Exchange-Coupled fct-FePd/α-Fe Nanocomposite Magnets Converted from Pd/Fe3O4Core/Shell Nanoparticles

High-Moment FeCo Magnetic Nanoparticles Obtained by Topochemical H2 Reduction of Co-Ferrites

Cobalt ferrite nanoparticles of different stoichiometries synthesized by a sol–gel autocombustion method were used as a starting material to obtain high-moment Fe50Co50 and Fe66Co34 metal nanoparticles by topochemical hydrogen reduction. Structural and magnetic investigations confirmed the formation of FeCo nanoparticles with crystallite sizes of about 30 nm and magnetization at 0.5 T of ~265 Am2/kg (0 K), which was larger than the expected bulk value, likely because of the incorporation in the body-centered cubic (bcc) FeCo structure of the residual C atoms present on the surface of the oxide particles. Temperature-dependent magnetization measurements in the H2 atmosphere were also performed to investigate in detail the reduction mechanism and the effect of an external magnetic field on the process efficiency.

Tip Interface Exchange-Coupling Based on "Bi-Anisotropic" Nanocomposites with Low Rare-Earth Content

Specially designed SmCo₅/Co magnetic nanocomposites have been fabricated by a "bottom up" process. SmCo₅ nanochips were first prepared by solution-phase chemical synthesis combined with reductive annealing and then coated by chemical deposition of Co nanorods. Both the SmCo₅ nanochips and Co nanorods are anisotropic and could be simultaneously aligned under the external magnetic field. Magnetic measurements applied on these "bi-anisotropic" SmCo₅/Co composites show high magnetic performance with the Co phase content in a wide range from 10 to 80 wt %. For the first time ever, the applicable exchange-coupled nanocomposites with a rare-earth content lower than 7 wt % was realized, which exhibits the coercivity close to 10 kOe and remanence 31% larger than that of single phase SmCo₅. 3-D micromagnetic simulations were performed to reveal that the reversal mechanism in the Co phase was transferred from the incoherent mode to the coherent mode under a tip interface exchange-coupling with a SmCo₅ surface.

Rare earth permanent magnetic nanostructures: chemical design and microstructure control to optimize magnetic properties

The routes for the optimization of the magnetic properties of rare earth permanent magnetic nanostructures are discussed, i.e. the control of microstructure, such as size and shape as well as the exchange-coupling interactions.

Magnetic Heterostructures: Interface Control to Optimize Magnetic Property and Multifunctionality

Generally, magnetic heterostructures are obtained by the growth of another component on the surface of seed nanoparticles. The direct electrical and magnetic interactions between the solid-state interfaces would endow the heterostructures with properties beyond the individual components. We have devoted the past few years to magnetic-optical, magnetic-catalytic, and exchange-coupled heterostructures, where the interface effects regulate and optimize the optical, catalytic, and magnetic properties, respectively. In this Spotlight on Applications, we describe our recent progress on magnetic heterostructures. Upon the understanding on the interface control, we then discuss our recent efforts to synthesize core-shell, dimer, and nanocomposite structures, while the regulation of their magnetic, optical, and catalytic properties is addressed in turn. Finally, we give the perspectives of magnetic heterostructures.

Harnessing strong metal-support interactions via a reverse route

Engineering strong metal-support interactions (SMSI) is an effective strategy for tuning structures and performances of supported metal catalysts but induces poor exposure of active sites. Here, we demonstrate a strong metal-support interaction via a reverse route (SMSIR) by starting from the final morphology of SMSI (fully-encapsulated core-shell structure) to obtain the intermediate state with desirable exposure of metal sites. Using core-shell nanoparticles (NPs) as a building block, the Pd-FeOx NPs are transformed into a porous yolk-shell structure along with the formation of SMSIR upon treatment under a reductive atmosphere. The final structure, denoted as Pd-Fe3O4-H, exhibits excellent catalytic performance in semi-hydrogenation of acetylene with 100% conversion and 85.1% selectivity to ethylene at 80 °C. Detailed electron microscopic and spectroscopic experiments coupled with computational modeling demonstrate that the compelling performance stems from the SMSIR, favoring the formation of surface hydrogen on Pd instead of hydride.

Chemical Synthesis of Magnetic Nanoparticles for Permanent Magnet Applications

Permanent magnets are a class of critical materials for information storage, energy storage, and other magneto-electronic applications. Compared with conventional bulk magnets, magnetic nanoparticles (MNPs) show unique size-dependent magnetic properties, which make it possible to control and optimize their magnetic performance for specific applications. The synthesis of MNPs has been intensively explored in recent years. Among different methods developed thus far, chemical synthesis based on solution-phase reactions has attracted much attention owing to its potential to achieve the desired size, morphology, structure, and magnetic controls. This Minireview focuses on the recent chemical syntheses of strongly ferromagnetic MNPs (H_c >10 kOe) of rare-earth metals and FePt intermetallic alloys. It further discusses the potential of enhancing the magnetic performance of MNP composites by assembly of hard and soft MNPs into exchange-coupled nanocomposites. High-performance nanocomposites are key to fabricating super-strong permanent magnets for magnetic, electronic, and energy applications.

Formation of strong L10-FePd/α-Fe nanocomposite magnets by visualizing efficient exchange coupling

Conceptual nanocomposite magnets (NCMs) composed of exchange-coupled hard/soft magnetic phases have been expected to show excellent magnetic performance based on simultaneous high coercivity (H _c) and high saturation magnetization (M _s). In our previous works, however, the H _c was considerably lower than its theoretical value (H _a), which prevented us from improving the performance of NCMs. Here, we show that the H _c of isolated particulate L1₀-FePd/α-Fe NCMs is dominated by their phase segregation into core/shell-like structures versus Janus-like structures. Using first-order reversal curve (FORC) analysis, we clearly distinguished a microscopically undetectable difference in the phase-segregation structure in the NCMs, finding both efficient and inefficient exchange coupling. The nanostructurally controlled NCMs dominated by core/shell-like structure with efficient exchange coupling showed the largest energy product ((BH)_max = 17.5 MGOe) in the Fe-Pd system and the highest H _c/H _a value (26.5%) among all NCM powders.

Magnetic Nanomaterials: Chemical Design, Synthesis, and Potential Applications

Magnetic nanomaterials (MNMs) have attracted significant interest in the past few decades because of their unique properties such as superparamagnetism, which results from the influence of thermal energy on a ferromagnetic nanoparticle. In the superparamagnetic size regime, the moments of nanoparticles fluctuate as a result of thermal energy. To understand the fundamental behavior of superparamagnetism and develop relevant potential applications, various preparation routes have been explored to produce MNMs with desired properties and structures. However, some challenges remain for the preparation of well-defined magnetic nanostructures, including exchange-coupled nanomagnets, which are considered as the next generation of advanced magnets. In such a case, effective synthetic methods are required to achieve control over the chemical composition, size, and structure of MNMs. For instance, liquid-phase chemical syntheses, a set of emerging approaches to prepare various magnetic nanostructures, facilitate precise control over the nucleation and specific growth processes of nanomaterials with diverse structures. Among them, the high-temperature organic-phase method is an indispensable one in which the microstructures and physical/chemical properties of MNMs can be tuned by controlling the reaction conditions such as precursor, surfactant, or solvent amounts, reaction temperature or time, reaction atmosphere, etc. In this Account, we present an overview of our progress on the chemical synthesis of various MNMs, including monocomponent nanostructures (e.g., metals, metal alloys, metal oxides/carbides) and multicomponent nanostructures (heterostructures and exchange-coupled nanomagnets). We emphasize the high-temperature organic-phase synthetic method, on which we have been focused over the past decade. Notably, multicomponent nanostructures, obtained by growing or incorporating different functional components together, not only retain the functionalities of each single component but also possess synergic properties that emerge from interfacial coupling, with improved magnetic, optical, or catalytic features. Herein, potential applications of MNMs are covered in three representative areas: biomedicine, catalysis, and environmental purification. Regarding biomedicine, MNMs can detect or target biological entities after being modified with specific biomolecules, and they can be applied to magnetic resonance imaging, imaging-guided drug delivery, and photothermal therapy. Apart from their magnetic features, the catalytic performance of some MNMs resulting from their highly specific chemical components and surface structure will be briefly introduced, highlighting its impact in the methanol oxidation reaction, the oxygen reduction reaction, the oxygen and hydrogen evolution reactions, and the Fischer-Tropsch synthesis. Finally, environmental purification, primarily for water remediation, will be highlighted with two main aspects: the effective capture of bacteria and the removal of adverse ions in wastewater. We hope that this Account will clarify the progress on the controllable preparation of MNMs with specific compositions, sizes, and structures and generate broad interest in the realms of biomedicine and catalysis as well as in environmental issues and other potential applications.

Controlled synthesis and magnetic properties of iron-cobalt-phosphide nanorods

A simple one-step solution-phase synthesis of iron-cobalt-phosphide ((Fe1-xCox)2P) nanorods (NRs) is reported in this paper. Through the control of the amount of Co in the samples, the crystal structure of (Fe1-xCox)2P NRs changes from a pure Fe-rich hexagonal Fe2P type structure to a mixture of Fe-rich hexagonal Fe2P and Co-rich orthorhombic Co2P type structures. These samples show superparamagnetic behavior at room temperature and ferromagnetic properties at 10 K. When the Co composition is 0.09, the (Fe0.91Co0.09)2P sample has the highest coercivity around 5.74 kOe at 10 K. The current route provides a new and general chemical method for tunable preparation of (Fe1-xCox)2P (x < 0.28) NRs, which are significant for the development of new iron- or cobalt-rich permanent magnet materials without rare-earth or noble metals.

Core-shell nanoparticles: synthesis and applications in catalysis and electrocatalysis

Core-shell nanoparticles (CSNs) are a class of nanostructured materials that have recently received increased attention owing to their interesting properties and broad range of applications in catalysis, biology, materials chemistry and sensors. By rationally tuning the cores as well as the shells of such materials, a range of core-shell nanoparticles can be produced with tailorable properties that can play important roles in various catalytic processes and offer sustainable solutions to current energy problems. Various synthetic methods for preparing different classes of CSNs, including the Stöber method, solvothermal method, one-pot synthetic method involving surfactants, etc., are briefly mentioned here. The roles of various classes of CSNs are exemplified for both catalytic and electrocatalytic applications, including oxidation, reduction, coupling reactions, etc.