Structural and magnetic properties of Co-N thin films deposited using magnetron sputtering at 523 K

In this work, we studied cobalt nitride (Co-N) thin films deposited using a dc magnetron sputtering method at a substrate temperature (T_s) of 523 K. We find that independent of the reactive gas flow (R_N2 ) used during sputtering, the phases of Co-N formed at this temperature seems to be identical having N at.% ∼ 5. This is contrary to Co-N phases formed at lower T_s. For example at T_s∼ 300 K, an evolution of Co-N phases starting from Co(N) → Co₄N → Co₃ N → CoN can be seen as R_N2 increases gradually to 100%, whereas when T_s increases to 523 K, the phase formed is fcc Co along with a minuscule Co₄N phase for R_N2 ≥ 25%. We used x-ray diffraction (XRD) to probe long range ordering, x-ray absorption spectroscopy (XAS) at Co absorption edge for the local structure, Magneto-optical Kerr effect (MOKE) and polarized neutron reflectivity (PNR) to measure the magnetization of samples. Quantification of N at.% was done using secondary ion mass spectroscopy (SIMS). Measurements suggest that the magnetic moment of Co-N samples deposited at 523 K is slightly higher than the bulk Co moment and does not get affected with the R_N2 used for reactive sputtering. Our results provide an important insight about the phase formation of Co-N thin films which is discussed in this work.