Surface functionalization of a series of nanosized iron oxide particles (average diameter around 6 nm) with oleic acid was realized in this study. The aim is to suspend the surface-functionalized nanoparticulated materials in insulating mineral oil and evaluate their colloidal stability as a function of time. Nanoparticulated samples presenting stoichiometry close to maghemite were obtained by oxidation of a freshly precipitated magnetite sample. Systematic variations observed in the Fe3+/Fe2+ ratio, average particle size, and lattice constant were attributed to differences in oxidation route and oxidation condition employed. Morphological, compositional, thermal, optical and magnetic characterization techniques were used in the investigation of native (P, PN1, PN2, POX1, POX3, and POX7) and surface-functionalized (POA, PN1OA, PN2OA, POX1OA, POX3OA, and POX7OA) samples. While suspending the oleic-acid-coated nanosized iron oxide particles in insulating mineral oil, the best colloidal stability was achieved at the oxidation profile of Fe3+/Fe2+ = 40 (5.9 nm average core diameter), leading to a surface grafting coefficient of about 75% of a full monolayer coating of chemisorbed species only and resulting in the smallest observed hydrodynamic radius (8.1 nm). Within the range of our investigation, our findings reveal the characteristics and the chemical protocol used to produce a magnetic fluid sample embodying long-term colloidal stability, thus representing a very much promising material for application as a refrigerating fluid in power transformers and related devices.

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We present precise measurements of the upper critical field ͑H c2 ͒ in the recently discovered cobalt oxide superconductor. We have found that the critical field has an unusual temperature dependence; namely, there is an abrupt change of the slope of H c2 ͑T͒ in a weak-field regime. In order to explain this result we have derived and solved Gor'kov equations on a triangular lattice. Our experimental results may be interpreted in terms of the field-induced transition from singlet to triplet superconductivity.

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The X-band EPR and magnetic susceptibility in the temperature range 4.2-300 K study of the shungite-I, natural nanostructured material from the deposit of Shunga are reported. Obtained results allow us to assign the EPR signal to conduction electrons, estimate their number, N P , and evaluate the Pauli paramagnetism contribution to shungite susceptibility. A small occupation (*5%) of the localized nonbonding p states in the zigzag edges of the open-ended graphene-like layers and/or on r (sp 2?x) orbitals in the curved parts of the shungite globules has been also revealed. The observed temperature dependence of the EPR linewidth can be explained by the earlier considered interaction of conduction p electrons with local phonon modes associated with the vibration of peripheral carbon atoms of the open zigzag-type edges and with peripheral carbon atoms cross-linking different nanostructures. The relaxation time T 2 and diffusion time T D are found to have comparable values (2.84 9 10-8 and 1.73 9 10-8 s at 5.2 K, respectively), and similar dependence on temperature. The magnetic measurements have revealed the suppression of orbital diamagnetism due to small amount of large enough fragments of the graphene layers.

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The magnetic measurements of the Josephson critical field and intergranular critical currents are presented for the Bi2223 compound. The investigations prove that the Josephson critical field value is mainly dependent on the intergranular critical current density. The other factors like the grain dimensions and shapes, sample porosity and density are less important. The relation between the Josephson critical field and critical current H cJ (J c ) is well described within the single junction model, H cj ∝ J c λ J . The temperature dependence of H cJ can also be well fitted according to the single junction model with a critical current dependence, J c ∝ (1 − T /T cJ ) α . The critical current density versus temperature is almost linear indicating the presence of superconductor-insulator-superconductor (SIS) type junctions.