Effect of interfacial interdiffusion on magnetism in epitaxial Fe$_4$N films on LaAlO$_3$ substrates

Epitaxial Fe$_4$N thin films grown on LaAlO$_3$ (LAO) substrate using sputtering and molecular beam epitaxy techniques have been studied in this work. Within the sputtering process, films were grown with conventional direct current magnetron sputtering (dcMS) and using a high power impulse magnetron sputtering (HiPIMS) process. Surface morphology and depth profile studies on these samples reveal that HiPIMS deposited film has the lowest roughness, the highest packing density, and the sharpest interface. We found that the substrate-film interface and the microstructure play a vital role in affecting the electronic hybridization and magnetic properties of Fe$_4$N films. La from the LAO substrate and Fe from the film interdiffuse and form an undesired interface. The magnetic moment (M$_s$) was compared using bulk, element-specific and magnetic depth profiling techniques. We found that M$_s$ was the highest when the thickness of the interdiffused layer was lowest and such conditions can only be achieved in the HiPIMS grown samples. The presence of a small moment at the N site was also evidenced by element-specific x-ray circular dichroism measurement in the HiPIMS grown sample. A large variation in the M$_s$ values of Fe$_4$N films found in the experimental works carried out so far could be due to such an interdiffused layer which is generally not expected to form in otherwise stable oxide substrate at a low substrate temperature ≈675 K. In addition, a consequence of substrate-film interdiffusion and microstructure resulted in the different kinds of magnetic anisotropies in Fe$_4$N films grown using different techniques. A detailed investigation of the substrate-film interface and microstructure on the magnetization of Fe$_4$N film is presented and discussed in this work