Structure and magnetism of Co:CoO core-shell nanoclusters

Transmission electron microscopy (TEM) and electron diffraction (ED) are used to investigate the nanostructures of two ensembles of Co:CoO core-shell particles. TEM images show that particles of size about 12 nm are almost fully oxidized, while particles with size about 18 nm have a core-shell structure where a Co core is surrounded by a shell of CoO. ED simulation confirms that the larger particles have an fcc-structured Co core and a rock-salt CoO shell structure, while the smaller particles mostly have the rock-salt CoO structure. The core-shell structure is responsible for the unusual magnetic properties of the Co:CoO nanoclusters, especially the occurrence of inverted hysteresis loops (proteresis), but previous research has been indirect, largely based on magnetic measurements and on a cross-comparison with granular materials. Our measurements show that the structures have ferromagnetic fcc Co cores of varying sizes down to 1 nm which are surrounded by antiferromagnetic rock-salt CoO shells. The core radii obtained from the TEM pictures are used to estimate the exchange interactions responsible for proteresis and to pinpoint the core-size window in which proteresis occurs.