H. L. Stadler

Nucleation and Growth of Ferroelectric Domains in BaTiO3 at Fields from 2 to 450 kV/cm

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 ...

Thickness Dependence of BaTiO3 Switching Time

The ferroelectric switching time of barium titanate single crystals has been measured at room temperature as a function of electric field and crystal thickness. The switching time was found to be independent of crystal thickness at high fields in disagreement with earlier work by Merz, who found the switching time proportional to thickness at the same high fields. At low fields the results are in substantial agreement with Merz and other previous work. At fields E greater than a threshold field of 1.4 (1+0.007 cm/d) kV/cm, where d is the crystal thickness, the switching time ts is given by 9E−1.4±0.1 (kV/cm)1.4 sec. Below this threshold the switching time is adequately represented by ts=0.8 exp[3.5(1+0.007 cm/d) kV/cm/E] μsec for d≳0.007 cm, but for d«.007 cm, the data are fitted better by ts=1.5×103 (Ed)−10.3±0.5 sec (V)10.3. Thus the data do not agree with the predictions of any known surface layer model of BaTiO3 switching.

Temperature Dependence of 180° Domain Wall Velocity in BaTiO3

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.

Random Nucleation of New Domains in BaTiO3 Crystals at 20 kV/cm

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 Landauers calculation of the probability of thermally activated new domains for a 180° wall energy of 0.4 erg/cm2.

Etched Hillocks in BaTiO3

Two kinds of mounds or hillocks have been found on BaTiO3 single crystals after etching. Rounded, flat‐topped hillocks were found after etching with 1% HF for 30 sec at room temperature. These hillocks were found where a 180° domain wall had been left for several days and had been removed shortly before etching. Their behavior can be explained by the ad hoc assumption that negative charges are attracted to the 180° domain wall at the crystal surface.Mounds shaped like Egyptian pyramids were found after etching BaTiO3 single crystals for 30 min in H3PO4 at 155°C. The edges between pyramidal faces always project onto the crystal surface (a 100 plane) in 110 directions.

A Card-Changeable Permanent-Magnet-Twistor Memory of Large Capacityt

The card-changeable permanent-magnet-twistor memory is a large capacity (ca 105 bits) storage media for information that is infrequently changed. The information is stored in the form of small bar magnets bonded to a removable plastic card. The magnets, when magnetized, inhibit the switching of a section of twistor wire at a twistor-wire-solenoid crosspoint. For maximum information density the magnet shape and strength must be optimized with respect to the magnets action on the inhibited crosspoint and the fringing action on neighboring crosspoints. The objective is a magnet with a small dipole moment, but with adequate inhibition of the twistor switching over a reasonable range of misposition. Suitable magnet shapes and a general discussion of the stray fields in a large array of magnets are given. For maximum capacity, the transmission characteristics of the twistor wire and the character of the access switch must be considered. Two novel structures of this memory permit increased information density and capacity. The feasibility of a random-access high-density memory submodule containing 360,000 bits in 0.7 foot3 with a cycle time of 5 ?sec has been demonstrated.