Specific features of magnetic structure formation in orbitally degenerate BiMnO 3 manganite
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DISORDER, AND PHASE TRANSITION IN CONDENSED SYSTEM
Specific Features of Magnetic Structure Formation in Orbitally Degenerate BiMnO3 Manganite1 L. E. Gonchara,b,*, T. O. Nikitinaa, and A. E. Nikiforova a
b
Ural Federal University, Yekaterinburg, 620002 Russia Ural State University of Railway Transport, Yekaterinburg, 620034 Russia *email: [email protected]
Abstract—The orbital structure and magnetic ordering of the Jahn–Teller multiferroic BiMnO3 manganite have been theoretically studied. It is shown that the orbital structure depends not only on the nearestneigh bor oxygen environment of manganese ions, but also on their nexttonearest neighbors. The orbital structure significantly influences the magnetic order that forms as a result of competition between ferromagnetic and antiferromagnetic exchange interactions. DOI: 10.1134/S106377611305018X 1
1. INTRODUCTION
The interest in Jahn–Teller magnetic oxides is increasing due to a number of specific effects that can be simultaneously observed in these crystals, including colossal magnetoresistance and numerous structural and magnetic phase transitions driven by temperature, magnetic and electric fields, and pressure. The unusual properties of these compounds are deter mined by the strong mutual relationship of crystalline, charge, orbital, and magnetic structures. In recent years, extensive research has also been devoted to mul tiferroelectric compounds in which magnetic order coexists with spontaneous polarization. Bismuth manganite BiMnO3 has been treated in many investigations as a multiferroelectric compound [1–6]. However, the results of recent experiments [7, 8] have shown that this ferromagnetic crystal belongs to the C2/c symmetry group, which excludes the exist ence of spontaneous polarization [5–9]. External fac tors (temperature, pressure) can alter the magnetic structure of BiMnO3 crystals so as to stabilize antifer romagnetism [8, 9]. The nonstoichiometry of a crystal with respect to oxygen also strongly influences the crystalline and magnetic structures: an increase in the amount of oxygen ions leads to a change in the mag netic order from ferro to antiferromagnetic [10]. It has been suggested that bismuth manganite, sim ilar to many rareearth manganites, possesses an orbital structure, competitive exchange interactions, and the related magnetic frustration [5–8]. The present work was aimed at studying the effect of orbital ordering on the magnetic structure of BiMnO3 in the presence of competitive exchange interactions.
2. CRYSTALLINE AND ORBITAL STRUCTURES OF BiMnO3 According to experimental data [7, 8], the crystalline lattice of bismuth manganite belongs to the C2/c sym metry group. In this monoclinic group, the vectors of the main lattice periods are not mutually orthogonal. Exper imental data [8] are summarized in Tables 1 and 2. The crystalline lattice of BiMnO3 is a distorted per ovskite structure, in which Mn3+ ion in a octahedral environment has an orbitally degenerate 5E ground state. In this case, the orbital degen
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