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

Abstract

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 N 2 ) 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 T s ∼300 K, an evolution of Co-N phases starting from Co(N)→Co 4 N→Co 3 N→CoN can be seen as R N 2 increases to 100%, whereas when the substrate temperature increases to 523 K, the phase formed is a mixture of Co and Co 4 N, independent of the R N 2 used during sputtering. 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 e ffect (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 N 2 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.

Structural and magnetic properties of stoichiometric Co4N epitaxial thin films

“…2) indicating that N out-diffuses almost completely from CoN. The implication of high T s on Co-N phases was also amply demonstrated recently [33]. Here, it was shown that when Co was sputtered with different amounts of N 2 (and even with N 2 alone) at 523 K, the resulting phase obtained was always fcc Co, independent of the amount of N 2 that was used during the deposition [33].…”

“…We find that in previous works the Co 4 N phase was always synthesized at T s 525 K [20][21][22][23][24][25][26][27]. Comparing the LP of Co 4 N films obtained in those works, we find that the LP was much smaller than the theoretically predicted value of 3.735 Å [18] [20,33]. In several other studies also, when T s > 525 K, LP remained at 3.54 Å [21,22].…”

“…The implication of high T s on Co-N phases was also amply demonstrated recently [33]. Here, it was shown that when Co was sputtered with different amounts of N 2 (and even with N 2 alone) at 523 K, the resulting phase obtained was always fcc Co, independent of the amount of N 2 that was used during the deposition [33]. This behavior can also be seen from the XRD, SIMS, x-ray photoelectron, and N K-edge x-ray absorption spectroscopy [42] results presented in the Supplemental Material [43] and references [44][45][46][47][48][49][50] therein.…”

“…A base pressure of 1 × 10 −7 Torr was achieved before deposition and the working pressure was maintained at 3 mTorr during deposition. More details about sample preparation can be found elsewhere [33][34][35].…”

Structure and magnetization of Co4N thin film

“…Theoretical values of enthalpy of formation (∆H • f ) for Fe 4 N is about -12 kJ mol −1 [22], whereas those for Co 4 N are slightly above or below 0 for hcp Co or fcc Co [11]. This also implies that at a higher T s , Co 4 N system will be less stable as compared to Fe 4 N. In a recent work, we studied the phase formation process in the Co-N system at T s = 300 K [23] and 523 K [24]. We found that at T s = 523 K, N incorporation in the Co-N system is minimal and the phases formed are similar to a fcc Co having LP∼3.52Å.…”