Temperature dependence of exchange biased multiferroic BiFeO₃/Ni₈₁Fe₁₉ polycrystalline bilayer

The temperature dependence of exchange bias properties are studied in polycrystalline ${BiFeO}_3/N{i}_{81}F{e}_{19}$ bilayers, for different ${BiFeO}_3$ 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 ${BiFeO}_3$ 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 ${BiFeO}_3$, indicating a driving mechanism independent of the long-range crystalline arrangement (i.e. epitaxial or polycrystalline). An intrinsic property of ${BiFeO}_3$ is proposed as being the driving mechanism for the thermal dependent magnetization reversal: the canting of the ${BiFeO}_3$ 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. (paper)