Synthesis and study of fcc-Co derived from isostructural Co4N

TL;DR

This study demonstrates a method to synthesize high-purity fcc-Co from Co4N by exploiting rapid N self-diffusion, enabling phase transformation at lower temperatures and avoiding impurity phases.

Contribution

It introduces a novel route for converting Co4N to fcc-Co via nitridation and N diffusion, reducing phase transition temperature and impurity formation.

Findings

N self-diffusion is fastest in Co4N among transition metal nitrides.

High-temperature annealing removes N, producing pure fcc-Co.

Structural and magnetic analyses confirm successful phase transformation.

Abstract

This work demonstrates synthesis and study of fcc-Co derived from an isostructural Co4N. Diffusion measurements carried out in this work, reveal that N self-diffusion is the swiftest in Co4N compared to other transition metal nitrides or even the mononitride CoN. By the application of a high substrate temperature (Ts) growth or thermal annealing temperature (Ta); N diffuses out from the fcc-Co4N above 573 K leaving behind a high purity fcc-Co phase. Generally, Co grows in a hcp structure and a (partial) hcp to 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 Co4N(111) thin films were grown using a CrN(111) template on a quartz substrate using a 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. It was found that Co-N bonding and N concentration get significantly reduced at a high Ts or Ta. Magnetization measurements combining ex-situ and in-situ magneto-optical Kerr effect showed differences in anisotropy and coercivity of Co4N 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 Co4N aided by an exceptionally high N self-diffusion in Co4N.