Mössbauer and magnetic study of Co _x Fe_3--x O₄ nanoparticles

Abstract

Magnetic nanoparticles of cobalt ferrites Co _x Fe_3−x O₄ (x = 1 or 2) have been obtained either by mechanical milling or thermal treatment of pre-prepared layered double hydroxide carbonate x-LDH–CO₃. Mechanical milling of the 1-LDH–CO₃ leads to the large-scale preparation of nearly spherical nanoparticles of CoFe₂O₄, the size of which (5 to 20 nm) is controlled by the treatment time. Core-shell structure with surface spin-canting has been considered for the nanoparticles formed to explain the observed hysteresis loop shift (from ZFC–FC) in the magnetic properties. Annealing treatment of the 2-LDH–CO₃ below 673 K results in the formation of nearly spherical pure Co₂FeO₄ nanoparticles. At 673 K and above, the LDH decomposition leads to the formation of a mixture of both spinels phases Co₂FeO₄ and CoFe₂O₄, the amount of the latter increases with annealing temperature. Unusually high magnetic hardness characterized by a 22 kOe coercive field at 1.8 K has been observed, which reflects the high intrinsic anisotropy for Co₂FeO₄.