National Institute Of Materials Physics - Romania

In this work, we present a new, fast and high yield synthesis method for obtaining microcrystalline yttrium orthoferrite YFeO3, based on hydrothermal technique. The advantage of the method consists in the direct crystallization of the material without the necessity of the calcination. The quality of the so-obtained compound is checked by X-ray diffraction and XPS measurements. Unusual behavior of the zero field cooled (ZFC) - field cooled (FC) magnetization curves (ZFC > FC) was also observed in YFeO3 and a possible explanation could be related to the effect of the synthesis methods on the magnetic properties of yttrium orthoferrite. (C) 2014 Elsevier B.V. All rights reserved.

Atomic intermixing processes in relation to structural aspects and phase formation in Be based thin films subjected to different annealing treatments simulating the case of re-deposited layered structures on plasma facing components in nuclear fusion devices are reported. Accordingly, bilayers of Be/W and Be/C have been deposited on Si(001) substrates with Fe buffer layers. The Fe films have been prepared by radiofrequency sputtering and further processed by annealing in hydrogen atmosphere at 300 degrees C, for 90 min, at a pressure of 10 bars of H-2. After the Be/W and Be/C bilayer deposition by means of thermionic vacuum arc method, annealing in vacuum at 600 degrees C, for 10 min has been applied to the complex structures. The influence of annealing on the phase composition and atomic intermixing processes in the complex structures has been studied by means of X-ray photoelectron spectroscopy (XPS) and conversion electron Mossbauer spectroscopy (CEMS). The layered structures present an oxidation gradient with oxide phases in the uppermost layers and non-oxidized phases in the lower layers, as observed from the XPS data. The CEMS results revealed that the as-deposited structures contain a main metallic Fe phase and secondary superparamagnetic Fe oxide phases at the Fe/Be interface, while annealed samples present a large contribution of Fe-Be and Fe-C mixtures. The annealing treatment induces considerable atomic interdiffusion, strongly dependent on the nature of the upper layer. In the case of Be/W system, the annealing provides a much rougher Be/W interface, while in case of the Be/C structure, the annealing treatment only homogenize the structure over the whole depth. (C) 2014 Elsevier B.V. All rights reserved.

Frequency-dependent AC susceptibility measurements have been used to estimate the critical current densities and pinning potential of two 1.1-mu m-thick YBCO films with nanoengineered pinning centres. The films were grown by pulsed laser deposition using a target with 4 % BaZrO3 nanoinclusions. Data were analysed in the framework of the Anderson-Kim model, collective pinning model, and a model due to Zeldov that implies a logarithmic dependence of pinning potential on the current density. It was found that the latter describes better our experimental data, and that the resulting pinning potential has reasonable high values in high magnetic field up to 9 T, indicating a high current-carrying capability in high magnetic fields.

Spark plasma sintering (SPS) was applied for fabrication of dense (relative density > 97 %) bulk MgB2 samples added with Ge. Mixtures of MgB2 and Ge powders with starting compositions (MgB2) Ge (x) , x=0.005, 0.01, and 0.03 were used. Added samples show enhancement of the critical current density at high magnetic fields when compared to a pristine reference sample. The optimum composition is for x=0.005 and for this sample J (c)(20 K) =10(2)A/cm (2) is obtained at 5 T, while for the reference sample is obtained at 3.9 T. Ge does not substitute in the crystal lattice of MgB2 and T (c,onset) or T (c,midpoint) from magnetization measurements scatter within 0.15 or 0.3 K, respectively.

AC susceptibility and DC magnetization relaxation measurements have been performed for YBa2Cu3O7 films containing BaZrO3 nanorods preferentially oriented along the c axis, with the external magnetic field perpendicular to the film surface. It was found that the effective vortex activation energy U (e) supplied by the standard analysis of the frequency dependent AC magnetic response remains very high even in the vicinity of the irreversibility line indicated by the DC relaxation. We show that the large U (e) values appearing in the AC signal are related to the pinning enhanced viscous drag and significant vortex velocities.

The Mossbauer spectra of a FeSe0.3Te0.7 single crystal grown by the Bridgman method were analysed across the superconducting transition by considering the interplay between the structure and electron configuration of the transition metal. The magnetically determined superconducting critical temperature is T-C similar to 14 K. The Fe-57 Mossbauer spectra collected in the temperature range from 5 to 200K mainly have an asymmetric doublet pattern, which was conveniently fitted by the full Hamiltonian method. No effective magnetic moment ascribed to the superconducting phase was observed down to 5K. The unusual behaviour observed below similar to 17K for the chemical isomer shift and quadrupole splitting may be associated with an electron reconfiguration process intimately related to an unusual lattice distortion accompanying the superconducting transition. The decreasing trend of the total absorption spectral area and second-order Doppler shift during cooling the sample below the critical temperature, point to enhanced phonon activation in the superconducting state.

xIrO(2)-(1-x)alpha-Fe2O3(x=0.1, 0.3 and 0.5) nanoparticle systems were successfully synthesized by mechanochemical activation of IrO2 and alpha-Fe2O3 mixtures for 0-12 h of ball milling time. The study aims at exploring the formation of magnetic oxide semiconductors at the nanoscale, which is of crucial importance for catalysis, sensing and electrochemical applications. X-ray powder diffraction (XRD), Mossbauer spectroscopy, magnetic measurements and simultaneous differential scanning calorimetry (DSC) and thermal gravirnetric analysis (TGA) were used to study the phase evolution of xIrO(2)-(1-x)alpha-Fe2O3 nanoparticle systems under the mechanochemical activation process. Rietveld refinement of the XRD patterns yielded the values of the particle size and lattice parameters as function of composition and milling times and indicated the presence of Ir-substituted hematite and Fe-doped iridium oxide for large x values and long milling times. The Mossbauer studies showed that the spectrum of the mechanochemically activated composites evolved from a sextet for hematite to sextets and a doublet upon duration of the milling process with iridium oxide. Magnetic measurements recorded at 5 K in an applied magnetic field of 40,000 Oe showed that the saturation magnetization of the milled samples increased with ball milling time while preserving a multidomain magnetic structure. The unmilled sample at 5 K showed a spinflop type metarnagnetic transition around 30,000 Oe. The Morin transformation was evidenced by zero-field cooling field cooling (ZFC-FC) measurements in 200 Oe and 1 T and the transformation characteristic temperatures were shifted to lower values. (C) 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Ribbons of Fe-Pd ferromagnetic shape memory alloys were prepared by rapid solidification via the melt-spinning technique. The effect of moderate and high temperature heat treatments on the martensitic transformation and the related changes of the magnetic behavior was analyzed by X-ray diffraction, differential scanning calorimetry and temperature, and field-dependent magnetometry. In addition, information about magnetic easy axis and magnetically field-induced strains (MFIS) were achieved from linear thermal expansion measurements performed under cooling at different applied fields. The observed low values of MFIS in the as-prepared ribbons are due, besides to the small size and random orientation of the grains, to the high atomic disorder inside the crystalline grains. The anisotropy field is enhanced by the reduced atomic disorder, as reflected by the increased MFIS values after thermal treatments.

Influence of Nd substitution for Ni on the magnetic properties and the martensitic transformation (MT) characteristics are investigated on Ni57-xNdxFe18Ga25 (x=0 divided by 4) ferromagnetic shape memory alloys (FSMAs) in bulk and also in ribbons prepared by melt spinning method and subjected to different thermal treatments. Increasing the Nd content induces a decrease of both the Curie and the MT temperatures. (C) 2015 The Authors. Published by Elsevier Ltd.

The synthesis routes for polycrystalline bulk Sr2FeMoO6 (SFMO), offer various possibilities, but in all the cases it is difficult to obtain a single phase of this compound. A new challenge in the field is to achieve mono-crystals using different growing routes and the Bridgman method represents one of them. In order to establish the optimal conditions of mono-crystals growing process, a complex thermal investigation of bulk double perovskite has been performed. Differential thermal analysis investigation in argon inert atmosphere, starting from room temperature up to 1650 degrees C provided information about melting and re-crystallization temperature range. Both, the activation energy of Sr2FeMoO6 re-crystallization process and the re-crystallization mechanism were comparatively analyzed by two free-model estimations (Friedman and Ozawa-Flynn-Wall analysis). The resulted data are very important in order to set up the heating program of Bridgman furnace.