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The effect of grain size on domain structure in unsupported thin films

The effect of grain size on domain structure in unsupported thin films with grains (70 - 250 nm) was studied by using TEM. It is found that there probably exists a critical grain size near 200 nm, at which an abrupt change in domain structure occurs. Above this size, domains, which are lamellar microtwins, appear in almost all of the coarse grains. The domain width decreases with grain size. Below this size, however, most of the grains are single-domain ones, while some irregular and curved domain walls appear only in a small portion of the fine grains. The number of the single-domain grains increases with decreasing grain size. Such a grain-size dependence of domain structure is correlated with the anomalous behaviours of the coercive field and dielectric constant in ferroelectric thin films. This interesting result has not been reported before.