Anomalous domain periodicity observed in ferroelectric PbTiO3 nanodots with 180o stripe domains

PbTiO3-based nano-scale dots and tubes have received a great deal of attention owing to their potential applications to nonvolatile memories and multi-functional devices. As for the size effect of 180o stripe domains in ferroelectric thin films, there have been extensive reports on the thickness-dependent domain periodicity. All these studies have revealed that the domain periodicity of 180o stripe domains scales with the film thickness (d) according to the classical Landau-Lifshitz-Kittel (LLK) scaling law down to the thickness of ~2 nm. In the case of PbTiO3 nanodots, however, we obtained a quite striking correlation that for the thickness less than a certain critical value, dc (~35 nm), the domain width even increases with decreasing thickness of the nanodot, which surprisingly indicates a negative value in the LLK scaling-law exponent. On the basis of theoretical considerations of dc, we attribute this anomalous domain periodicity to the presence of a nonferroelectric surface layer, in addition to the finite lateral-size effect of a ferroelectric nanodot.