National Institute Of Materials Physics - Romania

Hexagonal ferrites BaxSr1-xFe12O19 (x = 0.05, 0.15, 0.25, 0.35) were prepared by sol-gel and conventional solid state reaction techniques. Their magnetic properties, investigated by magnetometry and Mossbauer spectroscopy, were discussed in correlation to structural aspects. Whereas the hexaferrites obtained by sol-gel are almost single phase, the ones prepared by the conventional solid state reaction technique present a low amount of BaFe2O4 as secondary phase. All samples show a relative high coercive field and remanent magnetization, as specific features for permanent magnet behavior. The five Fe3+ sites of the specific elemental cell have been revealed and their relative occupancy has been derived from the Mossbauer spectra. The analyzed samples show a sensitivity of the magnetic properties (magnetic moment and anisotropy) with respect to the occupancy of the different Fe sites. (C) 2013 Elsevier B. V. All rights reserved.

Pure EuFeO3 and ceria were prepared via a sol-gel citrate route and were calcined at 700 degrees C. Separately, a 20 wt% EuFeO3/CeO2 was prepared via incipient wetness impregnation of calcined ceria with citrate precursors and calcined in the same conditions. SBET, XRD, toluene chemisorption, toluene-TPD and in situ Eu-151 and Fe-57 Mossbauer investigations were performed. All characterization data indicate the formation of a well-dispersed EuFeO3 at the surface of CeO2. During the oxidation of toluene Eu-151 and Fe-57 Mossbauer spectroscopy suggested a cooperative effect of the cations. The catalysts were tested in total oxidation of toluene. In terms of intrinsic activity for toluene combustion, supported catalysts were more active than bulk EuFeO3. All structural changes during operation time were reversible. (C) 2012 Elsevier B.V. All rights reserved.

The influence of milling type on the structural and magnetic behaviour of mechanically milled SmCo5/Fe65Co35 has been studied. By adjusting the mill speed parameters, (Omega/omega), three types of milling have been performed: shock mode process (SMP), friction mode process (FMP) and a combined shock followed by a friction mode process. It was found that in SMP conditions the remanent magnetization and composite coercivity increase with increasing milling time. The best exchange coupling, with a coercivity of 1.05 T, was obtained for samples milled for 8 h in SMP and annealed for 2 h at 550 degrees C. On the other hand, the low energy of FMP does not succeed in producing a microstructure adapted to favour the interphase exchange interactions between the soft and hard magnetic phases. A combined milling process does not change significantly the microstructure obtained previously after 8 h of SMP. This indicates that the microstructure of the composite is formed primarily during SMP while FMP, less energetic, does not drastically affect the crystallite size and crystallite shape of SMP samples. X-ray diffraction patterns were used to follow the evolution of the structure and microstructure. The diffusion of Fe and Co from Fe65Co35 to SmCo5 phase leads to the formation of Sm-2(Co,Fe)(17)/Fe-Co composite. The magnetic behaviour was checked from hysteresis curves and dM/dH vs. H plots. (C) 2013 Elsevier B.V. All rights reserved.

Within the framework of density functional theory, the electronic structure and magnetic properties have been studied for the Ti2FeSn full-Heusler compound. The ferromagnetic state is found to be energetically more favorable than paramagnetic and antiferromagnetic states. The spin-polarized results show that Ti2FeSn compound has half-metallic ferromagnetic character with a total spin moment of 2 mu(B) and a band gap in the minority spin channel of 0.489 eV, at the equilibrium lattice constant a = 6.342 angstrom. (C) 2013 Elsevier Ltd. All rights reserved.

Bi2O3 and Bi-metal powders were mixed with MgB2 powder. Starting compositions were (MgB2)(Bi2O3) x, x = 0.0025, 0.005, 0.015, and (MgB2)(Bi)(y), y = 0.01. Mixtures were processed by Spark Plasma Sintering (SPS) technique. As obtained composite samples show high density, above 94 % of the theoretical density. Samples with Bi2O3 for x = 0.0025 and 0.005 show higher critical current densities, J(c), at high fields and at 20 K than for the pristine sample. Their irreversibility field, H-irr, is also higher. On the other hand, sample added with Bi-metal has lower J(c) and H-irr at any temperature than for the pristine sample.

We investigated the temperature (T) variation of the normalized magnetization relaxation rate S and of the corresponding normalized vortex-creep activation energy U* = T/S for YBa2Cu3O7 films containing BaZrO3 nanorods, with the external magnetic field H oriented perpendicular to the film surface. It was found that by increasing the film thickness and using nanodot decorated substrates the high-T S(T) maximum appearing at low H is substituted by a minimum in S(T). As revealed by the analysis of the current density dependence of U*, this behaviour is due to the inhibition of vortex excitations involving double vortex-kinks and superkinks formation in the investigated thick films, owing to the large nanorod splay and pinning energy dispersion.

The electronic and magnetic properties of Ti2CoSi Heusler compound are investigated using density functional calculations. The optimized lattice constant is found to be 6.030 angstrom. The compound is a half-metallic ferromagnet with an energy gap in minority spin channel of 0.652 eV at equilibrium lattice constant, which leads to a 100% spin-polarization. The obtained total magnetic moment from spin-polarized calculations is 3.0 mu(B)for values of lattice constants higher than 5.941 angstrom. The half-metallicity is spoiled for a compressed volume of 4%, suggesting a possible application as pressure sensitive material. (C) 2012 Elsevier B.V All rights reserved.

The Bose glass theory for the vortex matter in superconductors with correlated disorder predicts the depinning of vortices due to the renormalization of the vortex pinning barriers by thermal fluctuations. For YB2Cu3O7 (YBCO) in external magnetic fields H oriented along the columnar pins generated by various techniques theoretical estimates give a depinning temperature T (dp) very close to the critical temperature T (c) (T (dp)similar to 0.95T (c)), whereas the results of standard magnetization relaxation experiments are repeatedly interpreted in terms of a much lower T (dp) (similar to 0.5T (c)). We investigated the temperature T variation of the normalized magnetization relaxation rate S for YBCO thin films containing BaZrO3 (BZO) nanorods preferentially oriented along the c axis, with H along the nanorods. The nonmonotonous S(T) variation below the matching field observed up to close to T (c) does not support a low T (dp). The often considered S(T) maximum occurring at relatively low T (which was connected to a disappointing T (dp)) is related to the occurrence of thermomagnetic instabilities. We show that the accommodation of vortices to the columnar pins in the presence of the T dependent macroscopic currents induced in the sample is signaled by a pronounced S(T) deep located at high T, in agreement with a T (dp) close to T (c). By increasing the film thickness and using the substrate decoration the BZO nanorods splay out, leading to the inhibition of (detrimental) vortex excitations involving double vortex kink or superkink formation, characteristic for high-quality thin films and single crystals with columnar pins along the c axis.

Ex situ spark plasma sintering was used to obtain dense MgB2 co-doped with SiC and Te. The composition (MgB2) + (SiC)(0.025) + Te-0.01 shows critical current densities J(c)(20 K, 5 T) of 3.5 x 10(2) A cm(-2) and J(c)(5 K, 9 T) of 2 x 10(3) A cm(-2). These values are higher than for pristine samples or those separately doped with Te or SiC. They are also comparable at high fields and temperatures with the best reported values for the co-doped MgB2 bulks produced by conventional in situ methods. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Highly textured ZnO thin films were implanted with Co ions at fluences of 1 x 10(16) and 1 x 10(17) ions cm(-2). Although their microstructure observed by analytical high-resolution electron microscopy was very different, SQUID magnetometer measurements showed qualitatively similar magnetic properties. In the low-fluence film, only small modifications, such as a slight amorphization, planar defects or very small (<1-1.5 nm) Co clusters, were hardly observable. Implantation at a higher fluence led to important changes in the film structure: heavy amorphization and metallic Co precipitates were identified by electron diffraction, energy dispersive x-ray spectroscopy (EDS), electron energy loss spectroscopy and scanning transmission electron microscopy spectrum imaging. EDS microanalysis additionally revealed the presence of similar to 2 at% Co atoms that were likely dissolved in the ZnO lattice at the atomic level. No Co oxides or other secondary oxide phases were detected. For both implanted samples, the dimensions of Co precipitates were below the superparamagnetic limit at room temperature. Thermo-magnetization curves and magnetic hysteresis loops measured in the temperature range from 5 to 300 K indicated two superimposed magnetic regimes, one predominant above 50 K and the other below 50 K. We analysed the possible mechanisms and quantitative contributions to explain the observed magnetic behaviour at low and room temperature.