Synthesis of fcc-Co from isostructural Co₄N

This work demonstrates the synthesis of fcc-Co derived from an isostructural $Co4N$ phase. When deposited at high substrate temperature (⁠$Ts$⁠) or thermal annealing (⁠$Ta$⁠) above 573 K, the out-diffusion of N from fcc-$Co4N$ occurs, leaving behind a high purity fcc-Co phase. Generally, Co grows in a hcp structure, and a hcp to fcc-Co transformation can be facilitated at high temperature or pressure. The proposed route by nitridation and diffusion of N not only brings down the transition temperature but an impurity present in the form of hcp-Co can be avoided altogether as well. Oriented $Co4N$(111) thin films were grown using a CrN(111) template on a quartz substrate using dc magnetron sputtering. Samples were grown at different $Ts$ or room temperature grown $Co4N$ samples were annealed at different $Ta$⁠. Analysis using x-ray diffraction, N K-edge x-ray absorption, x-ray photoelectron, and secondary ion mass spectroscopy confirmed the formation of fcc-$Co4N$ or fcc-Co phases. Furthermore, it was found that Co–N bonding and N concentration get significantly reduced at high $Ts$ or $Ta$ due to exceptionally high N self-diffusion taking place in $Co4N$⁠. Magnetic measurements using ex situ and in situ magneto-optical Kerr effect showed differences in saturation behavior and coercivity of $Co4N$ and fcc-Co samples. By combining structural, electronic, and magnetization measurements, it has been observed that a high purity fcc-Co can be conveniently derived from the isostructural $Co4N$ aided by an exceptionally high N self-diffusion in $Co4N$⁠.