Temperature dependence of exchange biased multiferroic $\mathbf{BiFe}{\mathbf{O}}_3$/$\mathbf{N}{\mathbf{i}}_{81}\mathbf{F}{\mathbf{e}}_{19}$ polycrystalline bilayer

Abstract

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