Controlling the sharpness of room-temperature metal-insulator transition in epitaxial Sm0.5Nd0.5NiO3 films

Sm0.5Nd0.5NiO3 (SNNO) films with metal-insulator transition (MIT) at room-temperature (∼300 K) have been grown on NdGaO3 (001) substrates by pulsed laser deposition. By modifying the parameters of oxygen pressure, substrate temperature, and film thickness, the role of oxygen vacancies and strain relaxation on the MIT of SNNO films was systematically analyzed. The strain status of the films was carefully characterized by means of high resolution x-ray diffraction. The results revealed that for the fully strained films (≤20 nm) an increment of deposition oxygen pressure (and/or temperature) would decrease the content of oxygen vacancies and Ni2+ in the films, leading to a sharp MIT. In contrast, the strain relaxation occurs in the thicker films (>20 nm) despite an optimized oxygen pressure (temperature) was adapted for the deposition, which results in an inferior transport property and surface morphology. Specifically, a broadening MIT and a doublet TMI was observed in the partially strained films, where one TMI kept a stable value around 300 K in analogues to that of fully strained film, and another one increased with the increment of the film thickness, reaching a highest value of 330 K. This might be induced by the coexistence of a fully strained part and a strain-relaxed portion in the thicker films that observed on high resolution X-ray reciprocal space mappings.