Effect of interstitial oxygen on the crystal structure and magnetic properties of Ni nanoparticles

The structure and magnetic properties of fine $\mathrm{Ni}$ nanoparticles ($\sim 65\mathrm{nm}$ diameter) having a spontaneous surface oxide layer have been studied. The particles were prepared by the chemical reduction of nickel ions in an aqueous medium, with sodium borohydride as the reducing agent. X-ray diffraction (XRD), transmission electron microscopy, and magnetization measurements (M-H plots and field cooled∕zero field cooled curves) have been used for characterizing the samples. No detectable change is observed in the M-H curves or in the XRD patterns of the “as prepared” sample and the sample annealed in air at $573\mathrm{K}$ . We have indexed both these patterns as $\mathrm{Ni}$ in a tetragonal crystal structure with lattice parameters $a=0.4905\mathrm{nm}$ , $c=0.5330\mathrm{nm}$ and $a=0.4890\mathrm{nm}$ , $c=0.5310\mathrm{nm}$ for the “as prepared” and $573\mathrm{K}$ annealed sample, respectively. This is a new report about the formation of $\mathrm{Ni}$ in a modified crystal structure. The M-H curves of both the samples show a clear hysteretic behavior but do not saturate, thereby suggesting the existence of both ferromagnetic and paramagnetic components in the magnetization. Large coercivity values $\approx 123\mathrm{Oe}$ as compared to $6\mathrm{Oe}$ in bulk $\mathrm{Ni}$ have been obtained. The magnetization results have been analyzed in correlation with X-ray diffraction and microstructure and satisfactorily explained on the basis of a core-shell model, where we consider each particle as a magnetically heterogeneous system consisting of a ferromagnetic core of $\mathrm{Ni}$ and an antiferromagnetic∕paramagnetic shell of $\mathrm{NiO}$ .