Charge, orbital and spin ordering in multiferroic BiMn2O5: density functional theory calculations

Abstract

Charge, orbital, and spin ordering of multiferroic BiMn₂O₅ are investigated by the full-potential linearized augmented plane-wave (FPLAPW) method as implemented in the WIEN2K package. Both the generalized gradient approximation (GGA) as well as GGA plus the one-site Coulomb interaction (GGA+U) methods are considered for the exchange–correlation energy functional. The obtained results show that BiMn₂O₅ is found stable in ferrimagnetic state with band gap about 1.23 eV. The results suggest that BiMn₂O₅ contains two kinds of manganese: the ionicity of Mn1 (Mn⁴⁺) is +3.6 with magnetic moment of 2.40 μ_B and the ionicity of Mn2 (Mn³⁺) is +3.4 with magnetic moment of 3.22 μ_B. While charge disproportion between Mn1 and Mn2 is small, the difference between e_g minority occupancies of Mn³⁺ and Mn⁴⁺ cations is large. Both these two properties give direct evidence of charge ordering. The analysis of the Born effective charge reveals that the partial ferroelectric polarization (P_ele) originates from the charge ordering, which is in agreement with a recent work by Brink and Khomskii [J. Phys.: Condens. Matter, 2008, 20, 434217].