Microstructural study of epitaxial platinum and permalloy/platinum films grown on (0001) sapphire

This paper reports a detailed transmission electron microscopy study of epitaxial Pt films grown by sputtering on single-crystal sapphire (0001) substrates. The orientation relationship between the Pt film and the substrate was determined to be. 111 + parallel to (0001)Al2O3, [110](Pt) parallel to [10 (1) over bar0](Al2O3). As a result of this relationship there are two orientation variants, related to each other by 60 degrees rotation about the [111] axis. Microstructural analysis using rose plots showed a large fraction (82%) of the grain boundaries oriented parallel to preferred facets along [110] directions. Although facet orientations occur every 60 degrees, facet junctions prefer 120 degrees angles. The films exhibited the mazed bicrystal structure, with larger grains displaying increasingly convoluted shapes, a behaviour that was quantified using the external form factor to describe grain morphologies. From high-resolution imaging of cross-section samples, the preferred boundaries were found to be perpendicular to the substrate and were identified as Sigma = 3, {112} symmetrical tilt boundaries. Quantitative analysis of high-resolution micrographs using the geometric phase technique shows a rigid shift of 0.3d({111}) parallel to the interface where the boundary is not constrained by the rigid substrate. Lattice images taken along the sapphire [1 (2) over bar 10] and [10 (1) over bar0] zone axes revealed a sharp interface between the Pt and the sapphire with no intermediate phases. The possible reasons for twinning in these films are discussed in detail, and it is concluded that the twin variants nucleate at the initial stages of island growth. Finally, we also report the growth of epitaxial Ni-Fe films on Pt. It is shown that the Ni-Fe film grows as a bicrystal with the twin boundaries correlated to those of Pt. These results clearly show the importance of being able to control the microstructure of the seed layer to grow defect-free multilayers.