An In Situ Anneal Study of Graded Anisotropy FePtCu Films

We provide a detailed study of how the anisotropy (K_u) gradient in a compositionally graded FePtCu film gradually develops as a function of the postannealing temperature (T_A). By utilizing the in situ annealing and magnetic characterization capabilities of a physical property measurement system, the evolution of the induced K_u gradient is elucidated. For low T_A, the sample primarily remains in the low-K_u A1 phase. At intermediate T_A, the gradual development of an A1 to L1₀ anisotropy gradient occurs. As T_A is further increased, a well-developed L1₀ gradient is realized. Finally, annealing temperatures greater than 475°C reduce the gradient until the film is effectively uniform for T_A ≥ 525°C and higher, presumably due to interdiffusion of the Cu through the film thickness. The resulting coercivity shows a nonmonotonic dependence on T_A with an initial steep increase as the L1₀ fraction of the sample increases, a local minimum at T_A = 525°C where the gradient vanishes, and a final increase as the uniform L1₀ film orders completely.