Atypical Ferrimagnetism in Ni$_4$Nb$_2$O$_9$

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 NiA and NiB sublattices, while sharing the same nominal valence, differ in their magnetic dimensionality NiA forms quasi one dimensional chains with enhanced p d hybridization and a reduced magnetic moment, whereas NiB 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.