Understanding the Nature of Ultrafast Polarization Dynamics of Ferroelectric Memory in the Multiferroic BiFeO 3 By Dhanvir Singh Rana,* Iwao Kawayama, Krushna Mavani, Kouhei Takahashi, Hironaru Murakami, and Masayoshi Tonouchi* The vast majority of data-storage devices are based on ferro- electric or magnetic materials. [1,2] The magnetic-memory effect is derived from the spin order, while the ferroelectric memories are based on the spontaneous polarization of electric dipoles. In the quest for superior efficiencies, the operating speed of the devices based on these memories has become a major focus of research. The time needed for the memory parameter to respond to ultrafast electric, magnetic, or optical stimuli is vital to determine its operating speed. [3–5] The ultrafast dynamics of spin (magnetic) memory over a picosecond time scale upon femtosecond excitation is well established. [3–11] Upon femtosecond excitation, the spin reorientation or the ultrafast demagnetization in ferromagnetic and antiferromagnetic materials occurs in the time scale of 1–2 ps, which suggests that when subjected to ultrafast stimuli, the magnetic memory can be manipulated at an exceptionally high speeds. However, the analogous situation of the ultrafast functionality of spontaneous polarization in ferro- electric/multiferroic materials remains unknown. The use of terahertz (THz) radiation-based spectroscopic techniques [12–18] has uncovered phenomena in a wide range of materials, such as semiconductors, [15,16] strongly correlated electron systems, [11–15] magnetic films, [9–11] and biological molecules. [19] The emission of THz radiation provides a direct measure of transient change in electric/magnetic fields over a picosecond time scale. [9–17] In any medium, THz emission according to the Maxwell wave equation is governed by r 2 E  " 0 m @ 2 E @t 2 ¼ mð @J @t þ @ 2 P @t 2 Þ (1) where E, eoˆ, m, and t are the electric field, electric susceptibility of free space, magnetic permeability, and time, respectively. In this relationship, THz emission can occur through a time-varying current density (J) and/or polarization (P). The polarization can be decomposed into P ¼ P S þ P NL , which indicates that THz emission can be due to time-varying spontaneous polarization P S (such as partial/complete depolarization) and/or optical rectifica- tion in non-linear medium (P NL ). [12,13,16,17] While the latter is a well-established source of THz emission, [13,16] the effect of P S is not known. Terahertz emission due to P S is of great importance, as it is a direct means of probing the ultrafast polarization dynamics of ferroelectric memories. [20] It should also provide a better understanding of the time scale at which the electric dipoles in the spontaneous polarized state are accessible for read or write operations. This time scale is crucial in determining the operating speed of ferroelectric memory devices, thus paving the way to improve their efficiency and ultimately resulting in their widespread adoption in various technological applications. To realize this, it is essential to utilize a ferroelectric system with a large P S . A choice popular with many researchers is the room-temperature ferroelectric antiferromagnet, BiFeO 3 . [21] This is because of its large spontaneous polarization, magnetoelectric coupling, and the possibility of epitaxial strain engineering of magnetoelectric properties in thin films. [21–26] Its ferroelectric polarization is oriented in the (111) crystallographic plane [24] (Fig. 1a), which implies that THz emission spectroscopy along all three (100), (110), and (111) orientations should show distinct features. In this paper, we report the ultrafast dynamics of P S of BiFeO 3 in (100)-, (110)-, and (111)-oriented thin films using THz emission spectroscopy. The axis of ferroelectric polarization, which is the (111) plane, is oriented at an angle of 54 8 from the surface normal in (100) films, and therefore an electric field applied in-plane results in spontaneous polarization (Fig. 1b). This also occurs in (110) films. However, for (111)-oriented films, an electric field applied normal to the (111) orientation does not result in any spontaneous polarization. As such, there is a difference in THz emission from (111) films and from (100) and (110) films. We present the ultrafast polarization dynamics of the room-temperature ferroelectric-antiferromagnet BiFeO 3 upon illumination by ultrafast band-gap laser pulses. Employing terahertz emission spectroscopy [9,10,12–15] to BiFeO 3 (100), (110), and (111) thin films, we demonstrate the ultrafast depolarization of the ferroelectric order over a picosecond time scale, followed by its nondestructive retrieval. The THz time-domain waveforms from all of the (100), (110), and (111) films (Fig. 1c) are only obtained when the energy of the excitation laser pulses is larger than the band-gap of BiFeO 3 , COMMUNICATION www.advmat.de [*] Dr. D. S. Rana, [+] Prof. M. Tonouchi, Dr. I. Kawayama, Dr. K. R. Mavani, [++] Dr. K. Takahashi, Dr. H. Murakami Institute of Laser Engineering, Osaka University 2-6 Yamadaoka, Suita 565-0871, Osaka (Japan) E-mail: dsrana@iiserbhopal.ac.in; tonouchi@ile.osaka-u.jp [ + ] Present address: Indian Institute of Science Education and Research, Bhopal, India. [ ++ ] Present address: Institute for Integrated Cell-Material Sciences, Kyoto University, Japan. DOI: 10.1002/adma.200802094 Adv. Mater. 2009, 21, 2881–2885 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2881