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.

In this minireview we discuss and provide a perspective on the novel systems, the synthesis routes and the interface-mediated properties that determine the heating efficiency of hybrid magnetic nanoparticles.

Epitaxial magnetic heterostructures of (soft-)Fe 3 O 4 /(hard-)CoFe 2 O 4 (001) have been fabricated with a varying thicknesses of soft ferrite from 5 to 25 nm. We report a change in the regime of magnetic interaction between the layers from rigid-coupling to exchange-spring behavior, above a critical thickness of the soft magnetic Fe 3 O 4 layer. We show that the symmetry and epitaxial matching between the spinel structures of CoFe 2 O 4 and Fe 3 O 4 at the interface stabilize the Verwey transition close to the bulk value even for 5-nm-thick Fe 3 O 4. The large interface exchange-coupling constant estimated from low-temperature M(H) data confirmed the good quality of the ferrite-ferrite interface and the major role played by the interface in the magnetization dynamics. The results presented here constitute a model system for understanding the magnetic behavior of interfaces in core/shell nanoparticles and magnetic oxide-based spintronic devices.

The control of the size of bimagnetic nanoparticles represents an important step toward the study of fundamental properties and the design of new nanostructured magnetic materials. We report the synthesis and the structural and magnetic characterization of bimagnetic CoO/CoFe2O4 core/shell nanoparticles. The material was fabricated by a seed-mediated growth high-temperature decomposition method with sizes in the range of 5-11 nm. We show that the core/shell morphology favours the crystallinity of the shell phase, and the reduction of the particle size leads to a remarkable increase of the magnetic hardening. When the size is reduced, the coercive field at 5 K increases from 21.5 kOe to 30.8 kOe, while the blocking temperature decreases from 388 K to 167 K. The size effects on the magnetic behaviour are described through a phenomenological model for strongly ferri-/antiferromagnetic coupled phases.

In this work, we studied the dynamic and static magnetic properties of ZnO-core/CoFe 2 O 4 -shell and CoO-core/CoFe 2 O 4 -shell nanoparticles. Both systems are formed by a core of ∼4 nm of diameter encapsulated in a shell of ∼2 nm of thickness. The mean blocking temperature changes from 106(7) to 276(5) K when the core is diamagnetic or antiferromagnetic, respectively. Magnetic remanence studies revealed the presence of weak dipolar interparticle interactions, where H int is approximately −0.1 kOe for ZnO/ CoFe 2 O 4 and −0.9 kOe for CoO/CoFe 2 O 4 , playing a minor role in the magnetic behavior of the materials. Relaxation experiments provided evidence that the magnetization reversal process of CoFe 2 O 4 is strongly dependent on the magnetic order of the core. At 10 K, activation volumes of ∼46(6) and ∼69(5) nm 3 were found for CoO/CoFe 2 O 4 and ZnO/CoFe 2 O 4 nanoparticles, respectively, corresponding to one-third and one-fifth of the total shell volume. While the magnetic behavior of ZnO/CoFe 2 O 4 nanoparticles is strongly affected by the surface disorder, the exchange coupling at the CoO/CoFe 2 O 4 interface rules the magnetization reversal and the nanoparticles' thermal stability by inducing a larger energy barrier and promoting smaller switching volume.

In this study we demonstrate that the effective coupling of the magnetic phases in core/shell nanoparticles can be promoted by an appropriate thermal annealing. In this way, the magnetization thermal stability of the hard ferrimagnetic CoFe 2 O 4 oxide can be increased up to room temperature when coupled to a CoO antiferromagnetic core in an inverse core/shell structure. In addition, the results show that, being encapsulated in a ~2 nm thick CoFe 2 O 4 shell, the CoO core is successfully protected against oxidation which is crucial for the effectiveness of the magnetic coupling at the interface.

Magnetite nanostructures gradually oxidize under environmental conditions. Here we demonstrate that a Zn-ferrite epitaxial coating protects magnetite cores from oxidation and provides a core/shell system with enhanced magnetic properties.