Sol–gel synthesis and anomalous magnetic behaviour of NiO nanoparticles

Abstract

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 T 1; (ii) presence of two maxima (broad and sharp at T 2 and T 3, respectively) in the M ZFC( T) and χ 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 T 3 towards lower and higher temperatures, respectively, in M ZFC( T) curves with increase of applied magnetic field (typical value of T 2 and T 3 at 20 kOe being 90 and 30 K, respectively, for average crystallite size ∼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 M FC( 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.