Ni2Mo3O8 : Complex antiferromagnetic order on a honeycomb lattice

Theoretical studies have predicted the existence of topological magnons in honeycomb compounds with stripy or zigzag antiferromagnetic (AFM) order. Here we report the discovery of AFM order in the layered and noncentrosymmetric honeycomb nickelate $\mathrm{N}{\mathrm{i}}₂\mathrm{M}{\mathrm{o}}₃{\mathrm{O}}₈$ through a combination of magnetization, specific heat, x-ray and neutron diffraction, and electron paramagnetic resonance measurements. The AFM order is complex, with a mixture of stripy and zigzag character on an integer spin noncentrosymmetric honeycomb lattice ($P{6}₃mc$). Further, each of the two sublattices of the bipartite honeycomb lattice is comprised of a different crystal field environment, i.e., octahedral and tetrahedral $\mathrm{N}{\mathrm{i}}²⁺$, respectively, enabling independent substitution on each. Replacement of Ni by Mg on the octahedral site suppresses the long-range magnetic order and results in a weakly ferromagnetic state. Conversely, substitution of Fe for Ni enhances the strength of the AFM exchange and increases the ordering temperature. Thus, $\mathrm{N}{\mathrm{i}}₂\mathrm{M}{\mathrm{o}}₃{\mathrm{O}}₈$ provides a platform on which to explore the rich physics of $S=1$ on the honeycomb lattice in the presence of competing magnetic interactions with a noncentrosymmetric, formally piezopolar, crystal structure.