Evolution of Ni:B2O3 core-shell structure and magnetic properties on devitrification of amorphous NiB particles in air

Abstract

The Ni:B2O3 nanoparticles have been synthesized by a novel two-step chemical reduction method, under ambient conditions and subsequent annealing in air. The evolution of this structure has been followed through a sequence of annealing treatments. Structural and magnetic investigations on as-prepared and annealed samples revealed that as-prepared samples are weakly magnetic and develop into spherical ferromagnetic entities with superior magnetic properties. The saturation magnetization (Ms∼42 emu/g) and coercivity (Hc∼482 Oe) at room temperature clearly suggest that the air annealed (500 °C) samples are protected from the oxidation due to formation of B2O3 protective layer. The magnetization results are consistent with the observed microstructural changes 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 nonmagnetic shell of B2O3. These results suggest that this kind of nanocomposite systems might have significant potential as a recording media.