Spinel to disorder rock-salt structural transition on (111) nickel ferrite thin films tailored by Ni content

(111)-oriented NixFe3−xOy thin films with varying Ni content (0.7 ≤ x ≤ 1.7) have been grown on c-plane (0001) α-Al2O3 substrates by ion beam sputtering. Epitaxial NixFe3−xOy thin films with spinel structure are obtained for x ≤ 1. For nickel content exceeding the value of the stoichiometric phase, i.e. NiFe2O4, a transition from an ordered spinel to a disordered rock-salt phase is identified, being the dominant one when x = 1.7. This result is confirmed by X-ray diffraction, Rutherford backscattering spectroscopy in channeling configuration, Raman spectroscopy, X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). We also report the first Raman spectra in NixFe3−xOy with mainly rock-salt structure. The evolution of the electrical and optical properties of the (111) NixFe3−xOy films as Ni content increases, supports the structural change. The electrical resistivity increases up to 103 ohm·cm, the direct and indirect band gaps also increase and only one direct band gap, instead of the two characteristic of the spinel structure, is present in the NixFe3−xOy film with x = 1.7.