Effect of air annealing on structural and magnetic properties of Ni/NiO nanoparticles

• Air annealing effects on structural and magnetic properties of Ni/NiO nanoparticles have been studied.
• The XRD spectra revealed two phases such as Ni and NiO.
• A phase diagram was developed between two phases versus annealing temperature using XRD analysis.
• The magnetic properties are well governed by ferromagnetic Ni phase.

We reported systematic study on structural and magnetic properties of nickel/nickel oxide (Ni/NiO) nanoparticles annealed under air atmosphere at different temperatures in the range 400–800 °C. The XRD spectra revealed two phases such as Ni and NiO. The average crystallite size increases with increasing annealing temperature. A phase diagram was developed between two phases versus annealing temperature using XRD analysis. At lower annealing temperatures, Ni phase is dominant which does not easily undergo oxidation to form NiO. The NiO phase increases with increasing annealing temperature. FTIR spectroscopy revealed an increase in the NiO phase content at higher annealing temperature, which is in agreement with the XRD analysis. SEM images showed that nanoparticles are well separated at lower annealing temperatures but get agglomerated at higher annealing temperatures. The ferromagnetic (FM) Ni phase content and saturation magnetization (Ms) showed nearly the same trend with increasing annealing temperature. The nanoparticles annealed at 500 °C and 800 °C revealed highest and lowest Ms values, respectively, which is in accordance with the XRD phase diagram. Coercivity showed an overall decreasing trend with increasing annealing temperature due to decreased concentration of FM Ni phase and increasing average crystallite size. All these measurements indicate that the structural and magnetic properties of Ni/NiO nanoparticles are strongly influenced by the annealing temperature.