Initial stages in the growth of {111}-faceted CoCr₂O₄ clusters: mechanisms and strained nanometric pyramids

Abstract

The epitaxial growth of ferrimagnetic CoCr₂O₄ films on MgAl₂O₄(001) results in self-organized {111}-faceted pyramids and hut clusters having base dimensions that can reach above a micron in size. This paper reports the morphology and structural characterization of the initial stages of film growth. Three distinctive stages are identified as the growth proceeds: in the initial stage the surface is homogeneous, in the second one nanometric strained pyramids appear, and in the third one there are relaxed pyramids and hut clusters that continue growing up to a micronic size. The islands maintain the {111}-faceted surfaces and the spatial order along 〈110〉 directions during the process. We discuss the mechanisms that are the origin of first the coherent pyramids and later the spectacular growth of part of the islands. Besides illustrating the formation process of the final structures, the results presented here demonstrate that arrays of strained {111}-faceted CoCr₂O₄ pyramids with nanometric dimensions can be fabricated.