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Structure and Magnetism of Co and CoAg Nanocrystals

Monodisperse, air-stable Co/CoO and CoAg55 nanoparticles with a mean diameter of about 11 nm have been synthesized using methods of colloidal chemistry. High resolution transmission electron microscopy (TEM) and Electron Energy-Loss Spectroscopy (EELS) element-specific TEM images reveal a multiply-twinned fcc Co metallic core covered with a 2-2.5 nm thick CoO shell. The lattice parameters are in agreement with those of bulk Co and CoO. A shift of the hysteresis loop of 0.4 T, induced by field CoOling of the Co/CoO particles, indicates a strong unidirectional exchange anisotropy due to the interaction between the ferromagnetic Co core and the antiferromagnetic CoO shell. CoAg55 composite particles consist of grains of fcc Co and fcc Ag. No evidence for alloy formation was observed. Electron energy-loss and X-ray microanalysis indicate that Co is predominantly found in the surface region of the particles. SQUID magnetometry shows that at room temperature the CoAg55 particles are superparamagnetic while at 90 K a hysteresis loop was detected with a coercive field of 0.07 T and a remanent magnetization of 32 % of the saturation value.