First Principles Investigation of Multiferroism in Perovskite Manganites
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Materials Department, University of California Santa Barbara, CA 93106-5050.
"* Dept. of Applied Physics, Yale University, P.O. Box 208284, New Haven, CT 06520-8284. ABSTRACT We present results of first principles calculations which indicate the simultaneous occurrence of ferromagnetism and ferroelectricity in perovskite structure bismuth manganite, BiMnO 3. INTRODUCTION Many rare earth perovskite manganites exhibit strong coupling between either their magnetic and structural, or magnetic and electronic order parameters, and the colossal magnetoresistive (CMR) effect[l] is thought to be related to the resulting magnetically induced structural and metal-insulator phase transitions. Similarly, the strong coupling between the ferroelectric and structural order parameters in many ferroelectric oxides has led to their widespread use in transducers and actuators[2]. However the existence of strong coupling between all three of the electric, magnetic and structural order parameters (leading to ferroelectricity, ferromagnetism and ferroelasticity) is rather rare. Bismuth manganite can be regarded as the "hydrogen atom" of such multiferroic[4] materials. Although information about BiMnO 3 is sparse[3], indications are that it is simultaneously ferromagnetic and ferroelectric at low temperatures. In addition, BiMnO 3 is strikingly different, both magnetically and structurally, from the rare earth perovskite manganites. BiMnO 3 is ferromagnetic with a triclinic structural distortion in its ground state, whereas the rare earth manganites are antiferromagnetic and either orthorhombic or hexagonal. The anomalous behavior of BiMnO 3 compared with conventional perovskite manganites could give rise to unusual and useful transport properties. The goal of the study described in this paper is to determine the origin of the differences between BiMnO 3 and the other perovskite manganites, both to understand the fundamental physics, and to assist in the optimization of perovskite manganite materials for novel device applications. To achieve this goal we evaluate the electronic and magnetic properties of perovskite manganites using a plane wave pseudopotential implementation of density functional theory within the local spin density approximation. Full details of our method are given in Ref. [5]. CALCULATED ELECTRONIC PROPERTIES OF BiMnO3 AND LaMnO 3 We investigate the origin of the differences between BiMnO 3 and the rare earth manganites by comparing the calculated electronic properties of BiMnO 3 with those of the prototypical rare earth manganite, LaMnO3 . We begin by calculating the electronic structure for the high symmetry cubic phases, without including magnetic effects (the paramagnetic (PM) phase), then lower the magnetic symmetry to the ferromagnetic (FM) phase. Finally we introduce structural distortions. This ability to isolate structural and magnetic distortions is unique to computational studies, and allows identification of the essential microscopic interactions which cause the observed macroscopic behavior. 157 Mat. Res.
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