Control of Ferroelectric Phases by 4 f Magnetic Moments in Multiferroic R MnO 3 Crystals
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0966-T06-02
Control of Ferroelectric Phases by 4f Magnetic Moments in Multiferroic RMnO3 Crystals H. Kuwahara, K. Noda, M. Akaki, F. Nakamura, and D. Akahoshi Dept. of Phys., Sophia Univ., 7-1 Kioi-cho, Chiyoda-ku, Tokyo, 102-8554, Japan
ABSTRACT We have investigated the magnetoelectric (ME) properties of the orthorhombic RMnO3 (R=mixed rare earth ions) crystals with and without the 4f magnetic moments, while keeping an average ionic size of R. In the presence of the magnetic contribution from the R ions ((Eu,Ho)MnO3), we have observed the ferroelectric polarization along the c axis (Pc) in a zero field, which arises from the bc spiral structure stabilized by the anisotropic exchange interaction between the R and Mn sublattices like TbMnO3. The Pc phase can be transferred to Pa by application of fields of ~1.5 T parallel to the b axis. The observed decrease of transition field compared with TbMnO3 suggests that the magnetic contribution and the local distortion of the R ions are key factors for governing the ME properties.
INTRODUCTION Since the discovery of magnetic-field control of ferroelectric polarization in orthorhombic TbMnO3 [1,2], a renewed interest has been attracted in magnetoelectric (ME) or multiferroic materials from the viewpoint of future device applications such as electric- (magnetic-) field controlled ferromagnetic (ferroelectric) memories or transducers. Stimulating experimental and theoretical studies have revealed that this intriguing ferroelectricity arises from an unusual noncollinear spin structure: the transversely-modulated spiral antiferromagnetic structure induces a uniform ferroelectric displacement of oxygen due to the inverse Dzyaloshinskii-Moriya r interaction [3-6]. The ferroelectric polarization ( P ) of magnetic origin is simply expressed as r r r r P = aeij × ( Si × S j ) , where a is the constant as determined by the spin exchange interaction and r
r
r
the spin-orbit interaction, eij means the unit vector connecting two sites, and Si × S j is the vector product of the adjacent canting spins. The P vector is orthogonal to both of the magneticr r r modulation wave vector ( eij ) and the vector of magnetic spiral ( Si × S j , the spin-current direction). This microscopic model is widely accepted as for many recently discovered multiferroics. However, the introduction of additional mechanisms is needed to explain the magnetic-field-induced flop of P. Although the theoretical model predicts that Pc disappears in the external magnetic field parallel to the b axis (Hbext), in fact, Pc has been observed to be flopped to Pa in TbMnO3 [1,2]. We focused this discrepancy, and investigated a role of magnetic contributions from the R ions, which should influence the ME properties. In this context, we have started investigating the dielectric and magnetic properties of the (Eu0.595,Y0.405)MnO3 ((Eu,Y)MnO3) crystal without the interference of magnetic contribution from the R ions, because the total 4f moment is quenched in Eu (7F0) and Y has no 4f electron. As previously reported [7,8], t
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