Evolution of NiO phase at the expense of metallic nickel: Structure, magnetic and electronic properties

Abstract

The role of oxidation on the structural, magnetic and electronic properties of a Ni-NiO compound is reported. XRD results indicate that NiO is formed at the expense of metallic Ni and complete NiO phase formation was observed at 1000 °C. The average crystallite size of NiO is found to increase from 12 to 35 nm upon calcination. Using transmission electron microscopy (TEM) the average particle size for the samples calcined at 1000 °C was found to be approximately 126 nm. Hence, comparing it to the crystallite sizes estimated from the XRD, it is clear that the powder samples are polycrystalline in nature, consisting of smaller crystallites clustered together. Both the ferromagnetic and AFM contributions from Ni and NiO, respectively, were observed in the magnetization data. The saturation magnetization and coercivity were found to decrease with increasing calcination temperature with a shifting of hysteresis loop towards the negative axis. The optical band-gap of the powders was found to be unchanged upon calcination (3.3 eV or 3.4 eV). The density of electrons near the Fermi level was found to be higher in the non-calcined and dried sample, resulting in a higher magnetic moment compared to the other samples.