Magnetization of Fe4N thin films: Suppression of interfacial intermixing using buffer layers

Abstract

We studied the magnetization (Ms) of 60 nm thick polycrystalline Fe4N thin films deposited using reactive dc magnetron sputtering. The optimum substrate temperature (Ts) to grow an stoichiometric Fe4N phase is about 673 K, in agreement with earlier works. However, at this Ts, we found that significant intermixing is taking place at the substrate-film interface, affecting the Ms of Fe4N film, adversely. We performed secondary ion mass spectroscopy (SIMS) measurements to extract the extent of such intermixing. It was found that Si from the SiO2 substrate get interdiffused into Fe4N film forming an undesired interface extending up to almost half of the film. As a result, the average Ms of Fe4N film get reduced by about 25% as compared to its theoretical value. To prohibit such intermixing, we placed thin layers of Ag, Cu, or CrN as a buffer layer between the substrate and Fe4N film. Such buffer layer suppress the extent of interdiffusion remarkably. From SIMS measurements, we found that among all buffer layers, CrN is most suitable as it reduces the extent of intermixing to a minimum level. Magnetization measurements performed using bulk magnetization and polarized neutron reflectivity (PNR) methods also clearly revealed that the Ms was the highest when CrN buffer layer was used. From these results, it becomes apparent that the intermixing taking place at the substrate-film interface plays a vital role in affecting the Ms of Fe4N thin films. To further suppress such intermixing an attempt was also made to bring down the Ts from 673 to 523 K. Obtained results are presented and discussed in this work.