Unravelling the role of cationic Ni2+ vacancies and Ni3+ ions in non-stoichiometric NiO: breakdown of anti-ferromagnetic ordering and large exchange bias

Nanoscale anti-ferromagnetic (AFM) NiO exhibits intriguing magnetic anomalies influenced by factors such as size, stoichiometry, surface chemistry, and lattice strain. In the present work, black-coloured NiO nanoparticles were synthesized by combustion method, incorporating cationic Ni2+ vacancies and Ni3+ species on their surface. Annealing NiO at high temperatures (400–800 °C) led to a colour change from black to green, indicating the transition from a non-stoichiometric to a stoichiometric state. The formation of Ni2+ vacancies disrupted AFM ordering between Ni2+ and O2− ions, while the presence of Ni3+ promoted double exchange interactions between Ni3+ and neighbouring Ni2+ ions, contributing to ferromagnetism (FM) in AFM NiO. The spin exchange interaction at AFM-FM core–shell interface in NiO resulted in an exchange bias effect. Notably, the NiO nanoparticles with smaller average crystallite size (~ 5 nm) and higher cationic Ni2+ vacancies on the surface exhibited strong ferromagnetism and higher coercivity values. In particular, the as-prepared NiO (N) and N-4 samples showed remarkable improvement in exchange bias field, measuring 2.5 kOe and 2.1 kOe, respectively, due to the reinforced spin-exchange interaction within the FM-AFM core–shell structure of NiO.