Cobalt based magnetic nanocomposites: Fabrication, Fundamentals and Applications.

摘要

Recently, magnetic nanocomposites have aroused significant interests due to their potential success in fundamental researches as well as in real applications. The research focus on magnetic nanocomposites now lies in developing efficient way for their synthesis, probing their fundamental physics and exploring possible related applications. In this dissertation, we have focused on three important topics in magnetic nanocomposite physics, namely, thermal, electrical and magnetic properties of cobalt based magnetic nanocomposites in three model systems.;First, as-synthesized Aucore-Coshell nanoparticles are thermally unstable. In an ex-situ annealing experiment, it was observed that the core-shells slowly transformed to stable peanuts structures via several intermediate states. The series of morphological transformations have been interpreted in term of a series of energy minimizations including the grain boundaries, Co/Au interface and strain.;Second, chemically prepared cobalt/poly (3-hexylthiophene, 2, 5-diyl) (P3HT) hybrid thin films are consisted of a crystalline P3HT matrix, interspersed with amorphous P3HT regions containing the cobalt nanoparticles. Temperature dependence of the resistance of these hybrid systems is well-fitted to the fluctuation induced tunneling (FIT) model. Under magnetic field, a magnetoresistance ratio of 3% was observed in 17 vol.%Co hybrid films at 10K. The magnetoresistance is interpreted by spin-dependent tuning between cobalt clusters across the crystalline P3HT.;Third, in cobalt ferrofluids, a second order blocking-unblocking and a first order melting transitions were observed and indicated by a broad peak and a sharp peak in the ZFC curves, respectively. When the blocking and melting transitions were superposed, the strongest intensity of the sharp peak at the melting point of the organic solvent was obtained. This observation is interpreted by strongest Brownian relaxation in the premelting stage Additionally, the first order melting is also a first order magnetic transition. The magnetocaloric effect in cobalt ferrofluid is also studied.;In addition, two mathematical models were also developed. In the first model, ZFC and FC curves of non-interacting and interacting cobalt nanoparticles are simulated by M-spectrum theory. In the second model, the traditional growth model of nanoparticles is revised without assumption of diffusional layer, which is only built upon Fick's law of diffusion and law of mass conservation.