Exotic honeycomb magnets with strong spin-orbit coupling

Honeycomb is a bipartite lattice and free from geometrical frustration. Nevertheless, exotic magnetic ground states have been predicted in honeycomb-based transition-metal oxides with strong spin-orbit coupling. We will present two honeycomb-based materials realizing such unconventional magnetism.

The first material is a new honeycomb iridate. Honeycomb iridates recently emerged as a possible materialization of Kitaev spin liquid. In iridates, the wave function of 5d electron is described by J_eff = 1/2 state produced by strong spin-orbit coupling. The magnetic coupling between J_eff = 1/2 isospins in a 90-degree Ir-O-Ir bond was proposed to take the form of bond-dependent ferromagnetic coupling, invoking a possible route for Kitaev spin liquid. The honeycomb iridates α-Na₂IrO₃ and α-Li₂IrO_3, in reality, undergo a magnetic ordering likely due to the competing magnetic interactions and lattice distortion. In order to realize a spin liquid ground state, development of new honeycomb iridates has been desired.

In search for new materials, we employed a topotactic ion-exchange method and obtained a honeycomb iridate H₃LiIr₂O₆. H₃LiIr₂O₆ does not show any magnetic order down to 40 mK despite large negative Curie-Weiss temperature of -105 K. The NMR measurements showed no broadening of spectra, indicating the absence of spin-glass freezing and hence spin liquid state.

Another exotic magnet was realized in Ru⁴⁺ oxide. Honeycomb ruthenate Li₂RuO₃ is known to display a spin singlet dimerization, and the ground state is non-magnetic insulator. By using ion-exchange reaction, we obtained Ag₃LiRu₂O₆ where Ru honeycomb is preserved. Ag₃LiRu₂O₆ displays no dimerization, and magnetism of Ru⁴⁺ ions seems recovered. Interestingly, we do not observe any signature of magnetic ordering down to 0.3 K. We argue that spin-orbit coupling of Ru yields J = 0 state in this honeycomb ruthenate and produces excitonic (Van Vleck) magnetism via excited J = 1 state.