Atypical ferrimagnetism in Ni₄Nb₂O₉

Ferrimagnetism typically emerges from chemically distinct magnetic ions or the same element at two inequivalent crystallographic sites, rendering unequal moments. In contrast, Ni₄Nb₂O₉ has been recently discovered to show a different mechanism, where identical Ni²⁺ ions with the same ligand coordination develop unequal magnetic moments purely due to differences in local environments. Here, we investigate the microscopic origin of this emergent mechanism through a synergy of powder neutron diffraction, inelastic neutron scattering, and first-principle-based calculations. We demonstrate that the Ni_A and Ni_B sublattices, while sharing the same nominal valence, differ in their magnetic dimensionality: Ni_A forms quasi-one-dimensional chains with enhanced p-d hybridization and a reduced magnetic moment, whereas Ni_B retains a nearly two-dimensional geometry and a full S = 1 moment. Our results underscore the pivotal role of spin dimensionality and local structural distortions in stabilizing ferrimagnetism in systems with electronically equivalent magnetic ions.