Effect of Impressing Rate of Field on Polarization Reversal in Mg Doped Near Stoichiometric Lithium Tantalate Single Cry
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Effect of Impressing Rate of Field on Polarization Reversal in Mg Doped Near Stoichiometric Lithium Tantalate Single Crystals Sarveswaran Ganesamoorthy, Masaru Nakamura, Shunji Takekawa, Somu Kumaragurubaran, Kazuya Terabe and Kenji Kitamura Opto Single Crystal Group, Advanced Materials Laboratory, National Institute of Materials Science, 1-1, Namiki, Tsukuba-Shi, Ibaraki 305 0044, Japan
ABSTRACT The coercive field measured from ferroelectric hysteresis loop for pure and Mg:SLT (0.5 mol% doped) crystals were found to be independent of ramp rate of voltage, while it depends strongly on ramp rate for Mg:SLT (1 mol% doped) crystals. The measured coercive field decreased monotonously depending on the Mg concentration and low Ec value of 6 kV/cm is obtained for Mg(1.0 mol%)SLT crystal. Internal field of about 1 kV/cm present in pure SLT completely gets vanished in Mg(1.0 mol%)SLT. Spontaneous polarization is insensitive to the Mg addition. The origin of the internal field and large changes in switching fields appear largely to be dependent on the [Li]/[Li+Ta] ratio in crystals. On repeated cycling, the coercive field is found to have a marginal variation in pure and Mg doped SLT crystals.
INTRODUCTION Lithium tantalate (LT) is an important single crystal oxide material in surface acoustic wave, electro-optic and waveguide applications. Periodically poled LT crystals find wide applications in quasi phase matched frequency conversion devices and have attracted much attention in recent years [1,2]. Higher domain inversion field (21 kV/mm) in the congruently melting LT (CLT) (Li/Ta=48.6/51.4) crystal restricts the feasibility of obtaining periodically poled domain structures. On the other hand, near stoichiometric LT (SLT) crystals grown from Li-rich melt by double crucible method exhibit remarkably low domain inversion field (1.7 kV/mm) [3]. To understand the underlying mechanism in poled structures, domain kinetic studies in these crystals have been investigated in recent years [4]. It has been observed that under the influence of an electric field the shape of the inverted domain is triangular for congruent lithium tantalate (CLT) and hexagonal for SLT samples. This hexagonal shape offers the possibility of a smooth domain wall structure in contrast to CLT samples. Pure SLT suffers from a limitation in some applications due to the photorefractive damage. Magnesium doping in LT has been demonstrated to suppress the optical damage [4]. In CLT higher (upto 5 mol %) MgO concentration is required to suppress optical damage, but however in near SLT crystals 0.5 to 1.0 mol % is sufficient to overcome the same. The intrinsic properties such as spontaneous polarization, coercive field and internal field of ferroelectric MgSLT have not been well understood. To get an insight into the effects of Mg doping and to obtain the optimum conditions for domain reversal, it is necessary to explore the ferroelectric switching
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properties. The most commonly used technique to invert the sign of the nonlinear coefficient o
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