Ferrimagnetism typically emerges from chemically distinct magnetic ions or the same element at two inequivalent crystallographic sites, rendering unequal moments. In contrast, ${\mathrm{Ni}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$ has been recently discovered to show a different mechanism, where identical ${\mathrm{Ni}}^{2+}$ 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 ${\mathrm{Ni}}_A$ and ${\mathrm{Ni}}_B$ sublattices, while sharing the same nominal valence, differ in their magnetic dimensionality: ${\mathrm{Ni}}_A$ forms quasi-one-dimensional chains with enhanced $p\text{−}d$ hybridization and a reduced magnetic moment, whereas ${\mathrm{Ni}}_B$ 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.

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• Ferrimagnetism
• Magnons
• Strongly correlated systems
• Transition metal oxides
• First-principles calculations
• Linear spin wave theory
• Magnetization measurements
• Neutron scattering
• X-ray absorption near-edge spectroscopy