Temperature dependence of exchange biased multiferroic BiFeO₃/Ni₈₁Fe₁₉ polycrystalline bilayer

The temperature dependence of exchange bias properties are studied in polycrystalline bilayers, for different 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 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 , indicating a driving mechanism independent of the long-range crystalline arrangement (i.e. epitaxial or polycrystalline). An intrinsic property of is proposed as being the driving mechanism for the thermal dependent magnetization reversal: the canting of the 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.