Study Of Strain Relaxation In Co/Ru(0001)

Properties of heteroepitaxial nanoislands on metal surfaces have attracted lots of research interests due to its wide application in magnetic sensor, data storage and other industries. In these systems, the strain induced by lattice mismatch in the nanoislands and the strain relaxation at the edge of nanoislands due to reduced coordination are very common. The strain plays a critical role in the epitaxial film growth process and sequential electronic, magnetic and chemical properties. The strain effect on the adatom diffusion and nucleation has been established for a long time. Despite these studies, as far as we know, the strain at the atomic scale in the nanoislands or the substrate has not yet been quantitatively measured in real space. For the most frequently researched system of Co nanoislands on Cu(001), the macroscopic mismatch is about 2%, which corresponds to around 5 pm difference of in-plane lattice spacing. However, in real space, the current STM does not provide the required lateral resolution of the order of 1 picometer.We select the research system of Co nanoislands on Ru(0001) because of the following reasons:(1)the macroscopic misfit is more than 7%, which corresponds to around 20 pm in-plane spacing difference; (2)long ordered single monolayer (1*1) coherent growth can be realized since cobalt and ruthenium share the same type of stacking and that close packed surface of ruthenium has a much larger surface energy than that of cobalt; (3)strain induced perpendicular magnetic anisotropy in this system holds promises for novel information-processing technologies. The shape and structure of Co islands on Ru(0001) are obtained by means of scanning tunneling microscopy (STM). Due to the hcp(100) structure, triangle nanoislands are found with two opposite orientations on adjacent terrace while they have the same direction on the same terrace. Utilizing molecular dynamics (MD) simulations with potentials formulated in the tight-binding second moment approximation (TB-SMA), we find that the missing coordination of the Co atom plays an important role in stain relaxation. In the direction perpendicular to the surface, the islands and the surface layers are not flat anymore. The islands are assumed to have a platelike shape and the edge atoms are highest. An adsorption concave appears in the surface just under the island. In the direction parallel to the surface, the average bond length of Co atoms in the center is closest to the Ru substrate and it decreases while reaching the edge of the island. The average bond length of Co atoms is reaching the Ru lattice while the island is getting larger. Along the edge direction at the step edges, the spacing between Co atoms becomes larger from the corner to the middle of the side. In the plane direction perpendicular to the edge, the atomic rows are curved inwards, the average spacing between atomic rows increases from the edge towards the island center and reaches its saturation at-6 atomic rows. The simulation results are in accordance with the strain relaxation anisotropy in the middle side of a 20 nm sized triangular Co island. Some edge atoms have a more than 10% (about 30nm) in-plane displacement from standard position due to strain relaxation in this system and this provides a new way to measure and prove strain in the real space by high resolved STM.