Effect of interfacial diffusion on microstructure and magnetic properties of Cu/FePt bilayer thin films

The crystal structure, microstructure, and magnetic properties for a series of Cu/FePt bilayer films were investigated. The samples were prepared by depositing a Cu top layer on a highly ordered L1₀ FePt film. To promote interdiffusion, the bilayer samples were annealed at a temperature T_d ranging from 300 to 800 °C. X-ray diffraction data indicate that observable diffusion occurs at 400 °C. The maximum coercivity thus obtained is 14.0 kOe, which is 24% larger than that of the ordered FePt film without a Cu top layer. The high H_c can be attributed to the diffusion of copper atoms through the grain boundaries of the magnetic films, which may produce extra pinning sites for domain-wall movement. The ΔM data measured from the Henkel plots of annealed Cu/FePt films change from negative to positive values as T_d is raised from 400 to 800 °C. This can result from the effects of demagnetization coupling and exchange coupling and is further explained from the variation of squareness ratios of hysteresis loops.