Structure and magnetic properties of oxygen-stabilized tetragonal Ni nanoparticles prepared by borohydride reduction method

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We report a comprehensive study on the structure and magnetic properties of ultrafine Ni nanoparticles prepared by the borohydride reduction method. A spontaneous surface oxide layer of NiO encapsulates the Ni particles, as these have been prepared under ambient atmosphere. From the x-ray diffraction (XRD) pattern, the ``as-prepared'' sample has been identified as Ni in a tetragonal crystal structure, stabilized by the incorporation of oxygen atoms in the Ni lattice. On annealing this sample in air at different temperatures, the XRD patterns showed an interesting feature: unexpected fcc Ni peaks appeared together with the usual NiO peaks. Anomalous behavior is also observed in the $M\text{\ensuremath{-}}H$ curves, with the as-prepared sample showing a linear response with field and low values of magnetization and the annealed samples showing ferromagnetism with large coercivity $(290\phantom{\rule{0.3em}{0ex}}\mathrm{Oe})$ and high magnetization values. These surprising and seemingly contradictory observations have been coherently explained on the basis of a proposed phenomenological model. Furthermore, we attribute the observed low magnetization values of the as-prepared sample to an antiferromagnetic superexchange interaction between some of the Ni atoms, mediated by the dissolved oxygen atoms in the Ni lattice.