Superparamagnetism and different growth mechanisms of Co/Au(111) and Co/Cu(111) multilayers grown by molecular-beam epitaxy

The magnetization of Co/Au(111) and Co/Cu(111) multilayers grown by molecular-beam epitaxy has been measured. For ultrathin Co layers, superparamagnetic behavior is observed, very similar to that reported previously for granular samples. For somewhat thicker Co layers, hysteresis effects occur, indicating the absence of superparamagnetism. The clear transition seen between these two modes of behavior is attributed to the growth of the Co particles from very small superparamagnetic clusters to larger islands and ultimately to a film, as the thickness of the Co layers is increased. Different magnetic properties are found for the Co/Au(111) and the Co/Cu(111) systems during the transition from granular to continuous layers. An investigation by reflection high-energy electron diffraction suggests that this difference is mainly due to the different growth modes of Co on Au(111) and on Cu(111).