Sol-gel synthesis and anomalous magnetic behaviour of NiO nanoparticles

Nickel oxide nanoparticles (size similar to 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 degrees C for 24 h to produce nickel oxalate dihydrate and subsequent calcination at 300 degrees C or above in air to decortipose and yield NiO powder. Their anomalous magnetic behaviour includes: (i) irreversibility in magnetization (M) vs temperature (7 curves under zero field cooled (ZFC) and field cooled (FC) conditions below the bifurcation temperature T-1; (ii) presence of two maxima (broad and sharp at T-2 and T-3, respectively) in the M-ZFC(T) and Z(ZFC)(T) curves; (iii) decrease of both T-1 (325-115 K) and T-2 (265-95 K) with increasing crystallite size; (iv) shift of T-2 and Tj towards lower and higher temperatures, respectively, in M-ZFC(T) curves with increase of applied magnetic field (typical value of T, and Tj at 20 kOe being 90 and 30 K, respectively, for average crystallite size similar to 6.8 nm); (v) bifurcation of M-ZFC(T) and M-FC(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 Mrc(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 T-2 is associated with the blocking process), shell contains randomly oriented spins with low co-ordination and are possibly responsible for sharp increase in M-ZFC below 30 K as a result of collective freezing effect. Finally, the increase of blocking temperature (T-B) as well as energy band gap (E-g) 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. (c) 2007 Elsevier Ltd. All rights reserved.