Here, we synthesize a Au@Fe₃O₄ core@shell system with a highly uniform unprecedented star-like shell morphology with combined plasmonic and magnetic properties. An advanced electron microscopy characterization allows assessing the multifaceted nature of the Au core and its role in the growth of the peculiar epitaxial star-like shell with excellent crystallinity and homogeneity. Magnetometry and magneto-optical spectroscopy revealed a pure magnetite shell, with a superior saturation magnetization compared to similar Au@Fe₃O₄ heterostructures reported in the literature, which is ascribed to the star-like morphology, as well as to the large thickness of the shell. Of note, Au@Fe₃O₄ nanostar-loaded cancer cells displayed magneto-mechanical stress under a low frequency external alternating magnetic field (few tens of Hz). On the other hand, such a uniform, homogeneous, and thick magnetite shell enables the shift of the plasmonic resonance of the Au core to 640 nm, which is the largest red shift achievable in Au@Fe₃O₄ homogeneous core@shell systems, prompting application in photothermal therapy and optical imaging in the first biologically transparent window. Preliminary experiments performing irradiation of a stable water suspension of the nanostar and Au@Fe₃O₄-loaded cancer cell culture suspension at 658 nm confirmed their optical response and their suitability for photothermal therapy. The outstanding features of the prepared system can be thus potentially exploited as a multifunctional platform for magnetic-plasmonic applications.

In this work, we demonstrate that the reduction of the local internal stress by a low-temperature solvent-mediated thermal treatment is an effective post-treatment tool for magnetic hardening of chemically synthesized nanoparticles. As a case study, we used nonstoichiometric cobalt ferrite particles of an average size of 32(8) nm synthesized by thermal decomposition, which were further subjected to solvent-mediated annealing at variable temperatures between 150 and 320 °C in an inert atmosphere. The postsynthesis treatment produces a 50% increase of the coercive field, without affecting neither the remanence ratio nor the spontaneous magnetization. As a consequence, the energy product and the magnetic energy storage capability, key features for applications as permanent magnets and magnetic hyperthermia, can be increased by ca. 70%. A deep structural, morphological, chemical, and magnetic characterization reveals that the mechanism governing the coercive field improvement is the reduction of the concomitant internal stresses induced by the low-temperature annealing postsynthesis treatment. Furthermore, we show that the medium where the mild annealing process occurs is essential to control the final properties of the nanoparticles because the classical annealing procedure (T > 350 °C) performed on a dried powder does not allow the release of the lattice stress, leading to the reduction of the initial coercive field. The strategy here proposed, therefore, constitutes a method to improve the magnetic properties of nanoparticles, which can be particularly appealing for those materials, as is the case of cobalt ferrite, currently investigated as building blocks for the development of rare-earth free permanent magnets.

The exploitation of the exchange coupling between hard and soft magnetic materials has been proposed for enhancing the magnetic performances of rare-earth free permanent magnets, with the aim of extending their use to all applications where moderate energy product (35–100 kJ m ⁻³ ) is required. Strontium hexaferrite (SFO)/spinel ferrite composites seem particularly promising to achieve this target, although the conditions to maximize the effect while using techniques easily scalable to industrial production have not yet been identified. Within this framework, the optimization of the structural, chemical, and magnetic properties of the two moieties before the coupling procedure is crucial to enhance the energy product of the final composite. Here we report the syntheses of both nanometric SFO with high coercivity (ca. 525 kA m ⁻¹ ) and quasi-bulk saturation magnetization (68 Am ² kg ⁻¹ ) and a series of nanosized zinc-doped ferrite (Zn _x Fe _{3− x} O ₄ , 0.0 ⩽ x ⩽ 0.4) through cheap, easily scalable and eco-friendly approaches. The structural and chemical stability of the two magnetic phases as a function of temperature were investigated up to 1100 °C, with the aim of finding the best compromise between preservation of the nanometric scale and magnetic properties. A very high-magnetization (106 Am ² kg ⁻¹ ) ferrite was obtained by annealing Zn _0.3 Fe _2.7 O ₄ nanopowder at the highest investigated temperature. A preliminary attempt at coupling the two phases, starting from a mixture of the nanopowders, was performed through a classic annealing process in the temperature range 500 °C–1100 °C. The adopted procedure allowed for obtaining an exchange coupled composite at 1100 °C where the two phases are intimately and homogeneously mixed, with micrometric (0.3–5 μ m) and nanometric (up to 50 nm) spinel ferrite particles. Despite these promising results, no enhancement of the energy product was found, highlighting the need for further experimental efforts to improve the coupling procedure.

Nanometric core@shell wüstite@ferrite (Fe_1−xO@Fe₃O₄) has been extensively studied because of the emergence of exchange bias phenomena. Since their actual implementation in modern technologies is hampered by the low temperature at which bias is operating, the critical issue to be solved is to obtain exchange‐coupled antiferromagnetic@ferrimagnetic nanoparticles (NPs) with ordering temperature close to 300 K by replacing the divalent iron with other transition‐metal ions. Here, the effect of the combined substitution of Fe^(II) with Co^(II) and Ni^(II) on the crystal structure and magnetic properties is studied. To this aim, a series of 20 nm NPs with a wüstite‐based core and a ferrite shell, with tailored composition, (Co_0.3Fe_0.7O@Co_0.8Fe_2.2O₄ and Ni_0.17Co_0.21Fe_0.62O@Ni_0.4Co_0.3Fe_2.3O₄) is synthetized through a thermal‐decomposition method. An extensive morphological and crystallographic characterization of the obtained NPs shows how a higher stability against the oxidation process in ambient condition is attained when divalent cation doping of the iron oxide lattice with Co^(II) and Ni^(II) ions is performed. The dual‐doping is revealed to be an efficient way for tuning the magnetic properties of the final system, obtaining Ni‐Co doped iron oxide core@shell NPs with high coercivity (and therefore, high energy product), and increased antiferromagnetic ordering transition temperature, close to room temperature.