57Fe 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 ⁵⁷Fe 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 N₂ 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 ⁵⁷Mn/⁵⁷Fe 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.