Magnetism of iron oxide based core-shell nanoparticles from interface mixing with enhanced spin-orbit coupling

Abstract

We show that the magnetism of core-shell nanoparticles (made of maghemite, $\ensuremath{\gamma}$-Fe${}_{2}$O${}_{3}$, cores and transition-metal and metal-oxide shells) is altered substantially by the interface, which is a doped iron-oxide layer formed naturally during the seed-mediated synthesis process, a route used typically to produce core-shell nanoparticles. Characteristics fundamental to useful applications, such as the anisotropy and superparamagnetic blocking temperature, were altered substantially with Cu, CoO, MnO, and NiO shells. To ascertain the origin of this behavior, the prototype $\ensuremath{\gamma}$-Fe${}_{2}$O${}_{3}$/CoO core-shell nanoparticles are described in detail. We show that the magnetism originates essentially from an interfacial doped iron-oxide layer formed via migration of shell ions, e.g., Co${}^{2+}$, into octahedral site vacancies in the surface layers of the $\ensuremath{\gamma}$-Fe${}_{2}$O${}_{3}$ core. For this system, an overall Fe ${m}_{\mathrm{orb}}/{m}_{\mathrm{spin}}=0.15\ifmmode\pm\else\textpm\fi{}0.03$ is measured (${m}_{\mathrm{orb}}\ensuremath{\sim}0$ for the Fe-oxides) and an enhanced Co ${m}_{\mathrm{orb}}/{m}_{\mathrm{spin}}=0.65\ifmmode\pm\else\textpm\fi{}0.03$ elucidates the origin of the unexpectedly high overall anisotropy of the nanoparticle. This interfacial layer is responsible for the overall (e.g., bulk) magnetism and provides a perspective on how the magnetism of core-shell nanoparticles manifests from the selected core and shell materials.