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Changes of the magnetic properties of ferromagnetic Co particles deposited at room temperature on a thin alumina film grown on a NiAl(110) substrate were investigated as a function of Pd coverage by subsequent deposition of Pd onto deposited Co particles. From previous x-ray photoelectron spectroscopy, IR, and temperature programmed desorption experiments it was concluded that Pd forms a shell on top of Co particles. However, the current experiments indicate that Pd does induce structural rearrangements within the Co particles which may also involve the intermixing of small amounts of Pd into the Co particles. The latter is inferred from a change in the g-value for small particles. The impact of a larger intermixture of Co and Pd on the magnetic properties will be emphasized by a reversed deposition order where Pd particles were deposited first and subsequently covered by Co. The reversal of the deposition order increases the magnetic anisotropy of the particles considerably. [ABSTRACT FROM AUTHOR]

Changes of the magnetic properties of ferromagnetic Co particles deposited on the [Square_Root]31×[Square_Root]31R±9° reconstructed α-Al2O3(0001) as well as on a thin alumina film grown on a NiAl(110) substrate were investigated as a function of thermal annealing. On the thin film changes of the magnetic response were found above 500 K which correlates with changes in the particle size distribution. Annealing to 870 K leads to a permeation of the metal though the oxide film which causes significant changes in the ferromagnetic resonance response. On the α-Al2O3 single crystal sintering of particles requires temperatures above 600 K being about 100 K higher as compared to the thin alumina film. For large clusters intraparticle redistribution takes place already below 600 K a phenomenon not observed for the small clusters. In addition, a significant dependence of the measured g values from the substrate as well as the thermal treatment is found which can be understood in terms of the structural properties of the systems. [ABSTRACT FROM AUTHOR]

Infrared reflection absorption spectroscopy of CO has been used as a surface-sensitive probe for the available binding sites of Co–Pd bimetallic particles supported on a thin alumina film. A high-coverage state is obtained on the Co particles in addition to atop sites, attributable to a M(CO)[sub n] species. Bridge and threefold sites are not detected from CO stretching on the Co particles. When both metals are deposited sequentially at 300 K, Pd easily forms a shell on existing Co particles. In the reverse order, much more Co is required to coat Pd particles, because it nucleates between Pd particles as well as on top of them. For both metals, atop sites are better preserved at various bimetallic compositions because they are statistically less vulnerable than threefold hollow sites. The stretching frequency of CO to a given site is nearly independent of the bimetallic composition. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Based on the work of the Somorjai group [Magni and Somorjai, Catal. Lett. 35, 205 (1995)] we have prepared a thin well ordered MgCl[sub 2](001) film by MgCl[sub 2] evaporation from a Knudsen cell. This film does not absorb TiCl[sub 4] at room temperature if it is not activated by increasing the defect density via electron or ion bombardment. The nature of some of the defects created is characterized by in situ ESR measurements and Auger spectroscopy. Paramagnetic surface defects are altered by the bonding of TICl[sub 4] to the surface as observed by ESR spectroscopy. Ti[sup 3+] centers are detected if particularly severely defected MgCl[sub 2] layers are prepared. Reactivity studies show however, that these species are not correlated with polymerization activity. Interaction with aluminum alkyl leads to the formation of the active catalyst and we observe for the first time directly ethyl radicals formed from trimethyl-aluminum in an abstraction process which may be formulated as TiCl[sub 4] / [sup surface] + A1Me[sub 3] → Me - TICl[sub 3] / [sup surface] + A1Me[sub 2]Cl, Me-TiCl[sub 3] / [sup surface] → TiCl[sub 3] / [sup surface] + Me-, and Me · +Me[sub 3]Al → C[sub 2]H[sub 5]- +AlH(Me)[sub 2]. The presence of the aluminum alkyl is observed via in situ IRAS in the same apparatus. [ABSTRACT FROM AUTHOR]

Examines the rotational motion of self-assembled stearic acid films adsorbed on Al[sub 2]O[sub 3]-thin film. Use of electron paramagnetic resonance (EPR) spectroscopy; Dependence of EPR spectra on temperature; Role of stearic acid films for the line shape analysis of EPR spectra.

A new ultrahigh vacuum (UHV) electron paramagnetic resonance (EPR) spectrometer operating at 94 GHz to investigate paramagnetic centers on single crystal surfaces is described. It is particularly designed to study paramagnetic centers on well-defined model catalysts using epitaxial thin oxide films grown on metal single crystals. The EPR setup is based on a commercial Bruker E600 spectrometer, which is adapted to ultrahigh vacuum conditions using a home made Fabry Perot resonator. The key idea of the resonator is to use the planar metal single crystal required to grow the single crystalline oxide films as one of the mirrors of the resonator. EPR spectroscopy is solely sensitive to paramagnetic species, which are typically minority species in such a system. Hence, additional experimental characterization tools are required to allow for a comprehensive investigation of the surface. The apparatus includes a preparation chamber hosting equipment, which is required to prepare supported model catalysts. In addition, surface characterization tools such as low energy electron diffraction (LEED)/Auger spectroscopy, temperature programmed desorption (TPD), and infrared reflection absorption spectroscopy (IRAS) are available to characterize the surfaces. A second chamber used to perform EPR spectroscopy at 94 GHz has a room temperature scanning tunneling micro-scope attached to it, which allows for real space structural characterization. The heart of the UHV adaptation of the EPR experiment is the sealing of the Fabry-Perot resonator against atmosphere. To this end it is possible to use a thin sapphire window glued to the backside of the coupling orifice of the Fabry Perot resonator. With the help of a variety of stabilization measures reducing vibrations as well as thermal drift it is possible to accumulate data for a time span, which is for low temperature measurements only limited by the amount of liquid helium. Test measurements show that the system can detect paramagnetic species with a density of approximately 5 x 1011 spins/cm², which is comparable to the limit obtained for the presently available UHV-EPR spectrometer operating at 10 GHz (X-band). Investigation of electron trapped centers in MgO(001) films shows that the increased resolution offered by the experiments atW-band allows to identify new paramagnetic species, that cannot be differentiated with the currently available methodology. [ABSTRACT FROM AUTHOR]

We report g tensors of surface color centers, so-called F(s) (+) centers, of MgO calculated with two density-functional approaches using accurately embedded cluster models. In line with recent UHV measurements on single-crystalline MgO film, we determined only small g-tensor anisotropies and negative shifts Deltag identical with g-g(e) for all F(s) (+) sites considered, namely, (001)-terrace, step, edge, and corner sites. The g values are very sensitive to the local structure of the defect: relaxation reverses the sign of Deltag. However, accounting for the spin-orbit interaction either self-consistently or perturbatively yields very similar results. In addition to the values of the tensor components, their direction with respect to the surface was determined. In contrast to edges, significant deviations from ideal C(2v) symmetry were found for F(s) (+) centers at steps. Recent data on single-crystalline thin films are reevaluated in the light of these results.

Changes of the magnetic properties of ferromagnetic Co particles deposited on the radical31 x radical31R +/- 9 degrees reconstructed alpha-Al2O3(0001) as well as on a thin alumina film grown on a NiAl(110) substrate were investigated as a function of thermal annealing. On the thin film changes of the magnetic response were found above 500 K which correlates with changes in the particle size distribution. Annealing to 870 K leads to a permeation of the metal though the oxide film which causes significant changes in the ferromagnetic resonance response. On the alpha-Al2O3 single crystal sintering of particles requires temperatures above 600 K being about 100 K higher as compared to the thin alumina film. For large clusters intraparticle redistribution takes place already below 600 K a phenomenon not observed for the small clusters. In addition, a significant dependence of the measured g values from the substrate as well as the thermal treatment is found which can be understood in terms of the structural properties of the systems.