$^{57}$Fe Mössbauer study of epitaxial TiN thin film grown on MgO (1 0 0) by magnetron sputtering

The properties and performance of TiN thin films are closely related to the concentration and mobility of lattice defects in the thin film structures of TiN. This makes a local atomic scale study of TiN thin films an ever-growing demand. Emission $^{57}$Fe Mössbauer spectroscopy (eMS) is a powerful tool in this regard, which we apply here to study an ultrathin TiN film epitaxially grown on MgO (1 0 0). With the help of theoretical calculations, our results show that most implanted Fe ions adopt a 2$^{+}$ valence state and locate at the Ti sublattice in the bulk-like single crystalline grains, with the rest Fe residing at the grain boundaries as interstitials. A small percentage of nitrogen point defects (vacancy V$_{N}$ and interstitial N$_{I}$) are observed in the bulk-like crystalline grains. A temperature-dependent, interstitial N$_{I}$ mediated site-exchange between N$_{I}$ and V$_{N}$ inside the crystal grain are deduced via a N$_{2}$ dimmer like diffusion of N$_{I}$ through the crystal grains in the temperature range of 540–620 K. This is interesting in the perspective of exploring the catalytic property of TiN nanostructures. The titanium vacancy(V$_{Ti}$) is only detected at the grain boundaries. Annealing up to 813 K, both the V$_{N}$ and N$_{I}$ are annihilated in the crystalline grains and the V$_{Ti}$ is fully recovered with healing of the grain boundaries. However, no evidence of ferromagnetism due to dilute implantation of $^{57}$Mn/$^{57}$Fe and or structural defects in the film is obtained. This suggests that the so far reported dilute magnetism and defect-induced ferromagnetism in TiN nanostructures requires a further systematic investigation.