Defect-Engineering by Solvent Mediated Mild Oxidation as a Tool to Induce Exchange Bias in Metal Doped Ferrites.

The crystal site occupancy of different divalent ions and the induction of lattice defects represent an additional tool for modifying the intrinsic magnetic properties of spinel ferrites nanoparticles. Here, the relevance of the lattice defects is demonstrated in the appearance of exchange-bias and in the improvement of the magnetic properties of doped ferrites of 20 nm, obtained from the mild oxidation of core@shell (wüstite@ferrite) nanoparticles. Three types of nanoparticles (Fe0.95 O@Fe3 O4 , Co0.3 Fe0.7 O@Co0.8 Fe2.2 O4 and Ni0.17 Co0.21 Fe0.62 O@Ni0.4 Co0.3 Fe2.3 O4 ) are oxidized. As a result, the core@shell morphology is removed and transformed in a spinel-like nanoparticle, through a topotactic transformation. This study shows that most of the induced defects in these nanoparticles and their magnetic properties are driven by the inability of the Co(II) ions at the octahedral sites to migrate to tetrahedral sites, at the chosen mild oxidation temperature. In addition, the appearance of crystal defects and antiphase boundaries improves the magnetic properties of the starting compounds and leads to the appearance of exchange bias at room temperature. These results highlight the validity of the proposed method to impose novel magnetic characteristics in the technologically relevant class of nanomaterials such as spinel ferrites, expanding their potential exploitation in several application fields.

Star-Shaped Magnetic-Plasmonic Au@Fe3O4 Nano-Heterostructures for Photothermal Therapy.

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.

3d Metal Doping of Core@Shell Wüstite@ferrite Nanoparticles as a Promising Route toward Room Temperature Exchange Bias Magnets.

Nanometric core@shell wüstite@ferrite (Fe1-x O@Fe3 O4 ) 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, (Co0.3 Fe0.7 O@Co0.8 Fe2.2 O4  and Ni0.17 Co0.21 Fe0.62 O@Ni0.4 Co0.3 Fe2.3 O4 ) 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.

Magnetic Property Enhancement of Spinel Mn-Zn Ferrite through Atomic Structure Control.

Temperature treatment of magnetic Mn-Zn ferrites with the composition Mn0.6Zn0.2Fe2.2O4 up to 1100 °C results in a tremendous enhancement of the saturation magnetization by more than 60%. Employing a robust combined Rietveld refinement of powder X-ray and neutron diffraction (PXRD and NPD) data, it is revealed how a reordering of the cations takes place during the annealing step, the extent of which depends on the annealing temperature. While Zn(II) exclusively occupies tetrahedral sites throughout the whole temperature range, as the annealing temperature increases up to 700 °C, the Mn(II) cation distribution shifts from 80(7)% of the total Mn content occupying the octahedral sites (partly inverse spinel) to Mn only being present on the tetrahedral sites (normal spinel). Above 700 °C, pronounced crystallite growth is observed, followed by an increase of the saturation magnetization. Complementary techniques such as energy dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM) confirm an even cation distribution and the particle growth with annealing temperature. The structural changes caused by annealing of spinel ferrites directly alter the magnetic properties of the materials, thus serving as an easy handle for enhancing their magnetic properties.

Unraveling the mechanism of the one-pot synthesis of exchange coupled Co-based nano-heterostructures with a high energy product.

The development of reproducible protocols to synthesize hard/soft nano-heterostructures (NHSs) with tailored magnetic properties is a crucial step to define their potential application in a variety of technological areas. Thermal decomposition has proved to be an effective tool to prepare such systems, but it has been scarcely used so far for the synthesis of Co-based metal/ferrite NHSs, despite their intriguing physical properties. We found a new approach to prepare this kind of nanomaterial based on a simple one-pot thermal decomposition reaction of metal-oleate precursors in the high boiling solvent docosane. The obtained NHSs are characterized by the coexistence of Co metal and Co doped magnetite and are highly stable in an air atmosphere, thanks to the passivation of the metal with a very thin oxide layer. The investigation of the influence of the metal precursor composition (a mixed iron-cobalt oleate), of the ligands (oleic acid and sodium oleate) and of the reaction time on the chemical and structural characteristics of the final product, allowed us to rationalize the reaction pathway and to determine the role of each parameter. In particular, the use of sodium oleate is crucial to obtain a metal phase in the NHSs. In such a way, the one-pot approach proposed here allows the fine control of the synthesis, leading to the formation of stable, high performant, metal/ferrite NHSs with tailored magnetic properties. For instance, the room temperature maximum energy product was increased up to 19 kJ m-3 by tuning the Co content in the metal precursor.