We studied the magnetization (M_s) of 60 nm thick polycrystalline Fe₄N thin films deposited using reactive dc magnetron sputtering. The optimum substrate temperature (T_s) to grow an stoichiometric Fe₄N phase is about 673 K, in agreement with earlier works. However, at this T_s, we found that significant intermixing is taking place at the substrate-film interface, affecting the M_s 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 M_s 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 M_s 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 M_s of Fe₄N thin films. To further suppress such intermixing an attempt was also made to bring down the T_s from 673 to 523 K. Obtained results are presented and discussed in this work.