Amanda A. Konieczna | Searcharxiv

arXiv2026

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

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.

cond-mat.str-el↗

arXiv2024

Understanding the microscopic origin of the magnetic interactions in CoNb$_2$O$_6$

Motivated by the on-going discussion on the nature of magnetism in the quantum Ising chain CoNb$_2$O$_6$, we present a first-principles-based analysis of its exchange interactions by applying an \textit{ab initio} approach with additional modelling that accounts for various drawbacks of a purely density functional theory ansatz. With this method we are able to extract and understand the origin of the magnetic couplings under inclusion of all symmetry-allowed terms, and to resolve the conflicting model descriptions in CoNb$_2$O$_6$. We find that the twisted Kitaev chain and the transverse-field ferromagnetic Ising chain views are mutually compatible, although additional off-diagonal exchanges are necessary to provide a complete picture. We show that the dominant exchange interaction is a ligand-centered exchange process - involving the $e_g$ electrons -, which is rendered anisotropic by the low-symmetry crystal fields environments in CoNb$_2$O$_6$, giving rise to the dominant Ising exchange, while the smaller bond-dependent anisotropies are found to originate from $d-d$ kinetic exchange processes involving the $t_{2g}$ electrons. We demonstrate the validity of our approach by comparing the predictions of the obtained low-energy model to measured THz and inelastic neutron scattering spectra.

cond-mat.str-el↗