Abstract
Na2IrO3, a honeycomb 5d5 oxide, has been recently identified as a potential realization of the Kitaev spin lattice. The basic feature of this spin model is that for each of the three metal–metal links emerging out of a metal site, the Kitaev interaction connects only spin components perpendicular to the plaquette defined by the magnetic ions and two bridging ligands. The fact that reciprocally orthogonal spin components are coupled along the three different links leads to strong frustration effects and nontrivial physics. While the experiments indicate zigzag antiferromagnetic order in Na2IrO3, the signs and relative strengths of the Kitaev and Heisenberg interactions are still under debate. Herein we report results of ab initio many-body electronic-structure calculations and establish that the nearest-neighbor exchange is strongly anisotropic with a dominant ferromagnetic Kitaev part, whereas the Heisenberg contribution is significantly weaker and antiferromagnetic. The calculations further reveal a strong sensitivity to tiny structural details such as the bond angles. In addition to the large spin–orbit interactions, this strong dependence on distortions of the Ir2O2 plaquettes singles out the honeycomb 5d5 oxides as a new playground for the realization of unconventional magnetic ground states and excitations in extended systems.
Highlights
The Heisenberg model of magnetic interactions, J Si · S j between spin moments at sites {i, j}, has been successfully used as an effective minimal model to describe the cooperative magnetic properties of both molecular and solid-state many-electron systems
The anisotropic, Kitaev type coupling stems from the particular form the superexchange between the Ir j = 1/2 pseudospins takes for 90◦ bond angles on the Ir–O2–Ir plaquette [2, 13, 14]
Multiconfiguration complete-active-space self-consistent-field (CASSCF) and multireference configuration-interaction (MRCI) calculations [26] were performed on embedded clusters made of two reference IrO6 octahedra
Summary
The Heisenberg model of magnetic interactions, J Si · S j between spin moments at sites {i, j}, has been successfully used as an effective minimal model to describe the cooperative magnetic properties of both molecular and solid-state many-electron systems. In model systems the KH Hamiltonian arises due to the destructive interference of different superexchange pathways that contribute to the effective intersite interaction [2, 13] This interference turns out to be rather fragile as even when we consider idealized structures with cubic IrO6 octahedra, orthogonal Ir–O–Ir bonds and D2h point-group symmetry of the Ir–Ir link, the computed low-energy magnetic spectrum does not support a pure KH model. For NN interaction parameters as derived in the QC study, we have further performed exact diagonalization (ED) calculations including finite AF second and third order Ir–Ir Heisenberg couplings These indicate the presence of zigzag AF order, in agreement with the experimentally observed spin texture [8, 9, 24]
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