Exotic honeycomb magnets with strong spin-orbit coupling
Invited
Abstract
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 Jeff = 1/2 state produced by strong spin-orbit coupling. The magnetic coupling between Jeff = 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-Li2IrO3, 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.
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 Jeff = 1/2 state produced by strong spin-orbit coupling. The magnetic coupling between Jeff = 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-Li2IrO3, 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.
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Presenters
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Tomohiro Takayama
Max Planck Institute for Solid State Research
Authors
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Tomohiro Takayama
Max Planck Institute for Solid State Research