The nucleation and movement of 180° domain walls in BaTiO3 crystals with salt-water electrodes has been observed directly by partial switching and repeated differential etching at applied fields from 2 to 450 kV/cm. Most of the polarization was found to be reversed by sidewise growth of new domains at these fields, as it is at lower fields, but the sidewise wall velocities are higher, as high as 105 cm/sec. The nuclei are originally circular cylinders, becoming squared up by sidewise growth whose lowest velocity is along the 100 crystal axis. The sidewise wall velocity and the nucleation rate each vary as E1.4, thereby causing the known E−1.4 variation of ts. The nucleation rate dependence is not understood, but the wall-velocity dependence agrees strikingly with a previously uncalculated prediction of the Miller-Weinreich theory. This indicates that, at both high and low applied fields, the apparent sidewise motion of 180° domain walls is due to the nucleation and forward growth of knife-shaped steps on the previously existing wall.
The velocity of 180° domain walls in BaTiO3 crystals with saltwater electrodes has been measured from 3° to 75°C at applied fields from 0.2 to 200 kV/cm by repeated partial switching and etching. Heating the crystal was found to lower the field required to attain any given wall velocity by a factor which is substantially the same for all wall velocities (or for all applied fields) and is determined wholly by the temperature change. The factor is about 3 for a change from 3° to 75°C. This behavior agrees with the Miller-Weinreich mechanism of thermally activated wall movement. Domain shapes are found to be very rounded at 50° and 75°C, becoming almost exactly circular at 75°C around 1 kV/cm, in marked contrast to the well-known square domains at 25°C and below.
The locations of domains appearing on the surface of a BaTiO3 crystal have been observed by repeated partial switching and etching to determine how frequently the domains appear at the same location on successive applied pulses. The ratio of the number of repeating locations to that of all locations is (10±1.6) % for a series of six pulses at 20 kV/cm on crystals with a positive internal bias and (28±5) % on crystals with a negative bias. The simplest interpretation of the results is that new domains can nucleate at points in the crystal remote from imperfections. This interpretation is in rough agreement with Landauer's calculation of the probability of thermally activated new domains for a 180° wall energy of 0.4 erg/cm2.
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