Exchange coupling interaction in L10-FePd/α-Fe nanocomposite magnets with large maximum energy products

Synthetic Approaches to Colloidal Nanocrystal Heterostructures Based on Metal and Metal-Oxide Materials

Composite inorganic nanoarchitectures, based on combinations of distinct materials, represent advanced solid-state constructs, where coexistence and synergistic interactions among nonhomologous optical, magnetic, chemical, and catalytic properties lay a basis for the engineering of enhanced or even unconventional functionalities. Such systems thus hold relevance for both theoretical and applied nanotechnology-based research in diverse areas, spanning optics, electronics, energy management, (photo)catalysis, biomedicine, and environmental remediation. Wet-chemical colloidal synthetic techniques have now been refined to the point of allowing the fabrication of solution free-standing and easily processable multicomponent nanocrystals with sophisticated modular heterostructure, built upon a programmed spatial distribution of the crystal phase, composition, and anchored surface moieties. Such last-generation breeds of nanocrystals are thus composed of nanoscale domains of different materials, assembled controllably into core/shell or heteromer-type configurations through bonding epitaxial heterojunctions. This review offers a critical overview of achievements made in the design and synthetic elaboration of colloidal nanocrystal heterostructures based on diverse associations of transition metals (with emphasis on plasmonic metals) and transition-metal oxides. Synthetic strategies, all leveraging on the basic seed-mediated approach, are described and discussed with reference to the most credited mechanisms underpinning regioselective heteroepitaxial deposition. The unique properties and advanced applications allowed by such brand-new nanomaterials are also mentioned.

Synthesis of mesoscopic particles of multi-component rare earth permanent magnet compounds

Multielement rare earth (R)-transition metal (T) intermetallics are arguably the next generation of high-performance permanent magnetic materials for future applications in energy-saving and renewable energy technologies. Pseudobinary Sm2Fe17N3 and (R,Zr)(Fe,Co,Ti)12 (R = Nd, Sm) compounds have the highest potential to meet current demands for rare-earth-element-lean permanent magnets (PMs) with ultra-large energy product and operating temperatures up to 200°C. However, the synthesis of these materials, especially in the mesoscopic scale for maximizing the maximum energy product (B H m a x ), remains a great challenge. Nonequilibrium processes are apparently used to overcome the phase-stabilization challenge in preparing the R-T intermetallics but have limited control of the material's microstructure. More radical bottom-up nanoparticle approaches based on chemical synthesis have also been explored, owing to their potential to achieve the desired composition, structure, size, and shape. While a great achievement has been made for the Sm2Fe17N3, progress in the synthesis of (R,Zr)(Fe,Co,Ti)12 magnetic mesoscopic particles (MMPs) and R-T/T exchange-coupled nanocomposites (NCMs) with substantial coercivity (H c ) and remanence (M r ) , respectively, remains marginal.

Magnetic spin exchange interaction in SmCo5/Co nanocomposite magnet for large energy product

Magnetic spin exchange-coupled magnets have been investigated for obtaining an enhanced energy product, however, approaches at the nanoscale have been greatly restricted because of the lack of consideration of the relationships among the individual components. Here, we suggest a facile strategy for fabricating exchange-coupled nanomagnets with a large energy product. As a bottom-up approach, this work introduces a combined thermal decomposition and reduction/diffusion process to obtain a magnetic spin exchange coupled SmCo₅/Co nanocomposite magnet. The SmCo₅/Co nanocomposite magnet was fabricated through a three-step approach: (1) chemical synthesis of Co@SmO_x nanoparticles and Co nanoparticles as hard and soft magnetic phases, respectively, (2) 3-dimensional alternating arrangement of both magnetic phases and (3) a reduction/diffusion process for the magnetic spin exchange interaction. Our results demonstrate that an effective magnetic spin exchange interaction strongly depends on the dimension and arrangement of the hard and soft phases, which were synthetically tuned to be within the magnetic domain wall size.

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.

Recent Novel Hybrid Pd–Fe3O4 Nanoparticles as Catalysts for Various C–C Coupling Reactions

The use of nanostructure materials as heterogeneous catalysts in the synthesis of organic compounds have been receiving more attention in the rapid developing area of nanotechnology. In this review, we mainly focused on our own work on the synthesis of hybrid palladium–iron oxide nanoparticles. We discuss the synthesis of Pd–Fe3O4—both morphology-controlled synthesis of Pd–Fe3O4 and transition metal-loaded Pd–Fe3O4—as well as its application in various C–C coupling reactions. In the case of rose-like Pd–Fe3O4 hybrid nanoparticles, thermal decomposition can be used instead of oxidants or reductants, and morphology can be easily controlled. We have developed a method for the synthesis of nanoparticles that is facile and eco-friendly. The catalyst was recyclable for up to five continual cycles without significant loss of catalytic activity and may provide a great platform as a catalyst for other organic reactions in the near future.

Discovery of Hexagonal Structured Pd-B Nanocrystals

We report on hexagonal close-packed (hcp) palladium (Pd)-boron (B) nanocrystals (NCs) by heavy B doping into face-centered cubic (fcc) Pd NCs. Scanning transmission electron microscopy-electron energy loss spectroscopy and synchrotron powder X-ray diffraction measurements demonstrated that the B atoms are homogeneously distributed inside the hcp Pd lattice. The large paramagnetic susceptibility of Pd is significantly suppressed in Pd-B NCs in good agreement with the reduction of density of states at Fermi energy suggested by X-ray absorption near-edge structure and theoretical calculations.

Ultrathin Interface Regime of Core-Shell Magnetic Nanoparticles for Effective Magnetism Tailoring

The magnetic exchange coupling interaction between hard and soft magnetic phases has been important for tailoring nanoscale magnetism, but spin interactions at the core-shell interface have not been well studied. Here, we systematically investigated a new interface phenomenon termed enhanced spin canting (ESC), which is operative when the shell thickness becomes ultrathin, a few atomic layers, and exhibits a large enhancement of magnetic coercivity (H_C). We found that ESC arises not from the typical hard-soft exchange coupling but rather from the large magnetic surface anisotropy (K_S) of the ultrathin interface. Due to this large increase in magnetism, ultrathin core-shell nanoparticles overreach the theoretical limit of magnetic energy product ((BH)_max) and exhibit one of the largest values of specific loss power (SLP), which testifies to their potential capability as an effective mediator of magnetic energy conversion.

Controlled synthesis and assembly into anisotropic arrays of magnetic cobalt-substituted magnetite nanocubes

Cubic cobalt-substituted magnetite CoxFe3-xO4 nanocubes (NCs) with uniform composition distributions of Co, Fe and O in the NCs, obtained via solution synthesis, are reported in this paper. Through the control of the reaction conditions, the size of the cubic NCs could be tuned from 35 to 110 nm. It was found that the cubic shape could easily induce the (400) orientation of the NCs on a Si substrate, and applying an external magnetic field in the out-of-plane direction could further enhance the (400) orientation of these NCs on the Si substrate. The highest coercivity of 2.07 kOe could be obtained by assembling the NCs in the external magnetic field. The reported magnetic cobalt-substituted magnetite NCs provide an ideal class of building blocks for studying ferrimagnetic nanoparticle (NP) assemblies with easily controlled magnetic alignment for magnetic tape recording with ever increased areal storage density.

Interdiffusion induced exchange coupling of L10-FePd/α-Fe magnetic nanocomposites

One-pot synthesis of FePd and FePd/Fe2O3 (core/shell) nanoparticles via interdiffusion is reported for the first time. It was found that the size of FePd particles and Fe2O3 shell thickness could be controlled by the ligand and iron precursor amounts, respectively. These FePd/Fe2O3 particles can be reductively annealed at 500 °C to produce exchanged coupled L10-FePd/α-Fe magnetic nanocomposites. The effect of the phosphine ligand on magnetic characteristics of synthesized particles and final annealed nanocomposite is discussed. Finally, it was found that the magnetic properties of the final L10-FePd/α-Fe nanocomposites could be tuned by Fe2O3 shell thickness and can reach a coercivity (Hc) of up to 2.4 kOe and a saturation magnetization (Ms) of 141 emu/g.

Room temperature ferromagnetic (Fe₁-xCox)₃BO₅ nanorods

Cobalt-doped ferroferriborate ((Fe1-xCox)3BO5) nanorods (NRs) are synthesized by a one-pot high-temperature organic-solution-phase method. The aspect ratios of the NRs are tuned by the heating rate. These NRs form via anisotropic growth along twin boundaries of the multiply twinned nuclei. Magnetic properties are dramatically modified by Co substitutional doping, changing from antiferromagnetic order at low temperatures to ferromagnetic above room temperature, with a greatly enhanced magnetic ordering temperature. These anisotropic ferromagnetic NRs with a high ordering temperature may provide a new platform for understanding nanomagnetism and for magnetic applications.

Hydrogen-induced structural transformation of AuCu nanoalloys probed by synchrotron X-ray diffraction techniques

In situ X-ray diffraction measurements reveal that the transformation of a AuCu nanoalloy from a face-centered-cubic to an L10 structure is accelerated under a hydrogen atmosphere. The structural transformation rate for the AuCu nanoalloy under hydrogen above 433 K was found to be 100 times faster than that in a vacuum, which is the first quantitative observation of hydrogen-induced ordering of nanoalloys.

A facile template synthesis of asymmetric gold silica heteronanoparticles

Silica hemispheres containing gold nanoparticle cores have been synthesized via immobilization of gold nanoparticles on a substrate and site-selective growth of silica followed by removal of the hemispherical particles. The structure of these asymmetric heteronanoparticles allows selective etching or overgrowth of the core gold seeds, which results in the respective formation of hemispherical capsules or gold homodimers.

Exchange-coupled nanocomposites: chemical synthesis, characterization and applications

This review summarizes the recent progress in the chemical synthesis and applications of exchange-coupled nanocomposites.

One-pot synthesis of urchin-like FePd-Fe3O4 and their conversion into exchange-coupled L1(0)-FePd-Fe nanocomposite magnets

We report a one-pot synthesis of urchin-like FePd-Fe3O4 nanocomposites, spherical clusters of FePd nanoparticles (NPs) with spikes of Fe3O4 nanorods (NRs), via controlled thermal decomposition of Fe(CO)5 and reduction of Pd(acac)2. The FePd NPs with sizes between 6 and 9 nm self-aggregate into 60 nm superparticles (SPs), and Fe3O4 NRs grow on the surface of these SPs. Reductive annealing at 500 °C converts the FePd-Fe3O4 into exchange-coupled nanocomposites L1(0)-FePd-Fe with their Hc tunable from 0.8 to 2.6 kOe and Ms controlled from 90 to 190 emu/g. The work provides a general approach to L1(0)-FePd-Fe nanocomposite magnets for understanding exchange coupling at the nanoscale. The concept may be extended to other magnetic nanocomposite systems and may help to build superstrong magnets for magnetic applications.

Cobalt-substituted magnetite nanoparticles and their assembly into ferrimagnetic nanoparticle arrays

A simple process to prepare monodisperse ferrimagnetic cobalt-substituted magnetite Co(x)Fe(3-x)O4 nanoparticles is reported. These ferrimagnetic nanoparticles are readily dispersed in hexane, forming a stable ferrimagnetic nanoparticle dispersion, and allowing easy nanoparticle self-assembly. When assembled under an external magnetic field (5.5 kOe), these nanoparticles show preferred magnetic alignment with their H(c) reaching 2.49 kOe.

Strongly exchange coupled inverse ferrimagnetic soft/hard, Mn(x)Fe(3-x)O4/Fe(x)Mn(3-x)O4, core/shell heterostructured nanoparticles

Inverted soft/hard, in contrast to conventional hard/soft, bi-magnetic core/shell nanoparticles of Mn(x)Fe(3-x)O(4)/Fe(x)Mn(3-x)O(4) with two different core sizes (7.5 and 11.5 nm) and fixed shell thickness (∼0.6 nm) have been synthesized. The structural characterization suggests that the particles have an interface with a graded composition. The magnetic characterization confirms the inverted soft/hard structure and evidences a strong exchange coupling between the core and the shell. Moreover, larger soft core sizes exhibit smaller coercivities and loop shifts, but larger blocking temperatures, as expected from spring-magnet or graded anisotropy structures. The results indicate that, similar to thin film systems, the magnetic properties of soft/hard core/shell nanoparticles can be fine tuned to match specific applications.

Synthesis and Magnetic Studies of Core-Shell FePt@Fe3O4 Nanowires and Nanoparticles

In this work, the core-shell FePt@Fe3O4 nanowires and nanoparticles as a new hard-soft composite magnetic materials were synthetized by reduction of platinum acetyl acetonate and iron pentacarbonyl together in the presence of oleic acid and oleyl amine stabilizers by polyol process. As-synthesized FePt nanowires and nanoparticles with 0.5-3 nm Fe3O4 shell were preparated by controlled addition of excess of Fe (CO)5 into the reaction mixture and air oxidation. The phase analysis, structure, and magnetic properties were determined by X-ray diffraction (XRD), High resolution transmission electron microscope (HRTEM), Scanning electron microscope (SEM) and vibrating sample magnetometer (VSM) techniques.