Raman studies of cation distribution and thermal stability of epitaxial spinel NiCo₂O₄ films Available to Purchase

Epitaxial thin films of spinel NiCo₂O₄ (NCO) grown on MgAl₂O₄ (001) substrates are reported to exhibit dramatic changes in the magnetic and transport properties with deposition temperature. While films grown at lower temperatures (⁠$<450°C$⁠) are ferrimagnetic with metallic characteristics, those grown at higher temperatures are non-magnetic and insulating. Detailed polarized Raman spectroscopy studies indicate that the higher temperature films have close to the ideal inverse spinel cation distribution, $Co3+[Ni2+Co3+]O42−$⁠, whereas those deposited at lower temperature are characterized by mixed cation/charge distribution at both the tetragonal (A) and octahedral (B) sites. Additionally, temperature-dependent Raman studies demonstrate that, unlike bulk polycrystalline samples, all the NCO films are robust against thermal treatment with full reversibility after annealing at $600°C$ in oxygen and air. However, partial decomposition is observed after annealing in vacuum.