LAUSR

Abstract This article reports on the oxygen vacancies and defects induced cooperative magnetism of 28 nm (as prepared) and 74 nm (annealed) sized nanocrystals of ZnO synthesized through sol- gel auto-combustion route. UV–Vis–NIR optical absorption and photoluminescence (PL) spectroscopic measurements showed marginal red shifting in the band gap energy and characteristic blue shift in the UV absorption peak position with the size reduction of ZnO nanocrystallites. PL analysis revealed that 28 nm sized ZnO nanocrystals have higher number of oxygen vacancies and lower interstitial defects in comparison to 74 nm (annealed) nanocrystals of ZnO. Both the sample display oxygen vacancy and defect mediated ferromagnetic behaviour at 300K but 28 nm ZnO nanocrystals are better ferromagnetic in comparison to annealed ZnO nanocrystals. The annealed ZnO nanocrystals also showed considerable diamagnetic character. High field (±7 T) magnetization measurements down to 5K and PL analysis establish presence, inter-conversion of oxygen vacancies and valence fluctuations in neutral, mono and divalent oxygen vacancies. The monovalent oxygen and zinc vacancies play important role in the mediation of d0 type cooperative magnetism in the ZnO nanocrystals.