  We have measured the tunnelling current in Nb/Nb$_x$O$_y$/Ni planar tunnel
junctions at different temperatures. The junctions are in the intermediate
transparency regime. We have extracted the current polarization of the
metal/ferromagnet junction without applying a magnetic field. We have used a
simple theoretical model, that provides consistent fitting parameters for the
whole range of temperatures analyzed. We have also been able to gain insight
into the microscopic structure of the oxide barriers of our junctions.%
