Coexisting long-range zigzag antiferromagnetic order and glassy dynamics in spin-1 honeycomb lattice...

Here, we report an interesting magnetic oxide Na$_3$Ni$_2$SbO$_6$, where the interplay between exchange interactions and lattice symmetry crucially influence its exotic ground state. This compound features a layered honeycomb lattice with magnetic Ni$^{2+}$ (spin-1) surrounding nonmagnetic Sb$^{5+}$ centers. The x-ray absorption and magnetic circular dichroism measurements confirm a Ni$^{2+}$ charge state with notable spin and orbital moments at room temperature. The material exhibits antiferromagnetic (AFM) ordering around 17 K ($T_N$) and a field-induced magnetic transition at lower temperatures. Our measurements reveal a low-temperature anomaly, likely linked to glassy behavior. The power-law magnetic specific heat suggests mixed magnon dispersion, while low magnetic entropy indicates a highly degenerate ground state. Neutron diffraction data point to a zigzag-type AFM ground state, with a reduced site moment of 1.34(3) $\mu_B/Ni^{2+}$ at 2.5 K that disappears near 17.2 K, consistent with bulk data. Lattice parameters show anomalies at $T_N$ and lower temperatures. Ab-initio calculations support zigzag spin order and imply a competing in-plane Ni$^{2+}$ interactions that causes spin frustration. Overall, our combined experimental and theoretical study uncovers the microscopic magnetic ground state of Na$_3$Ni$_2$SbO$_6$ as a mix of long-range zigzag AFM and a glassy state, advancing the understanding of frustrated honeycomb magnets.