Terahertz radiation from multiferroic BiFeO 3 thin films as a new approach for ferroelectric memory readout and ferroele
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0902-T06-02.1
Terahertz radiation from multiferroic BiFeO3 thin films as a new approach for ferroelectric memory readout and ferroelectric domain imaging microscopy Kouhei Takahashi1, Noriaki Kida2, and Masayoshi Tonouchi1 1 Institute of Laser Engineering, Osaka University, 2-6 Yamadaoka, Suita, Osaka 565-0871, Japan 2 Spin Superstructure Project (SSS), ERATO, Japan Science and Technology Agency (JST), c/o National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 4, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8562, Japan ABSTRACT We have observed novel terahertz (THz) radiation characteristics of multiferroic BiFeO3 thin films upon illumination of femtosecond laser pulses. The radiated THz pulses from BiFeO3 thin films were shown to originate from an ultrafast modulation of spontaneous polarization, which was introduced by the photoexcited charge carriers. Based on our findings, we briefly present new approaches to nondestructive readout for nonvolatile memory devices and ferroelectric domain imaging microscopy using THz radiation as a sensitive probe. INTRODUCTION Perovskite BiFeO3 is a multiferroic material displaying ferroelectricity below 1100 K and antiferromagnetism below 640 K. In addition to its multiferroic benefit, this material exhibits an extremely large spontaneous polarization Ps in thin film form providing its potential as a capacitor in various electronic devices [1,2]. Accordingly, studies on BiFeO3 have mainly been focused on developing its electronic functionality as a capacitor, and its optical functionality, on the other hand, has not been investigated intensively so far. Arima et al. have reported that optical gap in transition metal oxides tends to decrease with increasing the atomic number of the transition metal element [3]. One of the characteristics that the multiferroics exhibit is the “d-orbital occupancy”, which is essential for expressing magnetism. This gives rise to smaller energy gaps in multiferroics compared to the ordinary titanium-based ferroelectrics with “d0-ness” feature. In the case of BiFeO3 the energy gap is estimated to be about 2.5 eV [4], which is indeed small in contrast to the titanium-based ferroelectrics whose energy gap generally locates in the ultraviolet region. With exhibiting such a small energy gap, BiFeO3 allows free carrier excitation by commercially available femtosecond laser, and hence, is suitable for developing ultrafast ferroelectric optical devices as widely demonstrated in semiconductors. In the present work, we show a novel functionality of BiFeO3 thin films, i.e., terahertz (THz) electromagnetic pulse radiation via carrier excitation upon illumination of femtosecond laser pulses. THz radiation from BiFeO3 thin films were shown to result from an ultrafast modulation of Ps, which gave rise to a memory effect in a unique style and also enabled us to visualize the 180˚ ferroelectric domain structures. Based on our findings, we propose and demonstrate a new approach for ferroelectric nonvolatile memory as well as ferroelectr
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