Deterministically robust switching of ferroelectric polarization and spin cycloidal polarity in (111) monodomain BiFeO₃ thin films

(111) BiFeO₃ (BFO) thin film with single ferroelectric (FE), ferroelastic and antiferromagnetic (AFM) domains is desirable toward a deterministically switchable (by an electric field) memory device, as it has the highest ferroelectric polarization ever reported. However, it is extremely challenging to realize. Here, we show that epitaxial BFO film grown on orthorhombic NdGaO₃ (NGO) (011)_o substrate buffered by a thin SrRuO₃ layer exhibits single FE, ferroelastic and AFM domains as probed by P-E loop (and PFM), synchrotron X-ray and non-resonant X-ray magnetic scattering measurements. Importantly, its single domain nature is found to be very robust over 10,000 times electric field switching of FE polarization and associated spin-cycloidal polarity, which is remarkable as BFO grown on cubic SrTiO₃ is not truly single domain (single FE, but 3-AFM domains) and can degrade over less number of switching cycles. Unlike SrTiO₃, the symmetry of the (111)_pc plane is broken in case of NGO, which removes the degeneracy of the monoclinic domains, resulting associated single AFM domain with spin-cycloid along [1-10]. Also, the BFO film on NGO carries the orthorhombic distortion which helps in selecting a preferred path over others during electric field switching and thus, reducing the formation of charged domain wall which leads to robust and deterministic polarization reversal over many cycles. Thus, our study opens an avenue to explore (111) single domain BFO films toward prospective device applications.