National Institute Of Materials Physics - Romania

Single molecule magnets are of great interest due to a multitude of potential applications for some of which thin films are required. Traditional physical vapor deposition techniques are not suitable for the deposition of these fragile materials with low decomposition temperatures. Matrix Assisted Pulsed Laser Evaporation technique has been employed for the growth of thin films of the single molecule magnet Mn-12(Propionate) on Si and glass substrates. In this paper we report on the appropriate growth conditions and also the morphology, chemical composition and magnetic behavior of the films. Continuous Mn-12(Propionate) films with properties similar to bulk materials have been obtained. (c) 2011 Elsevier B.V. All rights reserved.

Amorphous titania thin films with increasing Fe content have been prepared by RF magnetron sputtering. X-ray absorption spectroscopy revealed modifications of both local structures/environment of the Ti and Fe atoms, with formation of phases containing amorphous material and a magnetite-like phase. The temperature dependence of the electrical conductivity of the films was investigated for temperatures higher than half of the Debye temperature (T > 391 K). It was found that the electrical conductivity in the amorphous Fe/TiO2 films obeys the Meyer-Neldel rule. The origin of this behavior is explained on the basis of the multiphonon-assisted hopping model.

The effects of the self-assembled 4,4'-thio-bis-benzenethiolate protected gold nanoclusters onto a p-GaAs (1 0 0) electrode were examined by AFM, XPS, SHG and EIS investigations. The AFM and XPS results revealed a well-ordered overlayer exhibiting a bi-modal highly correlated fractal behavior which, however, cannot fully protect the semiconducting surface against the oxidation in air. The EIS data pointed out the influence exerted by the gold-monolayer protected clusters (Au-MPCs) over the charging/discharging processes observed at p-GaAs (1 0 0) electrode. Although the applied potential is varied linearly, the potential drop within the semiconductor space charge region as well as that across the Au-MPCs layer undergoes stepped changes supposed to result in the discrete charging of the Au-MPCs. These effects point to an electronic equilibrium between the Au-MPCs and the semiconducting substrate. Fermi level pinning and enhancement of the SHG response in the potential range where the surface/interface states in the semiconductor band gap become electrically active bring further proof in this respect. (c) 2012 Elsevier Ltd. All rights reserved.

High quality pulsed laser deposited lead zirconate titanate (PZT) films are obtained by pulsed laser deposition on SrRuO3(111) single crystal layers and characterized by X-ray photoelectron spectroscopy (XPS), to determine the surface composition. It is found that a minor amount of Pb forms PbO2 at the surface and also some Pb is included into the contamination layer, in form of a Pb(CO3)2 layer of about 1?nm thickness, occupying about one quarter of the PZT surface. The stoichiometry of the outermost 45?nm layers yielded as PbZr0.25Ti0.80O2.5, which suggest the formation of an oxygen depleted, Brownmillerite-like layer at the surface, of at least 5?nm thickness.

Epitaxial growth of Bi1.5-xZn0.92-yNb1.5O6.92-1.5x-y (BZN) thin films was achieved on (100)(pc) LaAlO3 substrate by pulsed laser deposition (PLD) and by chemical solution deposition based on Pechini process. Effect of bismuth and zinc deficiency on the BZN thin films obtained by PLD was discussed, in relation with the starting target composition. Dielectric permittivity and bandgap values were determined from electrical and spectroscopic ellipsometry measurements performed on randomly oriented films grown on Pt/Si substrate. BZN thin films obtained by PLD exhibit, at 100 kHz, a dielectric constant of epsilon(r) = 203 and quite low dielectric losses of tan delta = 5 x 10(-2). Epitaxial ferroelectric - dielectric KTa0.65Nb0.35O3 (KTN) -Bi1.5-xZn0.92-yNb1.5O6.92-1.5x-y (KTN on BZN and BZN on KIN) bilayers were obtained by PLD on (100)(pc) LaAlO3 with the insertion of a suitable buffer layer of KNbO3 in the case of KTN on BZN. Such multilayer heterostructures with an epitaxial growth control of each layer are promising candidates for potential integration in microwave devices. (C) 2011 Elsevier B.V. All rights reserved.

Different hybrid structures were obtained by coating magnetic nanoparticles of perovskite type manganite at optimal doping (La0.67Sr0.33MnO3, LSMO) with different quantities of polypyrrole (PPy). The amorphous layer of polypyrrole surrounding the crystalline magnetic core was observed by high resolution transmission electron microscopy (HRTEM) and analyzed by using X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) measurements in near edge structure (XANES) techniques. By analyzing the magnetic behavior of the samples one can observe that the surface modification of magnetic nanoparticles by PPy results in an increase in the saturation magnetization of the composites. The process is ascribed to paired electrons transferred from the delocalized p states of the PPy into the outer disordered layers of the manganite. The analysis of pre-edge peak of the Mn K-edge XANES spectra in the case of PPy coated LSMO nanoparticles indicates that the charge transfer between polymer and nanoparticles is (directed) going to missing or distorted oxygen positions, hence increasing the 3d electrons' mobility and orbital hybridization between the neighboring manganese ion. As a consequence, within the surface layers of LSMO nanoparticles, both energy bands disrupted the structure, and the double exchange process between Mn ions was reestablished determining the saturation magnetizations and pre-edge features increase, respectively. (C) 2012 American Institute of Physics. [doi:10.1063/1.3686662]

This study presents a correlated study of structural, reactivity, and magnetic properties of ultrathin Fe layers grown on Si(001) by molecular beam epitaxy in ultrahigh vacuum. The interface reactivity is characterized by Auger electron spectroscopy. The surface structure is characterized by low electron energy diffraction with spot profile analysis. The magnetism of the synthesized layers is investigated by magneto-optical Kerr effect. At room temperature, metal Fe layers with poor long-range order are synthesized; these layers are ferromagnetic with an extremely low coercitive field (below 1 Oe). The reactivity with Si is low in this case, with formation of an interface layer of about 8 angstrom Fe equivalent thickness with about 7 at.% Si diffused. Samples synthesized at higher temperatures (500 degrees C) exhibit better long-range order, though the Fe reactivity with Si is higher and leads to the formation of an interface compound whose approximate stoichiometry is very close to Fe(3)Si. Once this compound is formed (for an equivalent Fe thickness of about 14 monolayers), disordered metal Fe islands are developing with subsequent Fe deposition, which contain also about 8 at.% Si diffused. These structures exhibit a much lower ferrimagnetism, with saturation magnetization about one order of magnitude lower than in the case of the room temperature synthesis. In this case of high temperature synthesis, two phases are observed, a ferrimagnetic one and a superparamagnetic one.

Structural aspects and atomic intermixing processes in Be/W bilayers deposited on Si(0 0 1) substrates with Fe buffer layers enriched in the (57)Fe Mossbauer isotope have been studied via atomic force microscopy, grazing incidence X-ray diffractometry, X-ray reflectometry, X-ray photoelectron spectroscopy and conversion electron Mossbauer spectroscopy. The mentioned investigations allowed a full sequential characterization of the involved interfaces. Various ionic configurations appeared for Fe or W, while an amorphous state was observed in the case of Be. It has been proven that the Be layer has a negative influence on the roughness of the whole structure, which however presents an oxidation gradient from more oxidized elements at the surface towards more reduced elements in deeper layers. A strong diffusion of the W atoms inside the Be layer, induced by the deposition method, as well as of the Fe atoms inside the Be layer, induced by thermal annealing, has been evidenced. (C) 2011 Elsevier B.V. All rights reserved.

Manganese is deposited onto GaAs(001) substrates in high vacuum conditions (10(-7) hPa), with substrates held at 300 degrees C. It is shown that this procedure yields to the diffusion of magnanese into gallium arsenide and the formation of a layer which exhibits room temperature ferromagnetism, with highly diluted Mn (below 1 atomic percent). X-ray absorption fine structure determinations at the Mn and Ga K-edges evidenced that Mn is not placed into substitutional Ga sites in GaAs. Most probably, Mn forms MnO clusters with rocksalt local structure. These clusters are the origin of the detected ferromagnetism.

The band (or Stoner) ferromagnetism in systems exhibiting a linear density of states, such as a graphene sheet, where the density of states g(e) is proportional to e, is investigated as function of temperature. The calculations are performed by neglecting spin-wave excitations and some arguments are presented for this approximation. It is found that the occurrence of ferromagnetism becomes easier at finite temperature than in the low temperature limit, especially for systems with low values of carrier density. An almost linear dependence of the magnetization on temperature is predicted; the temperature of vanishing magnetization is linear with the value of the equilibrium Fermi energy, irrespective of the value of the interaction (Hubbard) parameter. Also, for low values of the Hubbard energy an increase in the average value of the polarization with the temperature is predicted in the range of low temperatures. The most important application of such systems consists of control over the onset of magnetization by the charge density (the value of the Fermi energy).