Synthesis and study of fcc-Co derived from isostructural Co4N

Abstract

This work demonstrates synthesis and study of fcc-Co derived from an isostructural Co 4 N. Diffusion measurements carried out in this work, reveal that N self-diffusion is the swiftest in Co 4 N compared to other transition metal nitrides or even the mononitride CoN. By the application of a high substrate temperature (T s ) growth or thermal annealing temperature (T a ); N diffuses out from the fcc-Co 4 N above 573 K leaving behind a high purity fcc-Co phase. Generally, Co grows in a hcp structure and a (partial) hcp→fcc-Co transformation takes place around 700 K or above 70 GPa. The proposed route through nitridation and diffusion of N not only bring down the phase transition temperature, an impurity present in the form of hcp-Co can be avoided altogether. Oriented Co 4 N(111) thin films were grown using a CrN(111) template on a quartz substrate using a dc magnetron sputtering. Samples were grown at different T s or room temperature grown Co 4 N samples were annealed at different T a . Analysis using x-ray diffraction, N K-edge x-ray absorption, x-ray photoelectron and secondary ion mass spectroscopy confirmed the formation of fcc-Co 4 N or fcc-Co phases. It was found that Co-N bonding and N concentration get significantly reduced at a high T s or T a . Magnetization measurements combining ex-situ and in-situ magneto-optical Kerr effect showed differences in anisotropy and coercivity of Co 4 N and fcc-Co samples. Combining structural, electronic and magnetization measurements, it has been observed that a high purity fcc-Co can be conveniently derived from the isostructural Co 4 N aided by an exceptionally high N self-diffusion in Co 4 N.