  We experimentally investigated the magnetic properties of NiCo$_2$O$_4$
epitaxial films known to be conductive oxides with perpendicular magnetic
anisotropy (PMA) at room temperature. Both magneto-torque and magnetization
measurements at various temperatures provide clear experimental evidence of the
spin reorientation transition at which the MA changes from PMA to easy-cone
magnetic anisotropy (ECMA) at a certain temperature ($T_{\rm{SR}}$). ECMA was
commonly observed in films grown by pulsed laser deposition and reactive radio
frequency magnetron sputtering, although $T_{\mathrm{SR}}$ is dependent on the
growth method as well as the conditions. The cone angles measured from the
$c$-axis increased successively at $T_{\mathrm{SR}}$ and approached a maximum
of 45-50 degrees at the lowest measurement temperature of 5 K. Calculation with
the cluster model suggests that the Ni$^{3+}$ ions occupying the $T_d$ site
could be the origin of the ECMA. Both the magnetic properties and the results
of the calculation based on the cluster model indicate that the ECMA is
attributable to the cation anti-site distribution of Ni$^{3+}$, which is
possibly formed during the growth process of the thin films.
