Theory of Spin-Charge-Orbital States in the Frustrated System RFe 2 O 4
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0966-T03-33
Theory of Spin-Charge-Orbital States in the Frustrated System RFe2O4 Sumio Ishihara, Makoto Naka, and Aya Nagano Department of Physics, Tohoku University, Aramaki, Aoba, Sendai, 980-8578, Japan
ABSTRACT Electronic structure in multiferroic compound RFe2O4 is studied theoretically. We suggest that orbital degree of freedom in a Fe2+ ion is active. The effective Hamiltonian for spin, charge and orbital degrees of freedom is derived. Numerical analyses with the multi-canonical Monte-Carlo simulation show that the electric polarization is attributed to the charge order with momentum (1/3, 1/3). A magnitude of the polarization is enhanced around the magnetic ordering temperature due to the coupling between spin and charge. Conventional orbital order is not expected from the numerical calculation. We discuss possible orbital state at low temperatures. INTRODUCTION Several exotic phenomena observed in strongly correlated electron systems are attributed to the competition between multi degrees of freedoms in electron and lattice. Multiferroics mean phenomena where any two of the three, (anti)ferroelectricity, (anti)ferromagnetism and ferroelesticity, coexist. Recently coexistence of the electric and magnetic orders was observed in the mother compounds of the colossal magnetoresistive manganites, RMnO3, where R indicates a rare-earth ion [1]. d(3x2-r2) and d(3y2-r2) orbitals are alternately aligned at higher than 800K. Because of this orbital order and the GdFeO3-type lattice distortion, the ferromagnetic exchange interaction between the nearest neighbor (NN) Mn sites and the antiferromagnetic interaction between the next NN (NNN) sites become comparable. As a result, this system is recognized to be a frustrated spin system [2]. Non collinear spin structure realized in this frustrated system is considered to be an origin of the macroscopic electric polarization.
Figure 1: Schematic view of a pair of the FeO double layer structure. RFe2O4, would be a different type of the multiferroics and/or an exotic ferroelectric compound. Crystal structure in RFe2O4 consists of pairs of FeO layers and RO layers. Fe ions in a FeO layer form a two-dimensional triangle lattice (see Fig. 1). Electric, magnetic and lattice
structures have been examined experimentally [3,4]. A streak-type diffuse scattering in the electron diffraction is observed at (1/3, 1/3) below about 500K. Inside the streak, Bragg spots appear at (1/3, 1/3, m+1/2) below 320K [3]. Since a nominal charge valence of a Fe ion is 2.5+, this Bragg spot is interpreted to be a charge order of Fe2+ and Fe3+. The magnetic structure has been studied by the neutron scattering experiments. A ferrimagnetic three-dimensional order characterized by the (1/3, 1/3, m+1/2) appears below about 247K [5-7]. A macroscopic electric polarization was observed below 350K and its magnitude is confirmed to be increased around 270K [8,9]. It is considered that the electric polarization is related to the charge ordering of the Fe d electrons and is strongly influenced by the magnetic or
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