Polarization Reversal Anti-parallel to the Applied Electric Field Observed Using a Scanning Nonlinear Dielectric Microsc
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Polarization Reversal Anti-parallel to the Applied Electric Field Observed Using a Scanning Nonlinear Dielectric Microscopy Takeshi MORITA and Yasuo CHO Research Institute of Electrical Communication, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi, 980-8577, Japan [email protected] ABSTRACT It is well known that spontaneous polarization of ferroelectric material is an intrinsic property applied for nonvolatile memory devices. Poling direction can be reversed in a nanometer size area using the conductive cantilever of a scanning probe microscope. In order to detect nanodots patterns, scanning nonlinear dielectric microscope (SNDM) is superior to piezoresponse microscope in terms of resolution. In this paper, a real-time measuring method of a poling direction is proposed. Using this method, the domain reversal process was observed and an unexpected phenomenon was found , namely, that the poling directions were aligned antiparallel to the poling electric field. This antiparallel poling reversal took place when the film thickness was more than 350 nm in the case of lithium tantalate. At present, the reason and mechanism of the antiparallel poling reversal are uncertain, although it might be related to the concentrated electric field near the cantilever tip. INTRODUCTION Ferroelectric materials are intensively researched as a nonvolatile memory medium (FeRAM) due to its simple construction and good compatibility with integrated circuits. This application is based on the intrinsic property of the ferroelectric material, namely the controllability of the poling directions with external electric field. A FeRAM is composed of two plate electrodes and a ferroelectric medium. By replacing the top electrode with a conductive cantilever tip, a nanodot-domain-reversed pattern can be obtained using a conventional atomic force microscope. By using SNDM[1−3] , Cho et al. have demonstrated data storage in inverted domain dots in ferroelectric (lithium tantalate) film at a data density of 1.5 Tbit/inch2 [4,5] . The advantage of ferroelectric data storage over ferromagnetic storage is the thin domain wall that has been found to be only a few lattice constants wide by SNDM[6] . To realize a much higher speed, higher reliability nanodot writing and reading system, fundamental investigations into nanodot formation mechanisms are indispensable. Indeed, a number of unexplained phenomena have been observed in polarization reversal, such as a back-switching[7] and ring shaped poling reversal dots that are sometimes observed during nanodot patterning. Hence a conductive cantilever was fixed on the ferroelectric material and poling reversal phenomenon under the cantilever was observed. REALTIME POLING REVERSAL OBSERVATION Figure 1 shows an SNDM system for real-time poling reversal observation that is an atomic force microscope connected to a self oscillator (around 1.2 GHz), a FM demodulator and a lock-in amplifier. Ferroelectric materials exhibit strong nonlinear permittivity that is a function of applied electric field. For
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