Sol–gel synthesis and anomalous magnetic behaviour of NiO nanoparticles Original Research Article

Nickel oxide nanoparticles (size ∼4–22 nm) have been synthesized by sol–gel process using nickel acetate tetrahydrate and oxalic acid as precursors and ethanol as solvent. The process essentially involves formation of gel, drying at 110 °C for 24 h to produce nickel oxalate dihydrate and subsequent calcination at 300 °C or above in air to decompose and yield NiO powder. Their anomalous magnetic behaviour includes: (i) irreversibility in magnetization (M) vs temperature (T) curves under zero field cooled (ZFC) and field cooled (FC) conditions below the bifurcation temperature T1; (ii) presence of two maxima (broad and sharp at T2 and T3, respectively) in the MZFC(T) and χZFC(T) curves; (iii) decrease of both T1 (325–115 K) and T2 (265–95 K) with increasing crystallite size; (iv) shift of T2 and T3 towards lower and higher temperatures, respectively, in MZFC(T) curves with increase of applied magnetic field (typical value of T2 and T3 at 20 kOe being 90 and 30 K, respectively, for average crystallite size ∼6.8 nm); (v) bifurcation of MZFC(T) and MFC(T) curves even at high measuring fields; and (vi) stretched “S”- or eye-like shapes of M–H plots at 5 K with a shift of hysteresis loops MFC(H) by 590 Oe. Further, the present results provide evidence for the non-existence of spin-glass behaviour in NiO. Instead, the observations suggest core–shell description for NiO nanoparticles. While the core behaves like a ferrimagnet because of prevailing lack of spin compensation due to reduced size (peak at T2 is associated with the blocking process), shell contains randomly oriented spins with low co-ordination and are possibly responsible for sharp increase in MZFC below 30 K as a result of collective freezing effect. Finally, the increase of blocking temperature (TB) as well as energy band gap (Eg) observed with decreasing particle size seems to have common origin and caused by size effects, viz. confinement of charge carriers and increase in the uncompensated core spins, respectively.