National Institute Of Materials Physics - Romania

Beyond nano-science is angstrom-science, beyond nano-technology is angstrom-technology, beyond nano-devices are angstrom-devices. Beyond these new emerging fields in materials physics is to ride on electron, but this is almost pure phantasy.

We have studied the oxidation states of Fe and Mo and the presence of grain boundaries in the magneto resistive (MR) compounds Sr2FeMoO6 by means of x-ray photoelectron spectroscopy (XPS), Mossbauer spectroscopy and electrical resistivity measurements. XPS of the Mo 3d and Fe 3s core levels is indicating a mixed valence state involving around 30% Fe3+ - Mo5+ and 70% Fe2+ - Mo6+ states. Mossbauer studies confirm the presence of a valence fluctuation state and an essential amount of grain boundaries in the present Sr2FeMoO6 crystals. Resistivities and magnetoresistance studies evidenced strong grain boundary effects.

Two different Fe/MnF2 samples have been prepared by e-beam evaporation on MgO(001) substrates. The Fe layer in the samples includes a 10 angstrom thick Fe-57 probe layer either at the Fe/MnF2 interface (interface sample) or 35 angstrom away from the interface (center sample). The samples are characterized by X-ray diffraction, conversion electron Mossbauer spectroscopy (CEMS) and SQUID magnetometry. Fe-57 CEMS has been employed to study the depth dependent hyperfine interactions in Fe/MnF2 as a function of temperature between 18 K to 300 K. The hyperfine field B-hf has been obtained for the interfacial and off-interfacial Fe-57 layers. At the interface, besides B-hf of bcc-Fe, the presence of a component with a distribution P(B-hf ) is observed. The latter is assigned to interfacial Fe-57 atoms, indicating some (similar to 15%, equivalent to similar to 1 Fe atomic layer) intermixing at the Fe/MnF2 interface and a decrease of the average < B-hf > by 21%. The influence of the interface disappears as the Fe-57 probe layer is placed away from the interface. The temperature dependence of the average < B-hf > of the interface has been measured. The Fe spins, at remanence, are found to lie in the film plane.

Various closed nanocluster configurations based on the stoichiometric arsenic-chalcogen composition have been built and relaxed by a computer Monte Carlo procedure. They can be used in the construction of the glass network. A model with random packing of various large, closed nano-clusters has been devised and fairly good agreement with experimental data for As2S3 has been obtained.

It is proposed a quasi-linear algorithm for the solving the kinetic equations system, which describes the excitation and relaxation processes in the electronic and phononic subsystems of amorphous chalcogenides. The nonlinear interaction between an optical field and an amorphous chalcogenide medium is analyzed. The nonlinear interaction between an excitation incident optical field and the non-crystalline chalcogenide materials, evidences the specific nonlinear phenomena in amorphous semiconductors, which have no correspondent in the case of crystalline semiconductors. The model is limited to phenomena occurring in chalcogenide semiconductors, simultaneously with the optical field propagation, by neglecting the excitation phenomena.

Microstructure and the sintering behaviour of zirconia with twinned stabiliser ions: Ca2+ + Mg-2 in oxidative and in inert environment respectively obtained by solid state reactions method were studied. The specimens were cured at 1550 degrees C and 1750 degrees C in air in a kiln with methane gas. Samples with the same composition were sintered at 1550 degrees C in Argon in electrical furnace with (graphite electrodes). Their sinterability, microstructure as well as stability was studied The properties were compared as function of the processing route, the compositions stabilised in argon protective environment at 1550 degrees C or in air at 1750 degrees C, a totally stabilisation process was effective.

The ability of the Fe-57 Mossbauer technique to reveal the spin structures in exchange coupled magnetic phases containing iron is emphasized. Experimental results obtained on layered exchange-bias systems are presented. The influence of the ferromagnetic top layer on the out-of-plane spin component of the pinning antiferromagnetic-like layer, in both Fe/FeSn2 and Fe/Fe-Gd-B exchange bias systems, is discussed.

Recently, a double perovskite oxide, Sr2FeMoO6, has been reported to show a very sharp magnetoresistance (MR) response at relatively small-applied fields and at high temperatures. The half-metallic state of this compound ensures a theoretical magnetic moment of 4 mu(B) per formula unit. Ordering control is expected to be influenced by the synthesis route. In this regard, in this article, perovskite samples were obtained from the liquid phase, using oxalic acid as complexing agent. Application of a field above 0.08 T increases the zero-field-cooled (ZFC) magnetization. Below this field, for the ZFC-curves measured from 2 K up to 400 K, magnetization increases, reaching a peak at T-cups(ZFC), before decreasing cups gradually to a paramagnetic state. Saturation moment is relatively low (2.1-2.2 mu(B)), probably due to anti-site defects, as suggested in the literature.

GaAs-based Gunn diodes with graded AlGaAs hot electron injector heterostructures have been developed under the special needs in automotive applications. The fabrication of the Gunn diode chips was based on total substrate removal and processing of integrated Au heat sinks. Especially, the thermal and RF behavior of the diodes have been analyzed by DC, impedance and S-parameter measurements. The electrical investigations have revealed the functionality of the hot electron injector. An optimized layer structure could fulfill the requirements in adaptive cruise control (ACC) systems at 77 GHz with typical output power between 50 and 90 mW.