  We fabricated NiFe$_\textrm{2}$O$_\textrm{x}$ thin films on
MgAl$_2$O$_4$(001) substrates by reactive dc magnetron co-sputtering varying
the oxygen partial pressure during deposition. The fabrication of a variable
material with oxygen deficiency leads to controllable electrical and optical
properties which would be beneficial for the investigations of the transport
phenomena and would, therefore, promote the use of such materials in spintronic
and spin caloritronic applications. We used several characterization techniques
in order to investigate the film properties, focusing on their structural,
magnetic, electrical, and optical properties. From the electrical resistivity
measurements we obtained the conduction mechanisms that govern the systems in
high and low temperature regimes, extracting low thermal activation energies
which unveil extrinsic transport mechanisms. The thermal activation energy
decreases in the less oxidized samples revealing the pronounced contribution of
a large amount of electronic states localized in the band gap to the electrical
conductivity. Hall effect measurements showed the mixed-type semiconducting
character of our films. The optical band gaps were determined via
ultraviolet-visible spectroscopy. They follow a similar trend as the thermal
activation energy, with lower band gap values in the less oxidized samples.
