Effect of non-$180^{\circ }$ polarization invariants on the exchange bias of tetragonal $\langle 001\rangle$ and rhombohedral $\langle 111\rangle$ orientations of bismuth ferrite epitaxial thin films

Abstract

$\hbox {BiFeO}_{3}$ (BFO) is the only room temperature multiferroic material that has been extensively studied due to its multifunctional properties. BFO with a canted G-type antiferromagnetic (AFM) ordering exhibits strong exchange bias characteristics with NiFe which offers the potential to design and utilize devices working based on multiferroic features. In the past, it was known that the presence of $180^{\circ }$ domain walls of BFO hinders a plausible exchange bias interaction. To understand the role of the strain-induced effects on such $180^{\circ }$ domain walls and its effect on the exchange bias, NiFe, a soft ferromagnetic layer, was grown on the epitaxial BFO AFM layers. An approximately 80 nm BFO layer was grown epitaxially in both the tetragonal (001) phase on the $\hbox {LaAlO}_{3}$ (001) substrate and the rhombohedral (111) phase on the $\hbox {SrTiO}_{3}$ (111) substrate, with a thin (10 nm) layer of $\hbox {Ni}_{80}\hbox {Fe}_{20}$ on top of it. An exchange bias of 510 Oe was observed in the tetragonal phase of BFO with a c / a ratio of 1.22, which is comparable with the exchange bias shown by the (111) oriented rhombohedral phase (360 Oe). Both the tetragonal (001) and rhombohedral (111) layers possess ferroelectric polarization normal to the sample surface and so the domain walls are mostly $180^{\circ }$ oriented which is expected to have a minimum effect on the exchange bias. However, the weak strain-induced structural variants in the (111) oriented rhombohedral BFO and the monoclinic distortion present in the tetragonal BFO introduce non-$180^{\circ }$ domain walls in the system. These variants arising due to the structural distortion are expected to play a key role in defining the ferroelectric domain wall nature, thereby exhibiting exchange bias characteristics.