Origin and Shell-Driven Optimization of the Heating Power in Core/Shell Bimagnetic Nanoparticles

The magnetic properties of core/shell nanoparticles can be finely tuned through the exchange coupling at the interface, enabling large heating powers under alternating magnetic fields. However, the origin of their heating efficiency is still unclear due to the complex interplay of different heating mechanisms. Here, we show that monodisperse Fe3O4/CoxZn1-xFe2O4 core/shell nano particles can be designed to provide large heating powers for different field amplitudes and dispersion media conditions by modulating their shell composition and thickness. The fine control of the nanoparticles' effective anisotropy provided by the interface coupling between core and shell leads to values up to similar to 2400 W g(-1) for water colloids and similar to 1000 W g(-1) for immobilized particles at 80 mT and 309 kHz. A reduction in the shell thickness or Co/Zn ratio results in a transition from a viscous heating regime to a region governed by a collectivw behavior, characerized by chainlike formation due to interparticle interactions. These results shed light on the origin of the large heating powers core/shell ferrites and provide an empirical guide to design highly efficient magnetic nanoheaters.