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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 ...

Using high-resolution, aberration-corrected, scanning transmission electron microscopy and electron-energy-loss spectroscopy we show that in films of single-phase anatase Co:TiO2, the Co distribution and magnetic properties are strongly dependent both on the overall crystalline quality and postgrowth vacuum annealing process. The Co:TiO2 films are coherent, epitaxial anatase with no secondary phases or metallic Co. Films of lower crystalline quality reveal a relatively homogeneous Co concentration, while films of higher crystalline quality show a tendency for Co enrichment near the surface region, around grain boundaries, and the substrate interface. Both uniform and nonuniform samples show a notable enhancement in the saturation magnetization with annealing, while the coercive field is considerably higher in the samples with uniform Co distribution. These experiments confirm that films of single-phase anatase Co:TiO2 with both uniform and nonuniform Co distributions exhibit room-temperature ferromagnetism in the insulating ground state, while the compositional uniformity and film microstructure play a role in the bulk magnetic properties of the material and the mechanisms for ferromagnetic ordering.

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.

Measurement of the variation of relaxation frequency of Brownian rotation of magnetic nanoparticles in a ferrofluid due to binding of organic molecules is a possible tool for detection of biomolecules in solution. We investigate frequency- and temperature-dependent magnetic behavior of model ferro-fluids of surfactant-coated Co nanoparticles, 20nm in diameter, in dichlorobenzene in a wide concentration range. At room temperature the most diluted ferrofluids have a single relaxation frequency determined by the fluid viscosity and effective particle volume. Increasing concentration leads to an appearance of the second, low-frequency relaxation peak, attributed to the effective viscosity associated with interparticle friction.

Through nonequilibrium rf magnetron cosputtering of ZnO and Cr metal on a-plane Al2O3 we find ferromagnetic ordering with a room-temperature saturation moment of 1.4μB per Cr ion at a doping concentration of ∼9.5 at. % after UHV postanneal. No secondary phases are detected in the films via x-ray diffraction (XRD). Increased Cr doping causes disorder within the films resulting in decreased overall, and per Cr, moment. The Curie temperature exceeds 365 K, the maximum temperature reached in our experiment. All films are dielectric with a resistivity higher than 106Ωcm at room temperature. The lack of carriers indicates that the ferromagnetic mechanism is not carrier mediated.

We present a systematic approach to fabricate a variety of magnetic nanoparticles with desirable structure and controlled magnetic properties based on our studies of the process kinetics. The morphology of binary alloy particles is dependent on their bulk thermodynamics—for immiscible heterogeneous systems (Co–Au) core-shell structures are obtained while miscible systems (Fe–Pt) lead to alloy nanoparticles. The annealing effects on FePt nanoparticles show that the coercivity and magnetic anisotropy increase dramatically after annealing at temperatures above 650°C. Studies of Co–Au core-shell structure show that the core is magnetic, but the Au shell does not significantly affect its magnetic properties.

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.

The complex conductivity was measured on 3d granular NbC-KCI composite samples at varying metal volume fraction p, frequency ω and temperature above and below the superconductivity critical Tc. The observed frequency dispersion is anomalous in that it is not in accord with the scaling theory of percolation transition. The results are compared with a recently developed scaling theory, which takes both intercluster tunneling and intercluster capacitance into account. The experimental estimates for the new critical exponents are in reasonable agreement with the theory. The very low value of the crossover frequency can also be understood. We also present the data showing the dispersion of the complex conductivity well below the superconducting transition Tc of NbC.