Magnetic Coupling in Cobalt-Doped Iron Oxide Core-Shell Nanoparticles: Exchange Pinning through Epitaxial Alignment

Tuning the core-shell morphology of bimagnetic nanoparticles and its associated exchange bias behavior is a promising way to overcome the superparamagnetic limit and stabilize the particle moment in extended time and temperature ranges. The intraparticle magnetization distribution and magnetic coupling between the two phases, however, is still unclear. We report a significant nonzero magnetization in the Co_xFe_(1-x)O core of native core-shell bimagnetic nanoparticles that is typically considered antiferro- or paramagnetic. Co_0.14Fe_0.86O@Co_0.4Fe_2.4O₄ (6 nm@2 nm) and Co_0.08Fe_0.92O@Co_0.58Fe_2.28O₄ (12 nm@2 nm) core-shell nanoparticles have been synthesized by thermal decomposition of a mixed cobalt-iron oleate with a similar Fe/Co distribution throughout the nanoparticle. We determine the exact phase composition and the magnetization distribution in the core and shell using a combination of X-ray and neutron small-angle scattering. Core and shell magnetization are traced separately with a varying magnetic field. Our results reveal that the magnetization of the core and the spinel-type shell phases are coupled at room temperature, i.e., rotating coherently with the magnetic field. This is a mandatory condition to observe a significant exchange bias effect at low temperatures. These findings highlight the enormous potential of finite size and exchange coupling in bimagnetic nanoparticles to control the magnetic properties via interface-induced magnetization.