Homogenization and short-range chemical ordering of Co–Pt alloys driven by the grain boundary migration mechanism

Binary magnetic alloys like Co–Pt are relevant for applications as components of magnetic exchange coupled composites. Numerous approaches exist to tune the coercive field of Co–Pt alloys primarily relying on high-temperature processing aiming to realize chemically long-range ordered phases. The peculiarity of Co–Pt is that large coercive field and magnetic anisotropy can be achieved even in chemically disordered alloys relying on short-range order. Here, we study alloying of Co–Pt from bilayers of Pt(14 nm)/Co(13 nm) at temperatures up to 550 °С, where bulk diffusion processes are suppressed and the dominant diffusion mechanism is grain boundary migration. We demonstrate that grain boundary diffusion mechanism can lead to the realization of a homogeneous yet chemically disordered Co56Pt44 alloy at temperatures of 500 °С and higher. A pronounced increase of the coercive field for samples processed at temperatures higher than 400 °С is attributed to short-range ordering. With this work, we pinpoint the grain boundary diffusion as the mechanism responsible not only for the homogenization of binary alloy films but also as a driving force for the realization of short-range order in Co–Pt. Our results motivate further research on grain boundary diffusion as a mechanism to realize chemically long-range ordered phases in Co–Pt alloys.