Charge-ordering-induced polar domains and domain walls in a bilayered manganite _{Pr (Sr0.15 Ca0.85) 2} Mn2 O⁷

We have observed charge-ordering-induced polar domains and domain walls (DWs) in a bilayered manganite Pr (Sr0.15 Ca0.85) 2 Mn2 O⁷ using second-harmonic-generation microscopy. Around the transition temperature TC ∼306K, a nucleation process of the domains with spontaneous polarization is clearly observed. Well below TC, the characteristic band-shaped dark patterns are identified in the second-harmonic images. On the analogy of the nucleation feature, they can be assigned to the DWs separating antiparallel polar domains. The anisotropy in the DW is governed by the geometrical constraint of the charge and orbital ordering, not by the electrostatic stability as in conventional ferroelectric DWs.