Displacement Charge Patterns and Ferroelectric Domain Wall Dynamics Studied by In-Situ Tem
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manifestations of the same mechanism of domain wall motion. We argued that the shrinking or destruction of a domain requires the flow of a displacement charge. Such charge can be carried on the domain walls in the form of kinks, which may be envisaged as ripples of polarization that flow along the wall. The passage of kinks has the effect of moving the domain wall sideways, and causing a displacement current to flow. A non-uniform kink density causes curvature in the domain wall, with a corresponding displacement charge density, V- (P2 - PI), which depends on the local tilt of the domain wall relative to the polarizations P1 and P2 on either side of the wall.
The pointed tips of "zipping" domains carry a displacement current. A simple depiction of the flow of kinks on domain walls is presented in Figure 1. In this paper we present in-situ transmission electron microscopy (TEM) data of domain motion in BaTiO 3 and KNbO 3 under the application of electric fields. We show that curved domain walls move under the influence of electric fields, whereas charge-neutral domain configurations are more resistant to electric field-induced motion. We argue that charged domain walls within the bulk of the ferroelectric are important both for switching the polarization, and for maintaining a remanent polarization in the absence of electric field. We speculate that an intrinsic contribution to fatigue may arise from an increase in the number of lower energy, charge-neutral, domain configurations with multiple field cyclings.
161 Mat. Res. Soc. Symp. Proc. Vol. 596 ©2000 Materials Research Society
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"Izipping" motion Figure 1. Depiction of how the motion of charged kinks on a domain wall is equivalent to moving the domain wall. When the kink density is low, domain walls appear to move toward each other while remaining approximately parallel. When the kink density is non-uniform, curvature is introduced, with a corresponding build up of displacement charge. In this way, domain wall motion can carry the displacement current J from one electrode to the other. EXPERIMENT Single crystals of barium titanate were grown by the Remeika method [5]. All of the samples contain 0.2 % of Fe, and KF was used as the flux. The samples were annealed at 1160'C/I1l35'C for 8 hours, cooled at 20 'C per hour to 900 'C. 3mm disks were cut out and mechanically thinned to 1501im using a litm diamond slurry, and then floated on hot (-.140'C) phosphoric acid for thinning [6]. This usually caused the sample to break into several pieces but thin fragments could be recovered that were suitable for TEM analysis. The samples were rinsed in ethanol and distilled water prior to observation. Single crystals
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