Exchange bias and size effects in Ni nanoparticles embedded in a mesoporous carbon matrix

Abstract

The exchange bias effect has been studied in a series of carbon coated NiO (shell) / Ni (core) nanoparticles with diameters ranging from 3 to50 nm. Using an easy single-step reaction particles were embedded inside the pores of a mesoporous activated carbon matrix. A joint-analysis has been made in order to assess the effects of the coupling between the ferromagnetic Ni cores and the antiferromagnetic NiO shells. The structural characterization of the samples has been studied by x-ray diffraction, transmission electron microscopy, and x-ray absorption spectroscopy. In addition, magnetic properties have been extensively investigated by measuring hysteresis loops and magnetization measurements in the 2-300 K temperature range, in zero-field-cooling and field-cooling conditions. It has been found that Ni/NiO nanoparticles present a moderate exchange bias (~ 400 Oe) due to the exchange coupling at the interface between Ni and NiO layers. Finally, the temperature and field dependences of this effect have been discussed taking into account the anisotropy and magnetism of the NiO component.