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The methods of C-C bond formation between free or coordinated 1,10-phenanthrolines are examined, and a new structural class of cyanine tetraazachromophores, the electron-rich d-element 1,10-phenanthrocyanines is presented, the role of metal promoted Csp2H-Csp2H coupling reactions in the synthesis of new metal 1,10-phenanthrocyanines is discussed.

Electrically driven switching of resistance states is observed in a system prepared by depositing Co and Cu layers in trenches of widths 50-500 nm in SiO2, followed by application of a controlled high-density current. This `forming' process leads to both an increase of resistance and a transition from metallic conduction to activated tunnelling. In the resulting system, the value of resistance can be switched reversibly by positive or negative voltage. After applying a positive bias >1.3 V the low-bias resistance decreases, while after applying a negative bias of the same value it increases. The resistance ratio between the high-resistance and low-resistance states of the system is in the range ~1.5-6. A possible interpretation of the observed phenomena is discussed.

A systematic study of magnetic and magnetotransport properties of thin Co films on Si is reported in this work. The Co films of thicknesses 30, 160 and 440 A were prepared by magnetron sputtering from a Co target onto a Si(100) substrate held at room temperature. Resistance, magnetoresis-tance and Hall effect were measured in the temperature range 5-350 K. Magnetization was measured in the same temperature range using a SQUID magnetometer. Complete surface hysteresis loops were measured from 4 to 300 K by means of the magneto-optical Kerr effect, in order to follow the behavior of the coercivity and magnetic easy axes of the samples. The transport and magnetotransport properties display a peculiar effect as functions of temperature. The films behave as pure metallic Co below 250 K. However, the resistance drops with heating from 250 to 280 K, regaining its typical metallic behavior at temperatures higher than 280 K. The thinner the film, the larger is the resistance drop in the temperature interval 250-280 K, reaching a factor of 4.5 for the 30 A thick film. The Hall effect contains both ordinary and extraordinary contributions. From ordinary Hall effect measurements, one finds that the conduction is electronic at low temperatures and turns to hole-like above the transition point. Magnetoresistance changes from negative at low T to positive at high temperatures. These facts indicate that the observed effect is related to the metallic layer, which undergoes a conducting channel switching when the temperature is increased.

The synthesis of monodispersed 10 nm cobalt nanocrystals with controlled crystal morphology and investigation of the surface stability of these nanocrystals are described. Depending on the surfactants used, single crystalline or multiple grain nanocrystals can be reproducibly produced. The relative surface stability of these nanocrystals is analyzed using the temperature dependences of the dc magnetic susceptibility. The novel method, which allows sensitive monitoring of the surface stability, is based on the observation that, with particle oxidation, an anomalous peak appears at 8 K in zero-field-cooled magnetization measurements. It is found that the surfactant protective layer is more important for long-term stability at room temperature, while the high-temperature oxidation rate is controlled by the crystal morphology of the nanoparticles.

We found that in granular (NiFe) x (SiO 2 ) 1− x films, the ordinary Hall effect increases with decreasing x , in a way similar to the extraordinary Hall effect, with critical exponents greater than the theoretical value, while the resistivity increases with decreasing temperature as −log T for x slightly above the percolation threshold x c , and as T −0.2 for x ∼ x c . We suggest that the giant Hall effect results from a reduced effective carrier density due to weak localization at x ∼ x c .

Multilayers of Co and ZnO, with nominal layer thicknesses on the atomic scale with up to 25 bilayers, were deposited by ion beam sputtering on silicon and glass substrates at ambient temperature. Thick epitaxial CoxZn1−xO films on Al2O3(012) substrates were grown by metalorganic chemical vapor deposition using a liquid precursor delivery system. All were co-doped with Al. Comparative analysis of magnetization, resistivity, and magnetoresistance measurements, performed in the temperature range 2.5–300 K, is presented. At small thickness of Co layers in the multilayer samples, these structures are diluted magnetic semiconductor (DMS) superlattices, with properties close to the epitaxial films. A crossover from DMS to discontinuous magnetic metal/semiconductor multilayers is observed with increasing metal content in the multilayers. This leads to changes in conduction mechanisms, with increasing contribution of quasithree-dimensional or quasitwo-dimensional intergranular hopping, and superparamagnetism.

Synthesis and characterization of magnetic semiconductors Al1−xCrxN, in which the atomic fraction of chromium x is up to 0.357, are reported. The films, grown by reactive co-sputtering on silicon, glass, and kapton substrates, have a crystal structure of aluminum nitride. Magnetic and transport properties were studied in the temperature range of 50 to 340 K. The materials are in the dielectric regime and have variable-range-hopping type of conductance. The films are ferromagnetic at temperatures over 340 K.

Quasi-critical percolation systems are prepared from conducting metal-insulator granular composite samples by subjecting them to high-density current pulses. This treatment drastically reduces the room-temperature conductivity of the mixture, due to the explosion of a fraction of the metallic grains in percolation channels. Provided that the pulse current density is high enough, the samples show the hopping conductivity characteristic of an insulator. Abnormal dielectric properties of quasi-critical percolation systems have been previously investigated, and the objective of the present work is to study the transport and magnetotransport phenomena. Two distinctly different nearest-neighbour hopping (NNH) conduction regimes, which depend on the sample preparation conditions, are observed in the quasi-critical samples. The first type of conduction is dominated by hopping between adjacent metallic grains embedded in a dielectric host medium. The grain diameter, evaluated from the resistance versus temperature data, is found to be several nanometres. Negative orbital magnetoresistance (NMR) with a quasi-linear field dependence, is observed at both 77 and 300 K. In the sample of the second type (those which have been subjected to higher-current pulses or to several sequential pulses of the same amplitude), conduction is also found to be dominated by NNH between conducting sites in the host medium, but an evaluation of the 'grain' size shows them to be of atomic dimensions.

Magnetization, resistance and Hall effect are studied in granular magnetic Co–SiO2 nanocomposites in the temperature range 5–300 K and fields up to 6 T. Relative contributions from spin-independent and spin-dependent processes to the giant Hall effect near the metal–insulator transition are analysed.

The methods of C-C bond formation between free or coordinated 1,10-phenanthrolines are examined, and a new structural class of cyanine tetraazachromophores, the electron-rich d-element 1,10-phenanthrocyanines is presented, the role of metal promoted Csp2H-Csp2H coupling reactions in the synthesis of new metal 1,10-phenanthrocyanines is discussed.

A way of obtaining binary composite materials having some properties of the critical percolation system is considered. These materials can be obtained by high-density current action on an initially well conducting mixture. This action destroys part of the contacts between adjacent conducting granules, bringing the system to the quasi-critical state. It is shown that quasi-critical systems may have high values of dielectric constant epsilon approximately=102-106 in the frequency range 0

In this paper, we report a detailed investigation of temperature dependences of magnetization of powders of monodispersed /spl epsi/-Co particles. In the zero-field-cooled and field-cooled low field magnetization measurements, we observe two size-dependent and one size-independent features, corresponding to (I) blocking of moments of nanoparticles, (II) a second phase peak, and (III) a feature which may be associated with freezing of surface moments. For small particles, field-induced interaction effects on the blocking temperature (I) has also been observed.

Resistance, magnetoresistance and Hall effect were studied in sputtered cobalt films of varying thickness, deposited on silicon substrates, in the temperature range of 5–350 K and magnetic field up to 7 T. Contributions from both metallic cobalt and silicide layers are revealed. The latter undergo a metal–insulator transition in the temperature range of 260–280 K.

Following the synthesis of monodispersed 20nm cobalt nanoparticles via a thermal decomposition method we have successfully transferred these hydrophobic nanoparticles into the water phase using tetramethylamonium hydroxide pentahydrate as phase transfer agent and 12-aminododecanoic acid as a stabilizing agent. Frequency dependent ac susceptibility of water-based cobalt nanoparticle ferrofluids was measured at room temperature in the frequency range of 0.01–1000Hz. In addition to the “high” frequency magnetic relaxation peak at 200Hz, which is determined by the solvent viscosity and hydrodynamic volume of the nanoparticles, a lower frequency relaxation peak attributed to the interaction of surface coatings is observed. The position of the latter varies with the solution pH in the range of 0.02–0.05Hz. This variation is explained as due to the change of chemical and charge state of the surfactant molecules. The peak at low frequency may be potentially used to study protein configuration changes in solution ...