180° Ferroelectric Stripe Nanodomains in BiFeO₃ Thin Films

There is growing evidence that domain walls in ferroics can possess emergent properties that are absent in the bulk. For example, 180° ferroelectric domain walls in the ferroelectric-antiferromagnetic BiFeO₃ are particularly interesting because they have been predicted to possess a range of intriguing behaviors, including electronic conduction and enhanced magnetization. To date, however, ordered arrays of such domain structures have not been reported. Here, we report the observation of 180° stripe nanodomains in (110)-oriented BiFeO₃ thin films grown on orthorhombic GdScO₃ (010)_O substrates and their impact on exchange coupling to metallic ferromagnets. Nanoscale ferroelectric 180° stripe domains with {112¯} domain walls were observed in films <32 nm thick. With increasing film thickness, we observed a domain structure crossover from the depolarization field-driven 180° stripe nanodomains to 71° ferroelastic domains determined by the elastic energy. These 180° domain walls (which are typically cylindrical or meandering in nature due to a lack of strong anisotropy associated with the energy of such walls) are found to be highly ordered. Additional studies of Co_0.9Fe_0.1/BiFeO₃ heterostructures reveal exchange bias and exchange enhancement in heterostructures based on BiFeO₃ with 180° domain walls and an absence of exchange bias in heterostructures based on BiFeO₃ with 71° domain walls; suggesting that the 180° domain walls could be the possible source for pinned uncompensated spins that give rise to exchange bias. This is further confirmed by X-ray circular magnetic dichroism studies, which demonstrate that films with predominantly 180° domain walls have larger magnetization than those with primarily 71° domain walls. Our results could be useful to extract the structure of domain walls and to explore domain wall functionalities in BiFeO₃.