Synthesis, Characterization And Magnetic Properties Of Co And Co/CoO Magnetic Nanoparticles

Researches on exchange bias mainly focused on thin films system andnanoparticles-thin films system, and during the study, some laws and theoreticalsystems are summarized. But few researches on exchange bias in FM/AFM core-shellnanoparticles system have been done, which obtained, mostly, by partially oxidizing,nitriding and sulfiding ferromagnetic metal nanoparticles. In these systems, core andshell cannot be controlled independently, because the size of core and thickness ofshell will change at the same time. Therefore, we synthesize FM/AFM core-shellnanoparticles with size controlled and adjust magnetic properties such as exchangebias and coercivity through controlling core size and shell thickness.In this thesis, face-centered cubic (fcc) cobalt nanoparticles, hexagonal closepacked (hcp) cobalt nanoparticles and simple cubic (ε) cobalt nanoparticles aresynthesized by both pyrolysis method and high-temperature solution reduction, andCo/CoO core-shell nanoparticles are synthesized successfully. The microstructure,magnetic properties, and surface morphology of samples are studied. The mainconclusions are as follows:1. Face-centered cubic (fcc) cobalt nanoparticles with an average particle size ofaround18nm are synthesized by pyrolysis method, with Co2(CO)8as cobalt source,oleic acid and dioctylamine as surfactants. The samples are core-shell structure withfcc-Co nanoparticles as core and oleic acid and dioctylamine as coating layer, and thefcc-Co nanoparticles are single crystal with a mean crystal size of9nm. The resultsindicate that cobalt nanoparticles wih even size are well dispersed; The as-preparedsample at room temperature shows ferromagnetic behavior, with saturationmagnetization and the coercivity of the samples being43.4emu/g and115Oe,respectively, indicating that the superparamagnetic critical size should be far below9nm; after being annealed at300℃in hydrogen for1h, cobalt nanoparticles appearphase transformation from fcc-Co to hcp-Co, in which an endothermic process isobserved for the first time, indicating that the lower free energy and more stablestructure in fcc-Co compared with hcp-Co.2. Hexagonal close packed (hcp) cobalt nanoparticles are synthesized by high-temperature solution reduction method. Then, the influence of reaction conditionon the structure, size and magnetic properties are discussed. The results show that thecobalt nanoparticles obtained from different feeding mode are similar. Hcp-Conanoparticles of a size distribution at the range of3.0to15.8nm can be obtained bycontrolling surfactants, reaction temperature and the concenstration of reactants, andthe critical single domain size is around15nm.3. Simple cubic (ε) cobalt nanoparticles are synthesized by high-temperaturesolution reduction method, which show ferromagnetic behavior at room temperature,with effective saturation magnetization of127.37emu/g and the coercivity of275.07Oe. After being annealed at300℃, cobalt nanoparticles transform from ε-Co tohcp-Co.4. Basing on the hcp-Co nanoparticles, Co/CoO core-shell nanoparticles withcontrollable thickness are synthesized successfully by decomposing thecobalt-oleylamine complex. The size of hcp-Co core is about28nm and the thicknessof fcc-CoO shell is about0.8~2.3nm. The results indicate that below TN, covercivities,exchange bias and blocking temperature of the system increase, with increasing thethickness of antiferromagnetic CoO.