Ab initioinvestigation of the exchange interactions in Bi2Fe4O9: The Cairo pentagonal lattice compo

Abstract

We present the ab initio calculation of the electronic structure and magnetic properties of Bi 2 Fe 4 O 9 . This compound crystallizes in the orthorhombic crystal structure with the Fe 3+ 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 Bi 2 Fe 4 O 9 within the LSDA + U method. We obtain three different in-plane (J 3 = 36 K, J 4 = 73 K, J 5 = 23 K) and two interplane (J 1 = 10 K, J 2 = 12 K) exchange parameters. The derived set of exchange integrals shows that the realistic description of Bi 2 Fe 4 O 9 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 /J 4 = 0.49 < √ 2 [a critical parameter for the ideal Cairo pentagonal lattice, see Rousochatzakis, Läuchli, and Moessner, Phys. Rev. B 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.

Spin-reorientation transitions in the Cairo pentagonal magnet Bi4Fe5O13F

“…7). Furthermore, the calculated interactions (reported here and in [25]) place the two compounds nearly on the same spot in the phase diagram, and deep inside the orthogonal phase. Despite this remarkable similarity, the two Cairo materials show qualitatively different behavior.…”

“…The phase diagram contains three main phases, the coplanar orthogonal phase, the collinear ferrimagnetic phase, and a mixed phase in between, which is non-coplanar. In the latter phase, the projections of the spins along an axis yields the ferrimagnetic configuration while the pro- [25]) and the filled red dot (based on the parameters given above).…”

Spin‐singlet Quantum Ground State in Zigzag Spin Ladder Cu(CF3COO)2

“…One may see that Cu(CF 3 COO) 2 is an insulator with the band gap 3 eV, while spin moment on Cu was found to be 0.8 μ B . Slightly reduced magnetic moment on Cu is due to hybridization effects with oxygen, which are typical for different transition metal oxides . The top of the valence band is formed by the O 2p states.…”

Effect of Al3+ substitution on the structural, magnetic, and electric properties in multiferroic Bi2Fe4O9 ceramics

“…The dielectric anomalies observed at the magnetic transition temperature indicate a strong coupling between the magnetic and dielectric properties [11,12]. As the tetrahedral Fe 3 þ spins couple antiferromagnetically among themselves and with the octahedral Fe 3 þ spins and there is a ferromagnetic (FM) coupling within a pair of octahedral Fe 3 þ spins, the Fe 3 þ ions form Cairo pentagonal lattices which display noncollinear magnetic structure and lead to geometric frustration [13][14][15]. To achieve room temperature FM character, the study on the spin state of Fe 3 þ ions in Bi 2 Fe 4 O 9 as well as doping effect is desirable.…”