National Institute Of Materials Physics - Romania

The Stoner instability of the paramagnetic state, yielding to the occurence of ferromagnetism, is reviewed for electron density of states reflecting changes in the dimensionality of the system. The situations treated are one-dimensional (1D), two-dimensional (2D) and three-dimensional (3D) cases and also a special case where the density of states has a parabolic shape near the Fermi level, and is half-filled at equilibrium. We recover the basic results obtained in the original work of E.C. Stoner [Proc. Roy. Soc. London A 165, 372 (1938)]; also we demonstrate that in 1D and 2D case, whenever the Stoner criterion is satisfied, the system evolves spontaneously towards maximum polarization allowed by Hund's rules. For the 3D case, the situation is that: (i) when the Stoner criterion is satisfied, but the ratio between the Hubbard repulsion energy U and the Fermi energy epsilon(F) is between 4/3 and 3/2, the system evolves towards a ferromagnetic state with incomplete polarization (the polarization parameter is between 0 and 1); (ii) when U/epsilon(F), > 3/2, the system evolves towards maximum polarization. This situation was also recognized in the original paper of Stoner, but with no further analysis of the obtained polarizations nor comparison with experimental results. We apply the result of calculation in order to predict the Hubbard interaction energy. Finally, for the case of half-filled parabolic density of states a result quite similar with the case of the free electron 3D DOS is obtained, but here the Hubbard energy has to be compared with the band half-widh 6, whereas the absolute value of the Fermi energy has no relevance in this model. The Stoner criterion is U/delta > 4/3 and for U/delta < 2 the total energy is minimized for incomplete polarization. We compare the results obtained from the present theoretical study with the newest available experimental data.

The molecular dynamics of 4-n-octyl-4(')-cyanobiphenyl (8CB) confined inside the pores of a series of AlMCM-41 samples with the same structure, constant composition (Si/Al=14.7) but different pore sizes (diameter between 2.3 and 4.6 nm) was investigated by broadband dielectric spectroscopy (10(-2)-10(9) Hz) in a large temperature interval. Two relaxation processes are observed: one has a bulklike behavior and is assigned to the 8CB in the pore center. The relaxation time of the second relaxation process is essentially slower than that of the former one and this process is related to the dynamics of molecules in a surface layer with a paranematic order. Both relaxation processes are specifically influenced by the interaction of the molecules with the surface and by the confinement. Above the clearing temperature the temperature dependence of the relaxation rate of the bulklike process obeys the Vogel-Fulcher-Tammann (VFT) law. The Vogel temperature increases with decreasing pore size. This is explained by increasing influence of paranematic potential of the surface layer with decreasing pore size. The temperature dependence of the relaxation rate of the surface layer follows also the VFT formula and the Vogel temperature decreases with decreasing pore size. This temperature dependence is controlled by both the interaction of the 8CB molecules with the surface via hydrogen bonding and by spatial confinement effects. To discriminate between both effects the data for the surface layer of 8CB confined to the molecular sieves are compared with results concerning 8CB adsorbed as a quasimonolayer on the surface of silica spheres of aerosil. On this basis a confinement parameter is defined and discussed.

SBN thin films were grown on MgO and Silicon substrates by PLD and RF-PLD (radiofrequency assisted PLD) starting from single crystal Sr0.6Ba0.4Nb2O6 and ceramic Sr0.5Ba0.5Nb2O6 stoichiometric targets. Morphological and structural analyses were performed on the SBN layers by AFM and XRD and optical properties were measured by spectroellipsometry. The films composition was determined by Rutherford Backscattering Spectrometry. The best set of experimental conditions for obtaining crystalline, c-axis preferential texture and with dominant 31 degrees in-plane orientation relative to the MgO (100) axis is identified.

The paper presents the spectroscopic properties of electrolytic colored KCl:Tl and KCI:Tl+Ca crystals and the assignments of the new defects which are formed during electron injection process. The new formed defects Tl-0 and Tl-0+Ca2+ with the metal in the anionic sites, presents very high photoluminescence properties in the near infrared region. Together with those centers, the F-centers, their aggregates and the holes centers are formed during electrolytic coloring process. Their properties were studied by optical absorption (OA), photoluminescence (PL) and thermoluminescence (TL) measurements and compared with those obtained by ionizing irradiation in uncolored KCl:Tl crystals. The results suggest a more efficient Tl+-> Tl-0 process in the case of Ca2+ codoped samples due to the prolonged time of electrons injection, low temperature and high electric field.

Neutron irradiation is a promising method for a spectacular enhancement of the critical current density of the high temperature superconductors. The suppression of the mechanisms responsible for superconductivity under irradiation are analyzed and put side by side with the effects on the flux line lattice dynamics. Additionally, the evolution of the creation/generation balance of the defects of the crystal structure is connected to the evolution of the response of the vortex system with an emphasis on the ceramic materials.

The template method was used for preparing Zn1-xCoxO nanowires with x ranging from 0.01 to 0.05. Thus, electrochemical deposition was employed for filling up the pores of polycarbonate ion track nanoporous membranes with the desired material. The method allows a good control over the morphology and composition of the deposited nanowires, as evidenced by scanning electron microscopy (SEM) and energy dispersive X ray analysis (EDX). Measurements of the magnetic properties showed a paramagnetic behavior of the nanowire arrays for the whole set of temperatures and Co concentrations.

Despite many investigations already performed with respect to sintering stability and grain growth vs. annealing conditions in low yttrium doped alpha- Al2O3 (150 ppm) - some fundamental questions are still open. These questions concern mainly the influence of small level of impurities at grain boundary in the sintering behavior and grains growth of the materials, i.e. the microstructure function of the annealing time at annealing temperature of 1650 degrees C. The low yttrium doped alpha- Al2O3 (150 ppm) samples were sintered and annealed at high temperatures. Besides conventional micro structural studies, such as grain diameter and grain size functions of distribution of the annealing temperature were studied by electron microscopy investigations and statistical measurement. The implications of the different microstructures with respect to micro structural stability have discussed and compared to observations described in the literature.

PZT type films were prepared using the thermionic vacuum arc (TVA) method; a crucible filled PZT type powder (PbO/ZrO/TiO) was heated by a thermo-electron beam emitted by the circular cathode of the TVA gun. Applying high voltage potential (3000 +/- 200V) on the anode from a regulated HEINZINGER high voltage power supply the thermo-electrons were accelerated toward the anode crucible, evaporating the anode materials and initiating a plasma discharge. The morphology and the structure of the prepared films analyzed by transmission electron microscopy, (TEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy, showed nanostructured, smooth thin films with specific PZT stoichiometry.

We investigated the possibility of tuning luminescent spectra for a given Si/SiO2 nanoparticle size distribution in aqueous solution. The luminescence spectra of Si/SiO2 Suspensions in aqueous solutions of different particle/cluster concentrations were studied. The different Si/SiO2 nanoparticle/cluster concentrations were produced by both centrifugation and dilution. Depending on the centrifugal force we found a reversible shift up to 100 nm toward UV, regardless of the excitation wavelength. The shift could be reverse by sonication of the sample. A-scenario based on particle and cluster size variation is also discussed.

Ba0.6Sr0.4TiO3 (BST) bulk ceramic synthesized by solid state reaction was used as target for thin films grown by pulsed laser deposition (PLD) and radiofrequency beam assisted PLD (RF-PLD). The X-ray diffraction patterns indicate that the films exhibit a polycrystalline cubic structure with a distorted unit cell. Scanning Electron Microscopy investigations showed a columnar microstructure with size of spherical grains up to 150 nm. The capacitance-voltage (C-V) characteristics of the BST films were performed by applying a DC voltage up to 5 V. A value of 280 for dielectric constant and 12.5% electrical tunability of the BST capacitor have been measured at room temperature.