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 $Td$ ranging from $300to800°C$⁠. X-ray diffraction data indicate that observable diffusion occurs at $400°C$⁠. The maximum coercivity thus obtained is $14.0kOe$⁠, which is 24% larger than that of the ordered FePt film without a Cu top layer. The high $Hc$ 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 $Td$ is raised from $400to800°C$⁠. 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.