Novel synthesis and physical properties of CoFe[formula omitted]O[formula omitted]@CoFe[formula omitted] core@shell nanostructures

Abstract

In this work, CoFe2O4@CoFe2 (core@shell) nanoparticles were synthesized by an easy method without using special reagents or gases. Firstly, we have prepared glutaraldehyde crosslinked chitosan beads containing CoFe2O4 nanoparticles. Secondly, the beads were thermal treatment at high temperatures and in vacuum, at a pressure of 0.02 Torr and for 1 h. The CO gas released from chitosan during the thermal treatment helped to reduce the Fe3＋ and Co2+ to their zero valence state. Four samples were produced at temperatures of 400, 500, 600 and 700 °C, and their structure, particle size, morphology and magnetic properties were studied. The X-ray diffraction and Mössbauer spectroscopy showed the presence of CoFe2O4 and CoFe2 magnetic phases for samples prepared at 400 and 700 °C, respectively. For samples prepared at 500 and 600 °C we have obtained core@shell particles with crystallite sizes of 6.6–7.4 nm and 22.4–26.1 nm for CoFe2O4 and CoFe2 phases, respectively. The TEM images showed the presence of nanoparticles with a core@shell configuration, we found evidences that the shell CoFe2 is made of a single crystal. Magnetic measurements showed that the CoFe2O4 and CoFe2 phases, for sample treated at 600 °C, are exchange coupled leading to a (BH)max= 0.67 MGOe. We have tuned the thickness of the CoFe2 phase to be ∼9.0 nm, this value is smaller than the critical size of 10.2 nm obtained according to the Kneller-Hawig theory, therefore satisfying the physical condition for the magnetic coupling between CoFe2 and CoFe2O4 phases.