Revealing the microscopic origin of the magnetization plateau in Na$_3$Ni$_2$BiO$_6$

Abstract

Recent experimental studies of the spin-1 honeycomb antiferromagnet Na$_3$Ni$_2$BiO$_6$ have revealed a pronounced one-third magnetization plateau under applied magnetic fields, highlighting the presence of strong magnetic frustration and anisotropy in this material. Such behavior has been attributed to substantial bond-dependent Kitaev interactions in combination with single-ion anisotropy, placing Na$_3$Ni$_2$BiO$_6$ among honeycomb compounds of interest for unconventional magnetic phases. Motivated by these observations, we present a first-principles-based analysis of the magnetic interactions in Na$_3$Ni$_2$BiO$_6$. By combining density-functional calculations with microscopic modeling, we extract the relevant exchange parameters and construct an effective spin model that quantitatively reproduces both the elastic neutron-scattering spectra and the magnetization curve. The model captures the experimentally observed zero-field zigzag magnetic order, and proposes a $\textit{double-zigzag}$ state at intermediate magnetic fields, realizing the 1/3-magnetization plateau in a simpler way than suggested in previous works. Crucially, we show that the one-third magnetization plateau does not require Kitaev interactions; instead, it arises from the interplay of strong out-of-plane single-ion anisotropy and competing ferromagnetic nearest-neighbor ($J_1$) and antiferromagnetic third-neighbor ($J_3$) Heisenberg couplings. These results establish a consistent microscopic description of Na$_3$Ni$_2$BiO$_6$ and clarify the origin of its field-induced plateau phase.