Tailoring magnetic properties of core∕shell nanoparticles

Bimagnetic $\mathrm{FePt}∕{\mathrm{MFe}}_2{\mathrm{O}}_4\left(\mathrm{M}=\mathrm{Fe},\mathrm{Co}\right)$ core∕shell nanoparticles are synthesized via high-temperature solution phase coating of $3.5\mathrm{nm}$ $\mathrm{FePt}$ core with ${\mathrm{MFe}}_2{\mathrm{O}}_4$ shell. The thickness of the shell is controlled from 0.5 to $3\mathrm{nm}$ . An assembly of the core∕shell nanoparticles shows a smooth magnetization transition under an external field, indicating effective exchange coupling between the $\mathrm{FePt}$ core and the oxide shell. The coercivity of the $\mathrm{FePt}∕{\mathrm{Fe}}_3{\mathrm{O}}_4$ particles depends on the volume ratio of the hard and soft phases, consistent with previous theoretical predictions. These bimagnetic core∕shell nanoparticles represent a class of nanostructured magnetic materials with their properties tunable by varying the chemical composition and thickness of the coating materials.