Self-grown core/shell nanoparticles of cobalt: Correlation of structure, transport and magnetism

Abstract

The structure, electrical and magnetic transport properties of cobalt nanoparticles having core–shell structure are presented. The nanoparticles were prepared by a borohydride reduction method followed by heat treatments. X-ray diffraction shows that the as-prepared samples are amorphous while annealed samples are crystalline having a majority of fcc-Co along with metastable Co3B. The particles are spherical in shape and the average grain size increases with increasing anneal temperature. The core–shell structure is confirmed by high resolution transmission electron microscopy. The structural study reveals that the cores of the as-prepared and the annealed samples are of fcc-Co, while there is a profound microstructural change of the shells with annealing. A large change in the resistivity is observed between the as-prepared and annealed samples. The electrical transport properties at low temperature are interpreted in terms of tunneling between ferromagnetic cobalt cores through the non-magnetic shell. Improvements of the magnetic and the transport properties of the nanoparticles with annealing are observed with microstructural changes of the core–shell structure. The saturation magnetization (Ms=40emu/g) at room temperature suggests that air annealed (500°C) samples are protected from oxidation due to the formation of a B2O3 protective layer. These results suggest that this kind of nanocomposite systems might have significant potentiality in recording media and in medical diagnostic or therapy applications.