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 Fe 3 O 4 /Co x Zn 1−x Fe 2 O 4 core/shell nanoparticles 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 ∼2400 W g −1 for water colloids and ∼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 collective behavior, characterized by chainlike formation due to interparticle interactions. These results shed light on the origin of the large heating powers of core/shell ferrites and provide an empirical guide to design highly efficient magnetic nanoheaters.

In order to explore an alternative strategy to design exchange-biased magnetic nanostructures, bimagnetic core/shell nanoparticles have been fabricated by a thermal decomposition method and systematically studied as a function of the interface exchange coupling. The nanoparticles are constituted by a ∼3 nm antiferromagnetic (AFM) CoO core encapsulated in a ∼4 nm-thick CoZnFeO (x = 0-1) ferrimagnetic (FiM) shell. The system presents an enhancement of the coercivity (H) as compared to its FiM single-phase counterpart and exchange bias fields (H). While H decreases monotonically with the Zn concentration from ∼21.5 kOe for x = 0, to ∼7.1 kOe for x = 1, H exhibits a non-monotonous behavior being maximum, H ∼ 1.4 kOe, for intermediate concentrations. We found that the relationship between the AFM anisotropy energy and the exchange coupling energy can be tuned by replacing Co with Zn ions in the shell. As a consequence, the magnetization reversal mechanism of the system is changed from an AFM/FiM rigid-coupling regime to an exchange-biased regime, providing a new approach to tune the magnetic properties and to design novel hybrid nanostructures.