Phase transitions in Co nanoclusters grown by pulsed laser deposition

The crystalline structure of Co clusters embedded in an amorphous Al2O3 matrix was studied by transmission electron microscopy (TEM) and electron diffraction (TED). In the first stage of the growth a metastable structure (body-centred-cubic) is observed. A face-centred-cubic phase (fcc) is found when the size of the clusters increases (diameter > 4 nm). The hexagonal-close-packed phase arises in the fee phase by a succession of stacking faults at the largest sizes. The mechanisms of phase transformation have been determined by using high resolution electron microscopy (HREM). The chemical nature of the clusters: in particular the existence of Co-O bonds, was investigated by using electron energy loss spectroscopy (EELS).