Clamped Ferroelectric and Magnetic Domain Walls and their Application to Memory Engineering

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J6.4.1

Clamped Ferroelectric and Magnetic Domain Walls and their Application to Memory Engineering Eiichi Hanamura and Yukito Tanabe1 Chitose Institute of Science and Technology and Japan Science and Technology Agency, Chitose, Hokkaido 066-8655 Japan 1 Department of Applied Physics, University of Tokyo, Hongo, Tokyo 113-8656, Japan ABSTRACT A family of rare-earth metal manganites are antiferromagnetic (AFM) ferroelectrics and some of these may show also weak ferromagnetism. First we will show how to observe the ferroelectric (FEL) and AFM domain structures by the interference effects of second harmonic generation (SHG). Second, the observed clamping of the AFM domain wall (DW) to the FEL domain boundary (DB) is intuitively explained by the group theoretical consideration of the magnetic anisotropy energy depending upon the sign of the FEL polarization. Third, the application of these clamped DW-DB to the memory engineering will be briefly discussed. INTRODUCTION Although there are very few natural multiferroic crystals that are both FEL and ferromagnetic in a single phase, a number of AFM ferroelectrics have been found [1]. Some of those may show also weak-ferromagnetism. We will discuss a family of rare-earth metal manganites YMnO3 and ErMnO3, which have as high Curie temperatures for FEL as 1000 K and Néel temperature around 100 K. There are two kinds of the SHG tensor χ(i)αβγ and χ(c)αβγ which are invariant and changing its sign under time-reversal operation, respectively [2]. Here the first suffix α denotes the polarization direction of the SHG signal while the second β and third one γ mean those of the fundamentals. When we evaluate these nonlinear susceptibilities in terms of the eigen-functions Mn(3d) electrons in the paraelectric and paramagnetic phase, χ(i) is linearly proportional to the FEL order parameter while χ(c) is to the product of the FEL and AFM order parameters in the ordered phase [3]. Therefore we can determine the FEL and AFM domain structures by the interference signal of the SHG [4]. It was found by Fiebig et al. [5] that the FEL DB is always accompanied with the AFM DW while the AFM DW can exist by itself. The microscopic mechanism of this clamping was discussed theoretically [6]. The microscopic mechanism will be intuitively presented in the present paper basing upon the group-theoretical point of view. Then we will be able to understand why the FEL DB is always accompanied with the AFM DW while the AFM DW can exist independently. This group theoretical consideration will be applied to Ga1-xXxFeO3 crystal and we will discuss the possibility of memory control in which the magnetic memory will be switched by the electric field or vice versa.

J6.4.2

SECOND HARMONIC GENERATION The SHG from the rare-earth manganites can be described by χαβγ(ω), i.e., the second-order nonlinear susceptibility. We evaluate this susceptibility in terms of (3d)-orbitals in the paraelectric and paramagnetic state as bases and take into account the spin-orbit interaction HSO and the lower-symmetry crystalline fi

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