P. Luches

Morphology evolution and magnetic properties improvement in FePt epitaxial films by in situ annealing after growth

This paper presents a detailed study of structure, morphology, and magnetic properties in FePt thin films epitaxially grown on MgO(100) at intermediate temperatures (≅380°C). It focuses on the effects obtained by in situ annealing the FePt films after deposition. We have demonstrated that the annealing allows one to complete the ordering, thus obtaining an epitaxial L10-FePt film with large perpendicular anisotropy (up to 3×107erg∕cm3) and high perpendicular squareness and remanence ratio (both higher than 0.85). At the same time, we have found a peculiar morphology evolution by increasing the annealing time: the interconnected mazelike grains evolve towards interconnected dotlike grains of reduced size (down to 10nm). Due to the interconnection at the grains basis, the increase in the film perpendicular coercivity with the annealing process is moderate (up to 4.1kOe), leading to an increasing anisotropy/coercivity ratio with the annealing time. As a further step towards the understanding of the propertie...

Evidence of Catalase Mimetic Activity in Ce3+/Ce4+ Doped Bioactive Glasses

The ability of Ce-containing bioactive glasses to inhibit oxidative stress in terms of reduction of hydrogen peroxide, by mimicking the catalase enzyme activity is demonstrated here for the first time. The antioxidant properties of three bioactive glasses containing an increasing amount of CeO2 have been evaluated by following the degradation of hydrogen peroxide with time after immersion in H2O2 aqueous solutions with different concentration. XPS and UV-vis measurements allowed us to determine the Ce(3+)/Ce(4+) ratio in the bulk and on the glass surface, and to correlate it with the ability of the samples to show catalase mimetic activity. Interestingly, we have found that the bioactive glass with composition 23.2Na2O-25.7CaO-43.4SiO2-2.4P2O5-5.3CeO2 immersed in 0.1 M H2O2 aqueous solution is able to degrade 90% of it in 1 week. The reduction in bioactivity of the glasses with increasing CeO2 content is here rationalized in terms of a lower amount of phosphate groups available for the hydroxyapatite layer formation, after binding with cerium ions. In fact, classical molecular dynamics simulations revealed that the addition of CeO2 leads to the formation of cerium phosphate rich regions. The formation of an insoluble CePO4 crystalline phase is also observed by XRD analysis after thermal treatment of the glass samples.

Structure and morphology of ultrathin NiO layers on Ag(001)

Abstract The structure and morphology of thin NiO films prepared on Ag(001) by reactive growth at 460 K has been investigated as a function of the film thickness in the 3–20 monolayers range. Emphasis was on the study of the oxide layer misfit strain. Primary beam diffraction modulated electron emission and low energy electron diffraction experiments allowed the determination of the in-plane and out-of-plane strain in the oxide layer, while scanning tunneling microscopy, X-ray photoelectron spectroscopy and secondary electron imaging have been used to monitor the film morphology, stoichiometry and structure, respectively. The film strain begins to be removed at a critical thickness of 10 ML, while at 20 ML the film is fully relaxed. Strain analysis indicates that the Poisson ratio of the oxide layer is nearly equal to that of the bulk material.

Growth, structure and epitaxy of ultrathin NiO films on Ag(001)

NiO ultrathin films have been grown on Ag(001) by Ni deposition in an O2 atmosphere. The thickness range 5–50 ML has been investigated. X-ray photoelectron spectroscopy has been used to study the stoichiometric composition and chemical purity of the oxide films. We found completely oxidized stoichiometric NiO films. Their contamination has been found to be limited to the topmost layers. Photoelectron diffraction has given information concerning the local crystal structure of the films. The film atomic geometry has been found to be the same independent of thickness in the 0–50 ML range. The films have the expected (001) rock-salt structure with the same in plane orientation as the Ag(001) substrate. Specular X-ray reflectivity has allowed a very accurate thickness evaluation and has given information on the width of the density gradients at the film–substrate and vacuum–film interfaces, found to be of the order of a few atomic layers.

VARIATIONS IN THE LIFETIME OF 3d HOLE STATES IN ULTRATHIN Fe FILMS GROWN ON Cu(100) DEDUCED FROM THE LMM AUGER SPECTRA OF Fe

The Fe LMM Auger and Auger vacancy satellite spectra of ultrathin Fe films grown on Cu(100) have been measured using photon excitation energies above and below the L2 ionisation edge, and in coincidence with the Fe 2p3/2 and Fe 2p1/2 photoelectrons. A comparison of the relative intensities of the satellites and main Auger transitions indicates that the M4,5 holes of L3M4,5 double hole states are localised for longer than the L3 lifetime for the 0.3 and 10 ML coverages but have a lifetime comparable to that of L3 holes for the 1 ML coverage. The results for the 1 ML coverage are supported by the results of the coincidence experiments. The reasons for the variation in the lifetime of M4,5 holes are discussed in terms of changes in the number of Fe–Fe neighbours and the density of states at the Fermi energy in these systems.

Structure, morphology and reducibility of epitaxial cerium oxide ultrathin films and nanostructures

Cerium oxide is a very interesting material that finds applications in many different fields, such as catalysis, energy conversion, and biomedicine. An interesting approach to unravel the complexity of real systems and obtain an improved understanding of cerium oxide-based materials is represented by the study of model systems in the form of epitaxial ultrathin films or nanostructures supported on single crystalline substrates. These materials often show interesting novel properties, induced by spatial confinement and by the interaction with the supporting substrate, and their understanding requires the use of advanced experimental techniques combined with computational modeling. Recent experimental and theoretical studies performed within this field are examined and discussed here, with emphasis on the new perspectives introduced in view of the optimization of cerium oxide-based materials for application in different fields.

The Co/Si(111) interface formation: a temperature dependent reaction

We have investigated the reaction of Co with the Si(1 1 1) surface both at room temperature (RT) and at high temperature (500–650 C). The temperature evolution of the RT deposited 10 ML film has also been studied. The films, prepared by the different methods, have been structurally characterized by means of primary-beam diffraction modulated electron emission. Auger electron spectroscopy has been used to follow their stoichiometric evolution. For RT deposition the films have been found to have a B-type (180 rotated with respect to the underlying Si(1 1 1) surface) cubic structure with a Co content and an interlayer spacing increasing with thickness. After 650 C annealing, the films are completely reacted and have an unstrained B-type CoSi2 structure. High temperature (500 C) deposition of Co leads to the formation of stoichiometric CoSi2 films. Both annealed and high temperature grown films are found to be Si terminated. 2002 Elsevier Science B.V. All rights reserved.

Influence of defect distribution on the reducibility of CeO2−x nanoparticles

Ceria nanoparticles (NPs) are fundamental in heterogeneous catalysis because of their ability to store or release oxygen depending on the ambient conditions. Their oxygen storage capacity is strictly related to the exposed planes, crystallinity, density and distribution of defects. In this work a study of ceria NPs produced with a ligand-free, physical synthesis method is presented. The NP films were grown by a magnetron sputtering based gas aggregation source and studied by high resolution- and scanning-transmission electron microscopy and x-ray photoelectron spectroscopy. In particular, the influence of the oxidation procedure on the NP reducibility has been investigated. The different reducibility has been correlated to the exposed planes, crystallinity and density and distribution of structural defects. The results obtained in this work represent a basis to obtain cerium oxide NP with desired oxygen transport properties.

NiO/Fe(001): Magnetic anisotropy, exchange bias, and interface structure

The magnetic and structural properties of NiO/Fe epitaxial bilayers grown on MgO(001) were studied using magnetooptic Kerr effect (MOKE) and conversion electron Mossbauer spectroscopy (CEMS). The bilayers were prepared under ultra high vacuum conditions using molecular beam epitaxy with oblique deposition. Two systems were compared: one showing the exchange bias (100ML-NiO/24ML-Fe), ML stands for a monolayer, and another where the exchange bias was not observed (50ML-NiO/50ML-Fe). For both, the magnetic anisotropy was found to be complex, yet dominated by the growth-induced uniaxial anisotropy. The training effect was observed for the 100ML-NiO/24ML-Fe system and quantitatively described using the spin glass model. The composition and magnetic state of the interfacial Fe layers were studied using 57Fe-CEMS. An iron oxide phase (Fe3+4Fe2+1O7), as thick as 31 A, was identified at the NiO/Fe interface in the as-deposited samples. The ferrimagnetic nature of the interfacial iron oxide film explains the comple...

Local modifications of magnetism and structure in FePt (001) epitaxial thin films by focused ion beam : Two-dimensional perpendicular patterns

Focused ion beam was utilized to locally modify magnetism and structure of L10 FePt perpendicular thin films. As a first step, we have performed a magnetic, morphological, and structural study of completely irradiated FePt films with different Ga+ doses (1×1013–4×1016 ions/cm2) and ion beam energy of 30 keV. For doses of 1×1014 ions/cm2 and above a complete transition from the ordered L10 to the disordered A1 phase was found to occur, resulting in a drop of magnetic anisotropy and in the consequent moment reorientation from out-of-plane to in-plane. The lowest effective dose in disordering the structure (1×1014 ions/cm2) was found not to affect the film morphology. Taking advantage of these results, continuous two-dimensional (2D) patterns of perpendicular magnetic structures (250 nm dots, 1 μm dots, 1 μm-large stripes) were produced by focused ion beam without affecting the morphology. The 2D patterns were revealed by means of magnetic force microscopy, that evidenced peculiar domain structures in the ca...