Synthesis, structure and magnetization of Co$_4$N thin films

We reviewed magnetic tetra metal nitrides – Fe$_4$N and Co$_4$N for their structure, magnetization and the thermodynamics of phase formation. Opposed to Fe$_4$N, the formation of a stoichiometric Co$_4$N turns out to be extremely difficult. A review of the literature of Co$_4$N compound suggest that the experimental lattice parameter (LP) was always found to be smaller than the theoretical predicted value. It can also be seen that as the substrate temperature (T$_s$) increases, the LP of Co$_4$N film decreases. In this work, we deposited Co$_4$N films using molecular beam epitaxy (MBE), direct current magnetron sputtering (dcMS) and high power impulse MS (HiPIMS). Films were characterized using X-ray diffraction, X-ray reflectivity and atomic force microscopy. It was found that at high T$_s$, N out-diffusion significantly affects the growth of Co$_4$N phase. We found that the MBE deposited films did not show any signature of Co$_4$N phase when T$_s$ < 703 K but at T$_s$ = 703 K, the phase formed can be assigned to fcc Co rather than Co$_4$N. On the other hand, the dcMS and HiPIMS grown films clearly show the presence of Co$_4$N phase even at T$_s$ = 300 K. Detailed analysis of Co4N films grown using dcMS and HiPIMS reveals that HiPIMS grown films are single phase and have a denser microstructure. The density of HiPIMS deposited film was also found to be close to the theoretical value. Magneto optical Kerr effect and polarized neutron reflectivity measurements were carried out to study magnetic properties. Differences in the magnetic moment and magnetic anisotropy were correlated with structural parameters. Obtained results are presented and discussed in terms of involved thin film growth mechanism