Anomalous polarization inversion in ferroelectrics via scanning force microscopy

Local poling of ferroelectrics by the sharp conducting tip of a scanning force microscope (SFM) is studied experimentally and theoretically. The formation of the inverse domain under the SFM tip, where the polarization is oriented in the direction opposite to that of the poling field, is reported for bulk ferroelectrics (single crystals of solid solutions PbZn _1/3Nb_2/3O₃-PbTiO₃). This finding confirms earlier results on ferroelectric thick films, thus proving the universality of the anomalous polarization inversion in ferroelectric media. It is shown that the inverse domain grows with the increase of the poling voltage and duration and remains stable for a long time after the removal of electric field. The growth process is described by a dynamic model assuming that the appearance of inverse domains is due to a local internal electric field directed against the poling one. This field is attributed to the space charge formed beneath the SFM tip due to injection of charge carriers and their subsequent drift and trapping. Poling voltage and poling time dependences of the domain size are correctly described by the model. Implications of the anomalous polarization inversion for the domain engineering and dense data storage in ferroelectrics are discussed.