In this work, we measured N self-diffusion in the Co-N system and found an unexpected result that N diffuses out almost completely around 500 K, leaving behind fcc Co irrespective of the amount of N used to deposit Co-N. On the other hand, in previous attempts the ${\mathrm{Co}}_4\mathrm{N}$ phase has always been grown at 550 K or above. In view of our finding, it appears that fcc Co could have been mistaken for ${\mathrm{Co}}_4\mathrm{N}$, probably due to the closeness of their lattice parameters (LP; fcc Co = 3.54 $\AA$, ${\mathrm{Co}}_4\mathrm{N}$= 3.74 $\AA$). Therefore, ${\mathrm{Co}}_4\mathrm{N}$ – an interesting material for its high spin-polarization ratio and high magnetic moment remained unexplored. By bringing down the growth temperature, we report the growth of stoichiometric ${\mathrm{Co}}_4\mathrm{N}$ epitaxial thin films. Films were grown using a direct current reactive magnetron sputtering process on ${\mathrm{LaAlO}}_3$ (LAO mismatch $1.4\%$) and MgO (mismatch $11.3\%$) substrates and their structural and magnetic properties were studied. Precise magnetic moment ($M_s$) of ${\mathrm{Co}}_4\mathrm{N}$ samples were measured using polarized neutron reflectivity and compared with bulk magnetization results. We found that the $M_s$ of ${\mathrm{Co}}_4\mathrm{N}$ is higher due to a magnetovolume effect. Unlike previous findings, we observed that substrates induce misfit strain and strain inhomogeneity is the cause of modifications in magnetic ensemble such as coercivity, saturation magnetization, and magnetic anisotropy. A consequence of incoherent strain present in our samples is also reflected in the magnetic anisotropy leading to a superposition of strong fourfold and a small fraction of uniaxial magnetic anisotropy. Obtained results are presented and discussed in this work.

• Received 3 August 2018

DOI:https://doi.org/10.1103/PhysRevB.99.214109