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

Chemical engineering of cationic distribution in spinel ferrite nanoparticles: the effect on the magnetic properties

A set of ∼9 nm CoFe2O4 nanoparticles substituted with Zn2+ and Ni2+ was prepared by thermal decomposition of metallic acetylacetonate precursors to correlate the effects of replacement of Co2+ with the resulting magnetic properties. Due to the distinct selectivity of these cations for the spinel ferrite crystal sites, we show that it is possible to tailor the magnetic anisotropy, saturation magnetization, and interparticle interactions of the nanoparticles during the synthesis stage. This approach unlocks new possibilities for enhancing the performance of spinel ferrite nanoparticles in specific applications. Particularly, our study shows that the replacement of Co2+ by 48% of Zn2+ ions led to an increase in saturation magnetization of approximately 40% from ∼103 A m2 kg-1 to ∼143 A m2 kg-1, whereas the addition of Ni2+ at a similar percentage led to an ∼30% decrease in saturation magnetization to 68-72 A m2 kg-1. The results of calculations based on the two-sublattice Néel model of magnetization match the experimental findings, demonstrating the model's effectiveness in the strategic design of spinel ferrite nanoparticles with targeted magnetic properties through doping/inversion degree engineering.

Giant exchange bias field above room temperature in perovskite YbCr1-xFexO3 (x = 0.6-0.9)

Temperature-induced exchange bias is observed in perovskite YbCr1-xFexO3 (x = 0.6-0.9) compounds. This is ascribed to the ferromagnetic (FM) coupling between Fe3+/Cr3+ ions and Yb3+ ions. During demagnetization, the Yb3+ ions will show a tendency to rotate with the direction of the applied magnetic field. However, the antiferromagnetic coupling of Fe3+/Cr3+ ions has a pinning effect, hindering their rotation, and thus producing an exchange bias effect. With increasing Fe3+ ion content, the magnetization and exchange bias field gradually become larger. When x = 0.8 and 0.9, the exchange bias field reaches up to -9.7 kOe and -13.6 kOe at 300 K, respectively. This giant room temperature exchange bias field will be more conducive for practical applications in magnetoelectronic devices.

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.

A Giant Exchange Bias Effect Due to Enhanced Ferromagnetism Using a Mixed Martensitic Phase in Ni50Mn37Ga13 Spun Ribbons

The structure of a material is an important factor in determining its physical properties. Here, we adjust the structure of the Ni50Mn37Ga13 spun ribbons by changing the wheel speed to regulate the exchange bias effect of the material. The characterization results of micromorphology and structure show that as the wheel speed increases, the martensite lath decreases from 200 nm to 50 nm, the structure changed from the NM to a NM and 10M mixed martensitic structure containing mainly NM, then changed to NM and 10M where 10M and NM are approaching. Meanwhile, HE first increased and then decreased as the wheel speed increased. The optimum exchange bias effect (HE = 7.2 kOe) occurs when the wheel speed is 25 m∙s-1, mainly attributed to the enhanced ferromagnetism caused by part of 10M in NM martensite, which enhanced the exchange coupling of ferromagnetism and antiferromagnetism. This work reveals the structural dependence of exchange bias and provides a way to tune the magnitude of the exchange bias of Heusler alloys.

Cubic Mesocrystal Magnetic Iron Oxide Nanoparticle Formation by Oriented Aggregation of Cubes in Organic Media: A Rational Design to Enhance the Magnetic Hyperthermia Efficiency

Magnetic iron oxide mesocrystals have been reported to exhibit collective magnetic properties and consequently enhanced heating capabilities under alternating magnetic fields. However, there is no universal mechanism to fully explain the formation pathway that determines the particle diameter, crystal size, and shape of these mesocrystals and their evolution along with the reaction. In this work, we have analyzed the formation of cubic magnetic iron oxide mesocrystals by thermal decomposition in organic media. We have observed that a nonclassical pathway leads to mesocrystals via the attachment of crystallographically aligned primary cubic particles and grows through sintering with time to achieve a sizable single crystal. In this case, the solvent 1-octadecene and the surfactant agent biphenyl-4-carboxylic acid seem to be the key parameters to form cubic mesocrystals as intermediates of the reaction in the presence of oleic acid. Interestingly, the magnetic properties and hyperthermia efficiency of the aqueous suspensions strongly depend on the degree of aggregation of the cores forming the final particle. The highest saturation magnetization and specific absorption rate values were found for the less aggregated mesocrystals. Thus, these cubic magnetic iron oxide mesocrystals stand out as an excellent alternative for biomedical applications with their enhanced magnetic properties.

Emergent magnetism and exchange bias effect in iron oxide nanocubes with tunable phase and size

We report a systematic investigation of the magnetic properties including the exchange bias (EB) effect in an iron oxide nanocube system with tunable phase and average size (10, 15, 24, 34, and 43 nm). X-ray diffraction and Raman spectroscopy reveal the presence of Fe₃O₄, FeO, andα-Fe₂O₃phases in the nanocubes, in which the volume fraction of each phase varies depending upon particle size. While the Fe₃O₄phase is dominant in all and tends to grow with increasing particle size, the FeO phase appears to coexist with the Fe₃O₄phase in 10, 15, and 24 nm nanocubes but disappears in 34 and 43 nm nanocubes. The nanocubes exposed to air resulted in anα-Fe₂O₃oxidized surface layer whose thickness scaled with particle size resulting in a shell made ofα-Fe₂O₃phase and a core containing Fe₃O₄or a mixture of both Fe₃O₄and FeO phases. Magnetometry indicates that the nanocubes undergo Morin (of theα-Fe₂O₃phase) and Verwey (of the Fe₃O₄phase) transitions at ∼250 K and ∼120 K, respectively. For smaller nanocubes (10, 15, and 24 nm), the EB effect is observed below 200 K, of which the 15 nm nanocubes showed the most prominent EB with optimal antiferromagnetic (AFM) FeO phase. No EB is reported for larger nanocubes (34 and 43 nm). The observed EB effect is ascribed to the strong interfacial coupling between the ferrimagnetic (FiM) Fe₃O₄phase and AFM FeO phase, while its absence is related to the disappearance of the FeO phase. The Fe₃O₄/α-Fe₂O₃(FiM/AFM) interfaces are found to have negligible influence on the EB. Our findings shed light on the complexity of the EB effect in mixed-phase iron oxide nanosystems and pave the way to design exchange-coupled nanomaterials with desirable magnetic properties for biomedical and spintronic applications.