Probing vertically graded anisotropy in FePtCu films

Field-dependent polarized neutron reflectivity (PNR) and magnetometry are employed to study the magnetic properties of compositionally uniform and graded FePtCu films as a function of annealing temperature (T-A). The PNR results are able to directly probe the compositional and anisotropy variations through the film thickness. Further details about how the reversal mechanisms evolve are then elucidated by using a first-order reversal curve technique. The reversal of the graded sample annealed at 300 degrees C occurs by an initial rapid switching of the dominant soft A1 phase toward the surface of the film, followed by the gradual reversal of the residual hard phase components toward the bottom. This indicates that the anisotropy gradient is not well established at this low T-A. A fundamentally different mechanism is found after annealing at 400 degrees C, where the rapid switching of the entire film is preceded by a gradual reversal of the soft layers. This suggests that the anisotropy gradient has become better established through the film thickness. The field-dependent PNR measurements confirm the existence of an anisotropy gradient, where the lower (higher) anisotropy portions are now toward the bottom (top) of the film because of the Cu compositional gradient. However, after annealing at 500 degrees C, a single rapid reversal is found, indicating the formation of a uniform hard film. In this case, PNR demonstrates a more uniform magnetic depth profile that is consistent with a uniform reference sample, suggesting significant interdiffusion of the Cu is degrading the compositional and induced anisotropy gradient at this elevated T-A.