Paper SS-WeA2
Atomic-Scale Determination of the Crystallographic Stacking at the Technologically-Important Cobalt-Copper Interface

Wednesday, October 31, 2012, 2:20 pm, Room 21

The deposition of Co on Cu has been studied extensively due to the use of layered Co/Cu systems in giantmagnetoresistance devices and the application of magnetic Co nanostructures in spintronics. Co deposited on Cu follows a Volmer-Weber type growth mechanism, forming bilayer-high, triangular islands. These islands grow in two orientations that are rotated 60˚ with respect to each other. The formation of triangular islands is dictated by the six-fold symmetry of the underlying Cu lattice, and triangular growth is preferred to hexagonal growth due to favored diffusion of Co from the (100) to the (111) facet of the islands during deposition. The consequence of this diffusion is that there must be a difference in the crystallographic packing between the two orientations of the islands. It is thought that one packing configuration of the Co islands is fcc, in which Co follows the stacking of the underlying Cu, but the second packing structure is still debated. Here we use low-temperature scanning tunneling microscopy to explore the stacking of these islands through adsorption of hydrogen on their surfaces.

Hydrogen adsorbs dissociatively on Co, and prefers to bind to fcc three-fold hollow sites, although it is calculated that there is only a 0.01 eV difference in the binding energy of hydrogen to fcc hollows vs. hcp hollows. We show that at 80 K, hydrogen is present on the Co surface in both adsorption sites, and that there is an electronic difference between the two states that is apparent in our STM images. Through high-resolution imaging of the hydrogen at the boundary between the two adsorption sites, we have been able to deduce the stacking of the underlying Co island lattice. We confirm that the majority orientation of the islands is indeed fcc stacking, and the minority of the islands follow Co’s native hcp stacking. This has important ramifications in the development of Co/Cu/Co systems for GMR devices, as the interface between the two metals can significantly affect electron scattering.