Size-dependent magnetic properties of NiO nanoparticles synthesized via Ni–hydroxyacetate decomposition

This study focuses on a systematic investigation of the structural and magnetic properties of NiO nanoparticles (NPs) synthesized via the thermal dehydroxylation of a polyol-derived nickel layered hydroxyacetate salt (Ni–LHS). The turbostratic brucite-like Ni–LHS precursor undergoes a phase transition into pure NiO at 260–600 °C, yielding single-phase NPs with crystallite sizes ranging from ∼4.5 to 18 nm. Structural analyses reveal a transition from a sheet-like to a rougly spherical morphology and a progressive lattice contraction with increasing annealing temperature. Magnetic measurements at 5 K reveal characteristic signatures of nanoscale antiferromagnetism, including unsaturated hysteresis loops, high coercivity values, and pronounced exchange bias (EB). Both coercivity and EB exhibit a non-monotonic size dependence, reflecting a crossover from surface-disordered to core-ordered magnetic behaviour. These findings provide new insights into the interplay between finite-size effects and interfacial anisotropy in antiferromagnetic NiO NPs.