Improving Co distribution in acicular Fe–Co nanoparticles and its effect on their magnetic properties

We report on a procedure for increasing the Co content in acicular Fe–Co nanoparticles (70 nm length and ~5 axial ratio) obtained by thermal reduction of Co-doped goethite nanoparticles coated with Co(OH)2 layers. It has been found that the diffusion of Co cations located onto the particle surface of the precursor to the inner part can be promoted by separating reduction in two steps, first from the precursor to Co-doped magnetite and, finally, from this phase to metal. Furthermore, the uniformity in the Co distribution in the final metallic alloy could be improved by annealing the Co-doped magnetite obtained from the first reduction step, resulting in an important increase of coercivity. The enhancement of the magnetic properties is mainly discussed in terms of the reversal magnetization mechanism into the metallic alloy. The hard magnetic properties resulting from the higher Co content and its homogeneous distribution in the final Fe–Co alloy, along with the reduction in the particle size, make our acicular Fe–Co nanoparticles suitable for high-density magnetic recording applications.