Enhanced magnetic properties of NiO powders by the mechanical activation of aluminothermic reduction of NiO prepared by a ball milling process

We report the effect of mechanical activation on NiO–Al (x wt%) reduction reaction and resulting structural and magnetic properties by carrying out high-energy planetary ball milling. The pure NiO (un-milled) and milled NiO–Al (x≤2.5) powders exhibit face centered cubic structure, but the antiferromagnetic nature of pure NiO powder shows significant room temperature ferromagnetism with moderate moment and coercivity after milling due to non-stoichiometry in NiO caused by the defects, size reduction and oxidation of Ni. On the other hand, the addition of Al between 2.5 and 10% in NiO forms solid solution of NiO–Al with considerable reduction in the moment due to the atomic disorder. With increasing Al above 10%, NiO reduction reaction progresses gradually and as a result, the average magnetization increases from 0.57 to 4.3 emu/g with increasing Al up to 25%. A maximum of 91% reduction was observed for NiO–Al (40%) powders in 30 h of milling with a large increase in magnetization (~24 emu/g) along with the development of α-Al₂O₃. Thermomagnetization data reveal the presence of mixed magnetic phases in milled NiO powders and the component of induced ferromagnetic phase fades out with increasing Al due to the formation of Ni from the NiO–Al reduction reaction. The changes in the structural and magnetic properties are discussed on the basis of mechanical activation on the reduction of NiO by Al. The controlled reduction reaction with different Al content in NiO–Al is encouraging for the applications in catalysis and process of ore reduction. - Highlights: • Preparation of Ni/Al₂O₃ nanocomposite by mechanical activation of NiO–Al reduction. • Study the effect of Al addition on the reduction reaction of NiO–Al powders. • Understand the change in the structural and microstructural properties of NiO–Al. • Investigate role of reduction of NiO by Al on resulting magnetic property of NiO–Al. • Study the magnetic phase transition properties of NiO–Al powder at high temperature.