Orbital and spin physics in LiNiO2 and NaNiO2

TL;DR

This paper develops a spin-orbital Hamiltonian for Ni-based triangular lattice compounds, revealing complex frustrated interactions that influence magnetic properties and may explain differences between LiNiO2 and NaNiO2.

Contribution

It derives a detailed spin-orbital model considering Coulomb interactions and superexchange, highlighting the role of frustration and symmetry breaking in these materials.

Findings

Orbital interactions are strongly frustrated and select different orbitals along lattice directions.

In the orbital ordered phase, spin-orbital interactions can break U(1) symmetry and restore lattice symmetry.

Ferromagnetic interactions dominate but do not fully explain the absence of magnetic order in LiNiO2.

Abstract

We derive a spin-orbital Hamiltonian for a triangular lattice of e_g orbital degenerate (Ni^{3+}) transition metal ions interacting via 90 degree superexchange involving (O^{2-}) anions, taking into account the on-site Coulomb interactions on both the anions and the transition metal ions. The derived interactions in the spin-orbital model are strongly frustrated, with the strongest orbital interactions selecting different orbitals for pairs of Ni ions along the three different lattice directions. In the orbital ordered phase, favoured in mean field theory, the spin-orbital interaction can play an important role by breaking the U(1) symmetry generated by the much stronger orbital interaction and restoring the threefold symmetry of the lattice. As a result the effective magnetic exchange is non-uniform and includes both ferromagnetic and antiferromagnetic spin interactions. Since ferromagnetic interactions still dominate, this offers yet insufficient explanation for the absence of magnetic order and the low-temperature behaviour of the magnetic susceptibility of stoichiometric LiNiO_2. The scenario proposed to explain the observed difference in the physical properties of LiNiO_2 and NaNiO_2 includes small covalency of Ni-O-Li-O-Ni bonds inducing weaker interplane superexchange in LiNiO_2, insufficient to stabilize orbital long-range order in the presence of stronger intraplane competition between superexchange and Jahn-Teller coupling.