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 degrees 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 degrees 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 degrees stripe domains with {11 (2) over bar} 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 degrees stripe nanodomains to 71 degrees 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 Co0.9Fe0.1/BiFeO3 heterostructures reveal exchange bias and exchange enhancement in heterostructures based on BiFeO3 with 180 degrees domain walls and an absence of exchange bias in heterostructures based on BiFeO3 with 71 degrees domain walls; suggesting that the 180 degrees domain walls could be the possible source for pinned uncompensated spins that give rise to exchange bias. This is further confirmed by Xray circular magnetic dichroism studies, which demonstrate that films with predominantly 180 degrees domain walls have larger magnetization than those with primarily 71 degrees domain walls. Our results could be useful to extract the structure of domain walls and to explore domain wall functionalities in BiFeO3.