnetic properties have been investigated as a function of the morphology changes due to iron dopant (0-0.03 at.% Fe3+). The X-ray diffractograms of Zn1-xFexO powders indicated a hexagonal wurtzite polycrystalline structure. SEM images reveal the change of Zn1-xFexO grains shape from flower-like to hexagonal prisms as the Fe concentration (x) increases from 0 to 3 at.% Fe. Undoped ZnO shown weak room temperatures ferromagnetism, with high coercivity (H-c = 107 Oe) and saturation magnetization M-s of 1.5-10(-3) emu/g. ZnO doped with 1 and 3 at.% Fe presented a significant increase of the magnetization in comparison with the undoped ZnO. For ZnO doped with 3 at.% Fe, Ms = 32.5-10(-3) emu/g and M-rem = 0.78-10(-3) emu/g. Compared with other reports on magnetic properties of undoped and Fe doped ZnO, these results indicated higher coercivity and smaller magnetizations. The drop in the intensity of characteristic green-yellow photoluminescence band of ZnO at about 550-600 nm was attributed to the decrease of the number of oxygen vacancies and interstitial oxygen. By increasing the Fe concentration, the electron paramagnetic resonance (EPR) signal of undoped ZnO decreases due to the decrease of defects concentration. (C) 2017 Elsevier Ltd. All rights reserved.

292. Specific Changes in the Magnetoresistance of Ni-Fe-Ga Heusler Alloys Induced by Cu, Co, and Al Substitutions

Authors: Tolea, F; Tolea, M; Sofronie, M; Popescu, B; Crisan, A; Leca, A; Valeanu, M

Published: APR 2017, IEEE TRANSACTIONS ON MAGNETICS, 53, DOI: 10.1109/TMAG.2016.2628386

In this paper, we investigate the influence of Cu, Co, and Al substitutions on the transport properties, and in particular, the magnetoresistive effect in Ni-Fe-Ga ferromagnetic shape memory alloys (FSMAs) prepared as ribbons by melt spinning method and subjected to different thermal treatments. X-ray diffraction, differential scanning calorimetry, magnetometry, and magnetoresistive characterizations were performed. In the range of the martensitic transformation (MT), different FSMA compositions show a rich spectrum of different behaviors. For one of the compositions (Ni52Fe20Co2Ga23Al3), the magnetoresistance (MR) showed a local minimum or, on the contrary, a local maxima of reduced amplitude on cooling, in the range of the MT, depending on the performed thermal treatments. In the same composition, by replacing one Al atom with a Co one, no local extremes are seen, the alloy having a concomitant magneto-structural transition. When the magnetic field was varied, the MR showed a nonmonotonic variation in the martensite phase for some compounds, possibly due to the dynamics of the martensite variants realignment. From the studied compositions, the highest MR found on the MT of -9% for 5 T is for Ni50Fe20Ga27Cu3.

293. From an Anomalous Peak Effect to a Second Magnetization Peak in Nb-rich Nb-Ti Alloys

Authors: Ionescu, AM; Ivan, I; Enculescu, M; Grigoroscuta, M; Miu, D; Valeanu, M; Badica, P; Miu, L

Published: APR 2017, JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM, 30, 1108, DOI: 10.1007/s10948-016-3670-4

We investigated the evolution of the DC magnetic hysteresis curves of Nb0.89Ti0.11 alloys thermo-mechanically processed by intermediate heat treatments at 900 degrees C in vacuum and cold rolling (similar to 50% thickness reduction). Starting with a rectangular piece (similar to 0.4 mm thick) cut from the as-grown alloy, after the first thermo-mechanical treatment, the specimen exhibits over a wide temperature T interval a peak effect not far from the DC irreversibility line. With a supplemental thermo-mechanical treatment, the peak effect disappears and is substituted by a second magnetization peak ( where the characteristic fields are significantly lower) induced by pinning enhancement. The second magnetization peak was clearly seen at high temperatures only, due to the occurrence of thermo-magnetic instabilities in the low-T domain. In both cases, analysis of magnetic relaxation evidences a crossover towards plastic vortex creep accompanying the maximum in the effective critical current density. These results suggest a common nature of the observed effects, related to the disordering of the vortex system induced by pinning.

294. Structural, Magnetic, and Mossbauer Investigation of Ordered Iron Nitride with Martensitic Structure Obtained from Amorphous Hematite Synthesized via the Microwave Route

Authors: Palade, P; Plapcianu, C; Mercioniu, I; Comanescu, C; Schinteie, G; Leca, A; Vidu, R

Published: MAR 22 2017, INDUSTRIAL & ENGINEERING CHEMISTRY RESEARCH, 56, 2966, DOI: 10.1021/acs.iecr.6b04574

Amorphous hematite synthesized by a simple and fast microwave route was used to obtain Fe16N2 fine particles by reducing in 5% H-2/Ar gas flow, followed by long time nitridation in ammonia gas flow at temperatures below 200 degrees C. Depending on nitridation temperature, various amounts of metallic iron were present along with the alpha" -Fe16N2 main phase. A small amount of iron oxide was observed by Mossbauer spectroscopy, but it was undetected by X-ray diffraction due to its high degree of amorphization. Increased amounts of Fe3N and Fe4N phases were observed at a nitridation temperature above 150 degrees C, which had a detrimental effect on the magnetic properties. Structural information and phase composition were extracted from Rietveld refinement of the XRD data. Values of the magnetization at saturation measured at 40 kOe and 25 degrees C of 222 emu/g for alpha"-Fe16N2 and 192 emu/g for metallic iron were obtained via magnetic measurements, Rietveld, and Mossbauer analysis.

295. Nano-engineered pinning centres in YBCO superconducting films

Authors: Crisan, A; Dang, VS; Mikheenko, P

Published: FEB 15 2017, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 533, 132, DOI: 10.1016/j.physc.2016.06.011

For practical applications of superconducting materials in applied magnetic fields, artificial pinning centres in addition to natural ones are required to oppose the Lorentz force. These pinning centres are actually various types of defects in the superconductor matrix. The pinning centres can be categorised on their dimension (volume, surface or point) and on their character (normal cores or AK cores). Different samples have been produced by Pulsed Laser Deposition, with various thicknesses, temperatures and nanostructured additions to the superconducting matrix. They have been characterized by SQUID Magnetic Properties Measurement System and Physical Properties Measurement System, as well as by Transmission Electron Microscopy (TEM). Correlations between pinning architecture, TEM images, and critical currents at various fields and field orientations will be shown for a large number of YBa2Cu3Ox films with various types and architectures of artificial pinning centres. (C) 2016 Elsevier B.V. All rights reserved.

296. Magnetite-based glass-ceramics prepared by controlled crystallization of borosilicate glasses: Effect of nucleating agents on magnetic properties and relaxation

Authors: Sandu, V; Cimpoiasu, E; Greculeasa, S; Kuncser, A; Nicolescu, MS; Kuncser, V

Published: FEB 15 2017, CERAMICS INTERNATIONAL, 43, 3413, DOI: 10.1016/j.ceramint.2016.11.188

The specific magnetic structure and magnetic relaxation phenomena in magnetite nanoentities grown in a glassy matrix by controlled crystallization of Fe-containing borosilicate and boroaluminosilicate glasses in the presence of two types of nucleating agents, Cr2O3 and P2O5, were investigated. The structure, morphology and magnetic properties are strongly influenced by the nucleating agents. Cr2O3 generates magnetite-based glass ceramics with magnetite configurations showing an upward relaxation of magnetization at low and high temperatures but downward at intermediate temperatures. The magnetite grown with P2O5 displays only downward relaxation but with different signs of the temperature derivative of the relaxation rate S in different temperature ranges. The observed effects are discussed with respect to the following factors: i) the existence of a multimodal size distribution of the magnetite (nano)particles as revealed by high resolution electron microscopy; ii) the degree of occupation of different sublattices of the magnetite structure with Fe3+ and Fe2+ ions; and the interplay between the relaxation mechanisms in different temperature ranges.

297. A general perspective on the magnetization reversal in cylindrical soft magnetic nanowires with dominant shape anisotropy

Authors: Kuncser, A; Antohe, S; Kuncser, V

Published: FEB 1 2017, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 423, 38, DOI: 10.1016/j.jmmm.2016.09.066

Peculiarities of the magnetization reversal process in cylindrical Ni-Cu soft magnetic nanowires with dominant shape anisotropy are analyzed via both static and time dependent micromagnetic simulations. A reversible process involving acoherent-like spin rotation is always observed for magnetic fields applied perpendicularly to the easy axis whereas nucleation of domain walls is introduced for fields applied along the easy axis. Simple criteria for making distinction between a Stoner-Wohlfarth type rotation and a nucleation mechanism in systems with uniaxial magnetic anisotropy are discussed. Superposed reversal mechanisms can be in action for magnetic fields applied at arbitrary angles with respect to the easy axis within the condition of an enough strong axial component required by the nucleation. The dynamics of the domain wall, involving two different stages (nucleation and propagation), is discussed with respect to initial computing conditions and orientations of the magnetic field. A nucleation time of about 3 ns and corkscrew domain walls propagating with a constant velocity of about 150 m/s are obtained in case of Ni-Cu alloy (Ni rich side) NWs with diameters of 40 nm and high aspect ratio.

298. Sol-gel preparation and structural investigations of silico-phosphate glasses doped with Fe ions

Authors: Sava, BAC; Elisa, M; Boroica, L; Kuncser, V; Valeanu, M; Vasiliu, IC; Feraru, I; Iordanescu, R

Published: JAN 2017, JOURNAL OF SOL-GEL SCIENCE AND TECHNOLOGY, 81, 302, DOI: 10.1007/s10971-016-4192-z

Different aspects related to the sol-gel preparation and structural investigation of silico-phosphate glasses (SiO2-P2O5 system) doped with Fe ions are reported. During the preparation process, tetraethoxysilane was used as a precursor for SiO2 and phosphoric acid (H3PO4-HP) for P2O5. Ferric chloride was used as precursor for Fe ions, water as reagent for hydrolysis reaction and ethylic alcohol as solvent. The pH of the sols was modified by adding hydrochloric acid and ammonia. It was observed that a slight increase of the solution temperature (up to 40 degrees C) allows a drastic decrease of the gelation time (from days to hours). The structure of the obtained powders dried in air at room temperature and at 100 degrees C for 10 h and subsequently thermally treated at different temperatures was investigated by Fourier transformed infrared (FTIR) and Raman spectroscopy. Vibration modes specific to Si-O-Et, Si-OH, PO-P, P-O-Si, hydrogen bonds and H2O, as well as combined modes have been observed. The local structure and electron configurations of the doping Fe ions have been investigated by Fe-57 Mossbauer spectroscopy.

299. Thermal analysis on C6H10Ge2O7-doped MgB2

Authors: Aldica, G; Matei, C; Paun, A; Batalu, D; Ferbinteanu, M; Badica, P

Published: JAN 2017, JOURNAL OF THERMAL ANALYSIS AND CALORIMETRY, 127, 179, DOI: 10.1007/s10973-016-5877-9

Additives to MgB2 can improve the superconducting functional characteristics, such as critical current density (J (c)) and irreversibility field (H (irr)). Recently, we have shown that repagermanium (C6H10Ge2O7) is an effective additive, enhancing both J (c) and H (irr). To look into details of the processes taking place during the reactive sintering, a thermal analysis study (0.167 K s(-1), in Ar) is reported. We used differential scanning calorimetry between 298 and 863 K and simultaneous thermogravimetric-differential thermal analysis between 298 and 1233 K. Samples were mixtures of powders with composition 97 mol% MgB2 and 3 mol% C6H10Ge2O7. Up to 863 K, repagermanium decomposes by multiple steps and forms amorphous phases. A reaction with MgB2 is not observed. Above this temperature, partial decomposition of MgB2 occurs. Crystalline Ge and MgO are detected before formation of Mg2Ge and MgB4, when temperature approaches the melting point of Ge (1211 K). Carbon substitution for boron in the crystal lattice of MgB2 is observed for samples heated above 863 K. The amount of substitutional C does not significantly change with temperature.

300. Tuning the transmission phase by the dot size

Authors: Tolea, F; Tolea, M

Published: JAN 2017, PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, 85, 173, DOI: 10.1016/j.physe.2016.08.034

The phase shift of the electron's wave function after a tunneling event (i.e. the transmission phase) was at first measured for its fundamental or applicative relevance for quantum circuitry, but later the phase study self-motivated due to a number of unexpected results. One such result was the reduced increment of the phase on some resonances - with only fractions of pi - in the few-electrons "mesoscopic" regime. In this paper we address such a regime for a rectangular quantum dot and compute the total phase increase on the first four resonances by means of accurate configuration-interaction method and a generalized Friedel sum rule as proposed by Rontani (2006) [17]. Our findings confirm that the electronic correlations reduce the on-resonance phase growth which is also found to decrease quasi-linearly with the dot size, the decrease being more pronounced as the number of electrons on the dot is raised. Sudden jumps (of small amplitude) of the phase are found to accompany ground states spin transitions. (C) 2016 Elsevier B.V. All rights reserved.