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

Abstract

The structure and magnetic properties of fine Ni nanoparticles (∼65nm 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 573K. We have indexed both these patterns as Ni in a tetragonal crystal structure with lattice parameters a=0.4905nm, c=0.5330nm and a=0.4890nm, c=0.5310nm for the “as prepared” and 573K annealed sample, respectively. This is a new report about the formation of 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 ≈123Oe as compared to 6Oe in bulk 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 Ni and an antiferromagnetic∕paramagnetic shell of NiO.