Homogenization and Chemical Ordering in Co-Pt Thin Films

Binary alloys based on CoPt are attractive as a materials for spintronics, permanent magnets applications and data storage devices due to the high and tunable coercivity combined as well as an excellent corrosion resistance [1].
The formation of chemically ordered CoPt magnetic phases is intensively studied both in thin films and in nanoparticles [2, 3]. In Co-Pt alloys, a large coercive field and magnetic anisotropy can be achieved even in chemically disordered alloys due to short-range order [4]. We have implemented a systematic structural and magnetometry study of the diffusion-controlled formation of a homogeneous CoPt alloy by vacuum heat treatment of Pt/Co stacks, where diffusion processes are driven by diffusion-induced grain boundary migration mechanism.
Layered stacks of Pt(14 nm)/Co(13 nm)/Ta(3 nm) were magnetron sputter deposited and annealed in vacuum of 10‑6 mbar in the temperature range of 200 °С – 550 °С. The structure, chemical composition and magnetic properties of the films were analyzed by X-ray diffraction, secondary ion mass spectrometry, scanning transmission electron microscopy, energy-dispersive X-ray spectroscopy, and VSM magnetometry.
We demonstrate that a Co‑Pt alloy with a homogeneous structure is formed after annealing at temperature above 500 °C. Despite the fact that long-range chemical order in CoPt film was not formed, thermal treatment leads to an increase of the coercive field. We attribute the short-range chemical ordering as a mechanism responsible for the formation of a local anisotropy in Co‑Pt alloy. In this respect, our study suggests that the diffusion mechanism relying on grain boundary migration can be used to promote short-range ordering in binary magnetic alloys. These results will motivate further studies of diffusion processes and the formation of hard magnetic chemical