Polarization reversal and domain anisotropy in flux-grown KTiOPO 4 and isomorphic crystals
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M. Molotskii School of Physics and Astronomy, Beverly and Raymond Sackler Faculty of Exact Sciences, Tel Aviv University, Ramat-Aviv, 69978, Israel (Received 21 August 2000; accepted 5 March 2001)
Spontaneous polarization reversal and domain structures of flux-grown ferroelectric KTiOPO4 and isomorphic crystals were studied. Two temperature regions with dominant either ionic or electronic conductivity were found. It was shown that in the high-temperature region mobile cations contributed sufficiently to internal screening process. The ionic leakage current was suppressed at a specific temperature point for each studied crystal. High crystallographic asymmetry of domain wall velocity was observed. This shows that the electrode pattern should be properly oriented relative to the crystal axes of KTiOPO4 and its isomorphs for fabrication of periodically poled domain configurations used in nonlinear optical conversions.
I. INTRODUCTION
The primary property of ferroelectrics is the polarization reversal effect when an external electric field causes re-orientation of the spontaneous polarization (Ps) direction.1,2 An intensive development of several important applications, such as ferroelectric computer memory systems3 and specifically engineered ferroelectric domain configurations exploited for nonlinear optical converters, 4 showed that some basic problems of the phenomenon remain unsolved. The process of transition from one stable polarization state to another with opposite direction of macroscopic electrical dipole moment depends on minimization of the depolarization energy, or, in other words, on compensation of the unscreened charges generated during polarization inversion.2 Two quite different processes may contribute to the compensation of the depolarization field: internal screening process via ferroelectric crystal bulk and external screening occurring through the external short circuit. Previous studies showed that the polarization inversion is strongly influenced by electrical properties of the interface between the switching electrode and the ferroelectric crystal, which determine the external screening current. The effect was observed in BaTiO35,6 crystals where sidewise motion velocity of domain walls in metal electroded crystals was by several orders of magnitude less than that found for the same a)
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 16, No. 5, May 2001
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switching field with liquid (LiCl) switching electrodes. Asymmetric polarization switching current was obtained in triglycine sulphate (TGS) crystals inverted through the gap between the switching electrode and the polar surface of the ferroelectric.7 The effect of nonidentical electrodes was studied in the papers8,9 where one of the polar surfaces of TGS crystals was coated by Si-electrode. Dielectric hysteresis loops showing polarization switching in alternating electric field were greatly distorted because of unipolar conductivity of the dop
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