Ab initio investigation of the exchange interactions in Bi 2Fe4O9: The Cairo pentagonal lattice compound

Abstract

We present the ab initio calculation of the electronic structure and magnetic properties of Bi2Fe4O9. This compound crystallizes in the orthorhombic crystal structure with the Fe3+ ions forming the Cairo pentagonal lattice implying strong geometric frustration. The neutron diffraction measurements reveal nearly orthogonal magnetic configuration, which at first sight is rather unexpected since it does not minimize the total energy of the pair of magnetic ions coupled by the Heisenberg exchange interaction. Here, we calculate the electronic structure and exchange integrals of Bi2Fe4O9 within the LSDA+U method. We obtain three different in-plane (J3=36 K, J4=73 K, J5=23 K) and two interplane (J1=10 K, J2=12 K) exchange parameters. The derived set of exchange integrals shows that the realistic description of Bi2Fe4O9 needs a more complicated model than the ideal Cairo pentagonal lattice with only two exchange parameters in the plane. However, if one takes into account only the two largest exchange integrals, then according to the ratio x≡J 3/J4=0.49<√2 [a critical parameter for the ideal Cairo pentagonal lattice, see Rousochatzakis, Läuchli, and Moessner, Phys. Rev. BPRBMDO1098-012110.1103/PhysRevB.85.104415 85, 104415 (2012)] the ground state should be the orthogonal magnetic configuration in agreement with experiment. The microscopic origin of different exchange interactions is also discussed. © 2013 American Physical Society.