National Institute Of Materials Physics - Romania

Magnetization reversal in nanoscale (Sm-Co)/Fe (hard/soft) bilayer exchange-spring magnets with in-plane uniaxial magnetic anisotropy was investigated by magnetometry, conversion-electron Mossbauer spectroscopy (CEMS) and atomistic Fe spin-structure calculations. Magnetization loops along the easy direction exhibit signatures typical of exchange-spring magnets. In-field CEMS at inclined gamma-ray incidence onto thin (2 nm) Fe-57 probe layers embedded at various depths in the 20-nm-thick natural (soft) Fe layer provides depth-dependent information (via the line-intensity ratio R-23 as a function of the applied field H) about the in-plane rotation of Fe spins. A minimum in the R-23-vs-H dependence at (H-min, R-min) determines the field where Fe magnetic moments roughly adopt an average perpendicular orientation during their reversal from positive to negative easy-axis orientation. A monotonic decrease of H-min with distance from the hard/soft interface is observed. Rotation of Fe spins takes place even in the interface region in applied fields far below the field of irreversible switching, H-irr, of the hard phase. Formation of an Fe-Co alloy is detected in the interface region. For comparison, the noncollinear Fe spin structure during reversal and the resulting R-23 ratio were obtained by electronic-structure calculations based on a quantum-mechanical Hamiltonian for itinerant electrons. The coupling at the hard/soft interface is described by the uniaxial exchange-anisotropy field, h(int), as a parameter. Our calculated R-23 ratios as a function of the (reduced) applied field h exhibit similar features as observed in the experiment, in particular a minimum at (h(min), R-min). R-min is found to increase with h(int), thus providing a measure of the interface coupling. Evidence is provided for the existence of fluctuations of the interface coupling. The calculations also show that the Fe spin spiral formed during reversal is highly inhomogeneous. In general, our simulation of the Fe spin structure is applicable for the interpretation of experimental results on layered exchange-spring magnets.

Structural and magnetic properties of exchange spring magnets consisting of hard magnetic (FePt) and soft magnetic (Fe and Co) bilayers. prepared by ion beam sputtering method are studied via X-ray diffraction (XRD), magneto-optic Kerr effect (MOKE) and vibrating sample magnetometry (VSM). Thin tracer layers of Fe-57 were introduced in the soft layer in order to observe the Fe spin structure and interfacial diffusion by Conversion Electron Mossbauer Spectroscopy (CEMS). The observed in-plane exchange spring behavior extends also to the magnetic hard layer, whose switching field can be tuned in an unexpected manner via the top soft magnetic layer. To explain the observed phenomenon it is suggested that the increased switching field, found in the system with a Co/Fe bilayer acting as a single soft magnetic layer, is compatible with a peculiar behavior of the stiffness coefficient of the heterogeneous soft magnetic layer. According to this observation, possibilities to maximize the exchange spring effects via suitably chosen non-homogeneous soft magnetic layers are open. (C) 2011 Elsevier B.V. All rights reserved.

High quality nonlinear optical Gd0.84Lu0.11Nd0.05Ca4O(BO3)(3) (Nd: GdLuCOB) crystal has been grown by the Czochralski method. The high resolution and polarized spectroscopic data of Nd3+ in GdLuCOB crystal were carried out and analyzed in the framework of the Judd-Ofelt theory. The polarized emission cross-sections for the F-4(3/2) -> I-4(9/2) and F-4(3/2) -> I-4(11/2) Nd3+ transitions of special interest for laser application were determined. Near-infrared laser emission at 1061 nm was achieved under cw Ti: sapphire end pumping at 812 nm. For propagation along the Z-axis, laser emission polarized parallel to the Y-axis was obtained for the first time. This result offers the opportunity to achieve type I SFD of Nd3+ emission in the ZX plane which presents a non-linearity approximately three times larger than XY plane. (C) 2012 American Institute of Physics. [doi: 10.1063/1.3673801]

(Na0.5Bi0.5)TiO3 doped with 5 mol% BaTiO3 (NBT-BT0.05) nanopowder was prepared by sol-gel method. Structural and microstructural investigations revealed a powder crystallized in the Na0.5Bi0.5TiO3 rhombohedral phase form. Ceramics processed from this powder show good dielectric (epsilon(r) = 3940 and tan delta = 0.016) and piezoelectric characteristics (d(33) = 77 pC/N, k(p) = 0.091, epsilon(T)(33) = 1080 and Q(m) = 197.5). The as-obtained NBT-BT0.05 ceramic appears effective for piezoelectric resonator and high power applications. (C) 2011 Elsevier Ltd. All rights reserved.

The influence of Au nanoparticles dimension on the photocatalytic performances of the TiO2 aerogels-Au composites was evaluated. Structural and morphological peculiarities of the TiO2 aerogel, unloaded and loaded with 5 and 22 nm Au nanoparticles, were studied by transmission electron microscopy technique, X-ray diffraction and N-2-sorption measurements. UV-Vis diffuse reflectance measurements were performed to determine the band gap energies. The photocatalytic activity was evaluated by monitoring the salicylic acid photodegradation. It was found that the Au nanoparticles promote the anatase phase crystallization and produce the decrease of the specific surface area and band gap energy values. The porous composite with the smallest Au nanoparticles dimension exhibits the best photocatalytic performances.

Iron-carbon nanocomposites have gained interest due to their new engineering and biomedical applications. Carbon coated iron nanoparticles (Fe@C) were obtained continuously and in a single step using the laser pyrolysis method. The continuous wave CO2 laser beam was used to continuously heat a sensitized (with ethylene) precursor gas mixture, in which iron pentacarbonyl (vapor) and acetylene were the iron and carbon donors, respectively. The effect of varying the residence time in the reaction zone through the variation of the internal nozzle diameter was explored in order to improve the particle size and the phase distributions. At increased nozzle diameter, (i) the particle mean diameter increases (from about 3.5 to 10.5 nm), (ii) higher ordering of the crystallographic network seems to occur, (iii) the dominance of the alpha-Fe and iron carbide phases is revealed. Onion-like graphenic layers often cover the buried iron cores. Magnetic measurements and temperature dependent Mossbauer spectroscopy were used in order to find correlations concerning the magnetic behavior and the Fe phase composition of samples. Preliminary experiments for obtaining stable water-based magnetic nanofluids are discussed. (C) 2011 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

Superconducting (SC) MgB2 thin films were prepared in situ in one step by vacuum co-deposition at extremely low growth temperature (T-s similar to 110-150 degrees C) and at very low Mg deposition rate, far off from the MgB2 stability regime of the thermodynamic phase diagram. As-grown films are nanocrystalline and exhibit (002) texture, moderate surface roughness and SC onset up to T-c(on) = 18 K. This is the highest critical temperature ever observed in low-T-s as-grown MgB2 films. Our preparation method at very low T-s is favorable for the fabrication of layered SC hybrid electronic devices, where deposition and post-annealing at high temperature are detrimental to device functioning. Simple post-annealing of our films in ultrahigh vacuum enhances T-c(on) up to 22 K due to the improvement in structural properties.

The synthesis of a new garnet, Er3Li0.275Nb0.275Ga4.45O12 (ErLNGG), by solid-state reaction, is reported. A comparison of the fluorescence spectra of Er3+ in ErLNGG, Er-doped (0.1 at.%) calcium lithium niobium gallium garnet (CLNGG), and Er(50 at.%):CLNGG is presented. The narrowing of luminescence lines suggests an ordering of the partially disordered CLNGG with the increase of erbium concentration.

Output performances of passively Q-switched, composite Nd:YAG/Cr4+:YAG lasers that consisted of bonded, all-poly-crystalline ceramics Nd:YAG and Cr4+:YAG are reported. Laser pulses at 1.06 mu m with 2.5-mJ energy and 1.9-MW peak power are obtained from a 1.1-at % Nd:YAG/Cr4+:YAG ceramics that was quasi-continuous-wave (quasi-CW) pumped with a diode laser. Single-pass frequency doubling with LiB3O5 (LBO) nonlinear crystal at room temperature yielded green laser pulses at 532 nm of 0.36-mJ energy and 0.3-MW peak power, with a conversion efficiency of 0.27.

In this paper we present some comparative studies between the optical and electrical properties of the films prepared by vacuum evaporation from amidic monomeric compounds obtained from maleic anhydride and aniline derivative with different functional groups [CN; NO2], and the composite films prepared by spin-coating from a polycarbonate of bisphenol A matrix embedding clusters of these amidic monomers. We have emphasised the effect of the polymeric matrix on the properties of the synthesised monomers with the purpose to change the physical properties of the films (optical transmission, luminescence, electrical conduction, optical non-linear phenomena). The correlation between the morphology of the films and the optical and electrical properties has also been investigated. (C) 2011 Elsevier B.V. All rights reserved.