In-Situ Investigation of Ion Beam-Induced Crossover in Uniaxial Magnetic Anisotropy of Polycrystalline Fe Films

A combined role of structure and morphology of polycrystalline Fe thin films in the evolution of uniaxial magnetic anisotropy (UMA) subjected to several cycles of the ion beam erosion (IBE) process is investigated through a set of self-consistent in-situ experiments. In the initial stages of IBE, surface texturing of the Fe film imprints a recognizable magnetocrystalline anisotropy (MCA) with the easy axis of magnetization along the IBE direction. Further erosion results in development of a well-defined correlated morphology that incorporates reinforcement of stray dipolar fields and switches the easy axis of magnetization by 90°. The observed unusual crossover is inferred to be originated from an interplay between the relative weight of MCA and shape anisotropy, which is successfully explained by considering the modified surface structure, morphology, and the relative strength of the dipolar stray fields. The present phenomenological understanding provides a promising option of the fine tailoring of UMA strength by controlling crystallinity of the surface along with anisotropic morphology patterns.