Effect of annealing on the magnetic properties of ball milled NiO powders

• Preparation of fine NiO powder using top-to-bottom approach using planetary ball mill.
• Effect of milling on instituting room temperature ferromagnetism with size reduction.
• Stability of ferromagnetic properties at high temperatures in milled NiO powders
• Effect of annealing process on the structural properties of milled NiO powders.
• Understanding the origin of ferromagnetism at 300 K in NiO powders through annealing.

We report systematic investigations on structural and magnetic properties of nanosized NiO powders prepared by the ball milling process followed by systematic annealing at different temperatures. Both as-milled and annealed NiO powders exhibit face centered cubic structure, but average crystallite size decreases (increases) with increasing milling time (annealing temperature). Pure NiO exhibits antiferromagnetic nature, which transforms into ferromagnetic one with moderate moment at room temperature with decreasing crystallite size. The on-set of ferromagnetic behavior in the as-milled powders was observed at higher temperatures (>750 K) as compared to bulk Ni (~630 K). On the other hand, annealing of as-milled powders showed a large reduction in magnetic moment and the rate of decrease of moment strongly depends on the milling conditions. The observed properties are discussed on the basis of crystallite size variation, defect density, oxidation/reduction of Ni and interaction between uncompensated surfaces and particle core with lattice expansion.