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We have studied the shape of the distribution F(G) for the conductance G between points on the surface of a metal–insulator nanocomposite film and the conducting substrate by Conducting Atomic Force Microscopy and computer simulation. Random resistor networks with both metallic and tunneling bonds included are used to model the nanocomposite films. The shape of F(G) is determined mainly by the connectivity of metal particles and the maximum tunneling distance in the composite. By qualitatively comparing the simulation results with the experimental data on granular NiFe-SiO2 films near the metal–insulator transition, we find important implications for the understanding of microscopic conduction mechanisms near the metal–insulator transition.

Resistance of samples prepared by compressing submicron graphite powders was measured in the temperature range 1.8–300 K and magnetic field up to 4.5 T. The temperature dependence of resistance for some samples is satisfactorily described by a fluctuation-induced tunneling model R∝ exp [T 1 /(T+T 2 )] . In others resistance follows a power law R∝1/T α with α close to 1, and we propose a phonon-assisted bunch hopping (or tunneling) model for this case. The dominating mechanism is determined by the effective tunneling distance. Magnetoresistance (MR) is dominantly positive, with mesoscopic oscillations at low fields, in the fluctuation-induced tunneling regime, and has a negative component in the phonon-assisted bunch hopping regime. Our results confirm the existence of negative MR in systems with nearest-neighbor tunneling conduction and point the conditions for this effect. We interpret NMR as due to shifting of discrete electron levels towards the Fermi level in the magnetic field, which enhances the phonon-assisted bunch hopping.

Dielectric measurements were done on cosputtered metal-insulator nanocomposite films with metal volume fraction above the metal-insulator transition, in the frequency range between 20 Hz and 30 MHz. At intermediate and high frequency, the dielectric function can be qualitatively described in terms of the percolation theory. In the low-frequency region, we observe a sharp relaxation-type increase of the real part of dielectric constant with decreasing frequency, while the imaginary part is dominated by dc conductivity. We suggest that the low-frequency behavior may be due to a combination of metallic and tunneling conduction in the system.

We have imaged the conducting paths in a metal - insulator composite , with x ranging from 0.24 to 0.576 by conducting atomic force microscopy (AFM). The surface morphology and the electric current between the tip and sample have been obtained simultaneously on the nanometre scale. Measurable changes of current image have been observed for x below and above the percolation threshold . Our observations imply the importance of tunnelling for all samples. The high spatial resolution of AFM combined with a conducting tip provides new insight into electron transport behaviour in metal - insulator composites at the nanometre

Multilayers of Co/ZnO, with varying nominal thickness of metal (2–10 A) and semiconductor (2–20 A), were prepared by ion-beam sputtering. Magnetic, transport, and magnetotransport measurements were carried out over a temperature range of 2.5 to 300 K. Upon decreasing Co thickness and increasing ZnO thickness in the multilayer stack, the properties of the samples undergo a crossover from those of granular metallic Co/semiconductor multilayers to a dilute magnetic semiconductor superlattice. We interpret ferromagnetism in the latter case as due to ordering in the Co-rich layers, mediated by carriers from lightly doped, high carrier concentration layers.

Ferromagnetic properties have been observed in CuO doped with 3.5–15 at. % of Mn. The transition from ferromagnetic to paramagnetic phase at TC=80 K is associated with the metal–insulator transition. Magnetoresistance is weakly negative in the vicinity of the transition, but positive in a wide range of temperatures below TC. The experimental results suggest a possibility of interpretation in terms of the Zener double-exchange mechanism and strong electron–phonon interactions.

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.

We study the shape of distribution F(G) for the conductance G between a point on the surface of a metal–insulator nanocomposite film and the conducting substrate. Random resistor networks with both metallic and tunneling bonds included are used to model nanocomposite films. Our simulation results show explicitly that the shape of F(G) is determined mainly by the connectivity of metal particles and the maximum tunneling distance in the composite. By applying our results to the available experimental data on granular NiFe–SiO2, we find important implications for the understanding of microscopic conduction mechanisms near the metal–insulator transition.

Using complementary experiments we show that the room temperature ferromagnetism observed in anatase $\mathrm{Co}\ensuremath{\mathbin:}{\mathrm{TiO}}_{2}$ films is not carrier mediated, but coexists with the dielectric state. TEM and x-ray absorption spectroscopy reveal a solid solution of Co in anatase, where Co is not metallic but in the $+2$ state substituting for Ti. Measurements at 300 K yield a ${M}_{\mathrm{S}}$ of $1.1\text{ }\text{ }{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{Co}$ atom, while all films are highly insulating. The evidence of intrinsic ferromagnetism in the dielectric ground state of $\mathrm{Co}\ensuremath{\mathbin:}{\mathrm{TiO}}_{2}$ leads to new considerations for the origin of ferromagnetism in transition metal doped oxides.

We explore the effects of codoping with hydrogen on magnetism, conductivity, and spin polarization of carriers in Cr-doped ZnO. Zn0.99Cr0.01O:H films sputter deposited on sapphire show a correlation between magnetization and conductivity when H is introduced. In the first method, dielectric and weakly magnetic films grown in pure Ar are subsequently annealed at 400°C in a 5% H2 95% Ar 1atm flowing tube furnace. These films show increases in conductivity and saturation and remnant magnetization postanneal. In the second method, conducting ferromagnetic films are grown in the H∕Ar mixture. They are magnetic as grown but show a small decrease in saturation and remnant magnetization and conductivity post-H∕Ar anneal. Ferromagnetic CrO2 with TC=390K or antiferromagnetic phases are not detected in hydrogenated films. We studied spin polarization of carriers using anomalous Hall effect; however, initial experiments show no such signs, hence spin polarization is not yet confirmed. Hydrogen doped in dielectric Cr:...

We experimentally investigate the room temperature ferromagnetism observed in insulating Co doped anatase TiO2 thin films grown by sputter deposition. The Co was uniformly incorporated in the lattice as Co(II) with no evidence of Co metal. A series of annealing treatments were carried out to optimize the ferromagnetic ordering and a saturation moment of 1.1μB∕Co atom at 300 K was obtained with UHV annealing at 450 °C. Both as-deposited and annealed films were highly insulating at room temperature. Results show that the ferromagnetism is strongly dependent on the number and distribution of oxygen vacancies in the Co:TiO2 lattice.

The Hall effect in granular co‐sputtered ferromagnetic metal–insulator films was found to increase dramatically as the magnetic volume fraction decreases toward the metal–insulator transition. The saturated Hall resistivity is up to 160 μΩ cm at T=5 K, that is almost four orders of magnitude greater than that in a pure magnetic metal sample. Close to the metal–insulator transition, both magnetoresistivity and the saturated Hall resistivity decrease with increasing temperature. Correlations of the Hall resistivity with resistivity and magnetoresistivity are discussed.