Spin Distributions in Fe₃O₄@Co_xZn_1-xFe₂O₄ Core-Shell Nanoparticles

Magnetic nanoparticles (MNPs) are suitable for biomedical hyperthermia treatments; for this application important material parameters include saturation magnetization and anisotropy. Low anisotropy leads to undesirable spin canting and recent efforts to avoid this include synthesizing core / shell MNPs with magnetically hard and soft materials. In this work, CS MNPS consisting of an Fe₃O₄ core and a Zn-doped CoFe₂O₄ shell are studied with polarization analyzed small angle neutron scattering (SANS) in order to explore magnetic properties and spin coupling as a function of doping concentration. The higher anisotropy of Co-ferrite allows for a greater range of anisotropy variations and spin canting effects when doped with Zn than would be possible with Fe₃O₄ alone. Magnetometry of all MNPs reveal the existence of a Verwey transition (T_v) while SANS shows changes in magnetic ordering above and below this temperature. Below T_v the MNPs see both perpendicular and parallel magnetic ordering at low fields that diminishes with increasing temperature, disappears in the vicinity of T_v, and weakly reorders at higher temperatures while no change in spin ordering is seen at higher fields above or below T_v. Perpendicular magnetic moments confirm the presence of spin canting within the MNPs.