We have combined transmission electron microscopy (TEM) and magnetic measurements to characterize the growth of colloidal cobalt (Co) nanoparticles during synthesis and their subsequent stability in air. During the growth, TEM measurements have shown the formation of cobalt nanoparticles and magnetic measurements have shown the presence of both super-paramagnetic nanoparticles and a paramagnetic component. At the end of the synthesis, TEM shows that these freshly-prepared cobalt nanoparticles are mono-disperse and magnetization measurements demonstrate the solution is absent of the earlier detected paramagnetic component. The aging studies show that, over time, the magnetic moment of Co colloids stored in air decreases. A hysteresis loop shift under field-cooled conditions is observed, indicating the existence of the anti-ferromagnetic/ferromagnetic (AFM/FM) interfaces. High-resolution TEM results show that surface oxidation takes place for these Co nanoparticles and confirm the AFM/FM interfaces are respectively between face-centered cubic cobalt (II) oxide and epsilon-Co.
Citation: Crystal Growth & Design
Pub Type: Journals
Cobalt, Cobalt oxide, Exchange bias, Magnetic Property, Nanoparticles