  The temperature dependence of exchange bias properties are studied in
polycrystalline $ \mathrm{BiFeO_3} / \mathrm{Ni_{81}Fe_{19}} $ bilayers, for
different $ \mathrm{BiFeO_3} $ thicknesses. Using a field cooling protocol, a
non-monotonic behavior of the exchange bias field is shown in the
exchange-biased bilayers. Another thermal protocol, the Soeya protocol, related
to the $ \mathrm{BiFeO_3} $ thermal activation energies was carried out and
reveals a two-step evolution of the exchange bias field. The results of these
two different protocols are similar to the ones obtained for measurements
previously reported on epitaxial $ \mathrm{BiFeO_3} $, indicating a driving
mechanism independent of the long-range crystalline arrangement (i.e.,
epitaxial or polycrystalline). An intrinsic property of $ \mathrm{BiFeO_3} $ is
proposed as being the driving mechanism for the thermal dependent magnetization
reversal: the canting of the $ \mathrm{BiFeO_3} $ spins leading to a
biquadratic contribution to the exchange coupling. The temperature dependence
of the magnetization reversal angular behavior agrees with the presence of such
a biquadratic contribution for exchange biased bilayers studied here.
