Controlled synthesis and anomalous magnetic properties of relatively monodisperse CoO nanocrystals.

Monodisperse cobalt monoxide (CoO) nanocrystals ranging in size from 4.5 to 23 nm were prepared via the thermal decomposition of cobalt acetylacetonate (Co(acac)(2)) in oleylamine under vigorous stirring. The size control of the nanocrystals was achieved by tailoring the reaction temperature or by seed-mediated growth. The nanocrystals have been characterized by x-ray diffraction, transmission electron microscopy, and Fourier transform infrared and x-ray photoelectron spectroscopy. The as-prepared nanocrystals are stable because of the organic coating (oleylamine) that occurred in situ. Magnetic measurement reveals that all of the nanocrystals are superparamagnetic at high temperatures and show ferromagnetic interactions at low temperatures due to the existence of uncompensated moments on the surface of the nanoparticles. The weak ferromagnetic interactions increase with decreasing particle size.