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

TL;DR

This study investigates how temperature affects exchange bias in polycrystalline BiFeO3/NiFe bilayers, revealing non-monotonic behavior and a two-step evolution linked to BiFeO3's spin canting, independent of crystalline structure.

Contribution

It demonstrates that the thermal dependence of exchange bias in polycrystalline BiFeO3/NiFe bilayers is driven by intrinsic spin canting, consistent with epitaxial cases, and introduces a biquadratic exchange mechanism.

Findings

Non-monotonic exchange bias field with temperature.

Two-step evolution of exchange bias via the Soeya protocol.

Spin canting in BiFeO3 influences exchange coupling.

Abstract

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.