Surfactant effects on the growth and interfacial structure of giant magnetoresistive cobalt-copper multilayer systems.

摘要

Ab initio quantum mechanical calculations and atomistic simulations were carried out herein to determine the influence of metal surfactants (such as Ag) and background oxygen on the thin film growth and interfacial properties of giant magnetoresistance Co/Cu/Co spin valve multilayers. Density functional theoretical results show that (1) the presence of Ag atoms located at the island peripheries acts to lower the Ehrlich Schwoebel barrier for Co (Cu) from 0.10 (0.30) to 0.04 (0.03) eV. This should decrease the degree of the surface roughness at Co(Cu) thin film and thus decrease the degree of interfacial roughness at the Cu-on-Co (Co-on-Cu) interface. Silver atoms were found to diffuse on the Co(Cu) surface with high mobility, and prefer to adsorb at the island peripheries. In addition, Ag prefers to segregate to the surface rather than become trapped in the growing film. (2) The presence of oxygen can suppress the intermixing at the Co-on-Cu interface when the atomic oxygen coverage is beyond 0.360 monolayers. Oxygen suppresses the intermixing by stabilizing Co atoms at the surface and prevents Cu atoms from segregating out. This stabilization at the Co-on-Cu interface increases as the oxygen coverage increases. The Cu-on-Co interface with little intermixing is stable up to 0.362 monolayers of oxygen. Therefore, for the growth of Co/Cu/Co spin valve, at least 0.36 ML oxygen should be introduced into the system just after the deposition of the Cu layer. Furthermore, we developed a three-dimensional kinetic Monte Carlo algorithm which is based on the ab initio data to simulate thin film growth on lattice substrates. The simulation was used here to follow the influence of the operating conditions and the material specifications on the intermixing and interfacial roughness of the Co-Cu multilayer stacks. The simulation results indicate that the optimal conditions appear to be moderate deposition rates (such as 0.5 nm/s) and moderate temperatures (such as room temperature). The increase in the thickness of a metallic layer leads to the increase in the degree of interfacial roughness. The interfacial roughness increases with increase in the number of Co (Cu) layers in the overall stack.