Temperature dependence of exchange biased multiferroic $\mathbf{BiFeO_3}$/$\mathbf{Ni_{81}Fe_{19}}$ polycrystalline bilayer

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.

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