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Physical characteristics and cation distribution of NiFe2O4 thin films with high resistivity prepared by reactive co-sputtering

We fabricated NiFe2O thin films on MgAl2O4 (001) substrates by reactive dc magnetron co-sputtering in a pure oxygen atmosphere at different substrate temperatures. The film properties were investigated by various techniques with a focus on their structure, surface topography, magnetic characteristics, and transport properties. Structural analysis revealed a good crystallization with epitaxial growth and low roughness and a similar quality as in films grown by pulsed laser deposition. Electrical conductivity measurements showed high room temperature resistivity (12 Ohmm), but low activation energy, indicating an extrinsic transport mechanism. A band gap of about 1.55 eV was found by optical spectroscopy. Detailed x-ray spectroscopy studies confirmed the samples to be ferrimagnetic with fully compensated Fe moments. By comparison with multiplet calculations of the spectra we found that the cation valencies are to a large extent Ni2+ and Fe3+.