Composition-induced structural, electrical, and magnetic phase transitions in AX-type mixed-valence cobalt oxynitride epitaxial thin films

Synthesis of mid- to late-transition metal oxynitrides is generally difficult by conventional thermal ammonolysis because of thermal instability. In this letter, we synthesized epitaxial thin films of AX-type phase-pure cobalt oxynitrides (CoO_xN_y) by using nitrogen-plasma-assisted pulsed laser deposition and investigated their structural, electrical, and magnetic properties. The CoO_xN_y thin films with 0 ≤ y/(x + y) ≤ 0.63 grown on MgO (100) substrates showed a structural phase transition from rock salt (RS) to zinc blend at the nitrogen content y/(x + y) ∼ 0.5. As the nitrogen content increased, the room-temperature electrical resistivity of the CoO_xN_y thin films monotonically decreased from the order of 10⁵ Ω cm to 10^-4 Ω cm. Furthermore, we observed an insulator-to-metal transition at y/(x + y) ∼ 0.34 in the RS-CoO_xN_y phase, which has not yet been reported in Co²⁺/Co³⁺ mixed-valence cobalt oxides with octahedral coordination. The low resistivity in the RS-CoO_xN_y phase, on the 10^-3 Ω cm order, may have originated from the intermediate spin state of Co³⁺ stabilized by the lowered crystal field symmetry of the CoO_6-nN_n octahedra (n = 1, 2,⋯5). Magnetization measurements suggested that a magnetic phase transition occurred in the RS-CoO_xN_y films during the insulator-to-metal transition. These results demonstrate that low-temperature epitaxial growth is a promising approach for exploring novel electronic functionalities in oxynitrides.