Exploring the direct synthesis of exchange-spring nanocomposites by reduction of CoFe2O4 spinel nanoparticles using in situ neutron diffraction

Abstract

In situ neutron powder diffraction (NPD) was employed for investigating gram-scale reduction of hard magnetic CoFe₂O₄ (spinel) nanoparticles into CoFe₂O₄/CoFe₂ exchange-spring nanocomposites via H₂ partial reduction. Time-resolved structural information was extracted from Rietveld refinements of the NPD data, revealing significant changes in the reduction kinetics based on the applied temperature and H₂ available. The nanocomposite formation was found to take place via the following two-step reduction process: Co_xFe_3−xO₄ → Co_yFe_1−yO → Co_zFe_2−z. The refined lattice parameters and site occupation fractions indicate that the reduced phases, i.e. Co_yFe_1−yO and Co_zFe_2−z, initially form as Co-rich compounds (i.e. y > 0.33 and z > 1), which gradually incorporate more Fe as the reduction proceeds. The reduction depletes the Co-content in the parent spinel, which may end up becoming magnetically soft Fe₃O₄ at high temperature (T = 542 °C), while at lower temperatures there may be a co-existence of Fe₃O₄ and γ-Fe₂O₃ or Co_xFe_3−xO₄. The macroscopic magnetic properties of the products were measured by vibrating sample magnetometry (VSM) and revealed the hard and soft magnetic domains in the nanocomposites to be effectively exchange-coupled. An increase of approximately 70% in specific saturation magnetisation, remanence magnetisation, and coercivity compared to the parent CoFe₂O₄ material was achieved for the best sample.