Fabrication, characterization, and magnetic properties of exchange-coupled porous BaFe8Al4O19/Co0.6Zn0.4Fe2O4 nanocomposite magnets

Tuning the magnetic properties of hard-soft Ba0.5Sr0.5Fe10Al2O19and Ni0.1Co0.9Fe2O4nanocomposites via one pot sol-gel auto combustion method for permanent magnet applications

Permanent magnets generate magnetic fields that can be sustained when a reverse field is supplied. These permanent magnets are effective in a wide range of applications. However, strategic rare-earth element demand has increased interest in replacing them with huge energy product (BH)max. Exchange-coupled hard/soft ferrite nanocomposites have the potential to replace a portion of extravagant rare earth element-based magnets. In the present, we have reported the facile auto combustion synthesis of exchange-coupled Ba0.5Sr0.5Fe10Al2O19and Ni0.1Co0.9Fe2O4nanocomposites by increasing the content of soft ferrite over the hard fromx= 0.1 to 0.4 wt%. The XRD combined with Rietveld analysis reflected the presence of hexaferrite and spinel ferrite without the existence of secondary phases. The absorption bands from the Fourier transform infrared spectrum analysis proved the presence of M-O bonds in tetrahedral sites and octahedral sites. Rod and non-spherical images from TEM represent the hexaferrite and spinel ferrite. The smoothM-Hcurve and a single peak of the switching field distribution curve prove that the material has undergone a good exchange coupling. The nanopowders displayed an increase in saturation magnetization and a decrease in coercivity with the increases in the spinel content. The prepared nanocomposites were showing higher energy products. The composite with the ratiox= 0.2 displayed a higher value of (BH)maxof 13.16 kJ m-3.

In situ annealing achieves an ultrafast synthesis of high coercive strontium ferrite foams and beyond

Strontium ferrite nanostructures have attracted intensive interest recently due to the increasing demand for cost-effective features and good chemical corrosion resistance of magnetic materials, yet the ultrafast synthesis of strontium ferrite with desired coercivity is still experiencing a severe challenge. Herein, porous strontium ferrite foams with a coercivity up to 23.35 kOe were prepared by ultrafast in situ annealing for 1 min based on an auto-combustion strategy. The high coercivity of strontium ferrite benefits from the increasing magnetocrystalline anisotropy caused by the ion substitution and the appropriate grain size close to the critical single-domain size of strontium ferrite. In addition, this ultrafast synthesis can be extended to prepare a series of porous spinel, lanthanide-based perovskites, and their high-entropy counterpart foams. We also demonstrate that this strategy is feasible for preparing biphasic composite oxide foams. Furthermore, this work provides important guidance for the design of porous permanent magnet materials and the efficient preparation of porous oxide foam materials.

Short-Range Diffusion Enables General Synthesis of Medium-Entropy Alloy Aerogels

Medium-entropy alloy aerogels (MEAAs) with the advantages of both multimetallic alloys and aerogels are promising new materials in catalytic applications. However, limited by the immiscible behavior of different metals, achieving single-phase MEAAs is still a grand challenge. Herein, a general strategy for preparing ultralight 3D porous MEAAs with the lowest density of 39.3 mg cm^-3 among the metal materials is reported, through combining auto-combustion and subsequent low-temperature reduction procedures. The homogenous mixing of precursors at the ionic level makes the short-range diffusion of metal atoms possible to drive the formation of single-phase MEAAs. As a proof of concept in catalysis, as-synthesized Ni₅₀ Co₁₅ Fe₃₀ Cu₅ MEAAs exhibit a high mass activity of 1.62 A mg^-1 and specific activity of 132.24 mA cm^-2 toward methanol oxidation reactions, much higher than those of the low-entropy counterparts. In situ Fourier transform infrared and NMR spectroscopies reveal that MEAAs can enable highly selective conversion of methanol to formate. Most importantly, a methanol-oxidation-assisted MEAAs-based water electrolyzer can achieve a low cell voltage of 1.476 V at 10 mA cm^-2 for making value-added formate at the anode and H₂ at the cathode, 173 mV lower than that of traditional alkaline water electrolyzers.

Enhancement of the maximum energy product in Ba0.5Sr0.5Fe12O19/Y3Fe5O12 nanocomposites synthesized by the co-precipitation method

Ba_0.5Sr_0.5Fe₁₂O₁₉/Y₃Fe₅O₁₂ (BSFO/YIG) nanocomposite ferrite with hard (BSFO) and soft (YIG) magnetic phases, were prepared by the two-step co-precipitation method. The soft magnetic phase was introduced in different weight ratios (x= 0.0, 0.1, 0.2, 0.3, 0.4 and 1) in the (1-x)Ba_0.5Sr_0.5Fe₁₂O₁₉/(x)Y₃Fe₅O₁₂ nanocomposite ferrites. The structural, morphological and magnetic properties of nanocomposite ferrites were analyzed by x-ray diffraction (XRD), high-resolution transmission electron microscope (HR-TEM) and room temperature vibrational sample magnetometer (VSM). The presence of the hard and soft phase have been confirmed without any secondary phase from the XRD analysis, indicating the formation of nanocomposite ferrite. The crystallite size is found to be in the range of 45-55 nm calculated by Scherrer's formula. The HR-TEM revealed hexagonal platelets of BSFO with YIG particles with an average particle size of 90 nm formed at the surface of the (0.9)BSFO/(0.1)YIG nanocomposite. The room temperature magnetic properties of the nanocomposite, such as saturation magnetization (M_s), squareness ratio (M_r/M_s), coercivity (H_c) and nucleation field (H_n) were evaluated by employing VSM. The magnetic measurements have displayed an enhancement in coercivity and magnetization for (0.9)BSFO/(0.1)YIG. Compared with pure BSFO, the optimized (0.9)BSFO/(0.1)YIG nanocomposite showed 57% enhancement in energy product (BH)_max, indicating that the nanocomposite possessed excellent exchange coupling. To investigate the exchange coupling between the hard and soft magnetic phases, dM/dH values were plotted using the demagnetization curves which indicated the effective exchange coupling effect between the hard and soft phases.

A unique synthesis of rare-earth-Co-based single crystal particles by "self-aligned" Co nano-arrays

In this study, anisotropic SmCo₅ magnets were prepared by a distinctive method, which is the high-temperature reductive annealing of Co@Sm₂O₃ with a specially designed nanostructure. High resolution transmission electron microscopy and elemental mapping show that the precursor self-assembly is composed of hcp-structured Co nano-rods with a coherent crystallographic orientation. During high temperature reduction, the Sm₂O₃ shell preserves the original morphology and alignment of these anisotropic Co nano-arrays, providing a template for hcp-structured SmCo₅ single crystal particle synthesis. The as-prepared SmCo₅ magnets exhibit well-controlled size and morphology, and a high coercivity of 30.9 kOe at room temperature. No stabilizer coating is necessary to prevent the formation of polycrystals in this synthesis.

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.

Exploring the direct synthesis of exchange-spring nanocomposites by reduction of CoFe2O4 spinel nanoparticles using in situ neutron diffraction

In situ neutron powder diffraction (NPD) was employed for investigating gram-scale reduction of hard magnetic CoFe2O4 (spinel) nanoparticles into CoFe2O4/CoFe2 exchange-spring nanocomposites via H2 partial reduction. Time-resolved structural information was extracted from Rietveld refinements of the NPD data, revealing significant changes in the reduction kinetics based on the applied temperature and H2 available. The nanocomposite formation was found to take place via the following two-step reduction process: CoxFe3-xO4 → CoyFe1-yO → CozFe2-z. The refined lattice parameters and site occupation fractions indicate that the reduced phases, i.e. CoyFe1-yO and CozFe2-z, initially form as Co-rich compounds (i.e. y > 0.33 and z > 1), which gradually incorporate more Fe as the reduction proceeds. The reduction depletes the Co-content in the parent spinel, which may end up becoming magnetically soft Fe3O4 at high temperature (T = 542 °C), while at lower temperatures there may be a co-existence of Fe3O4 and γ-Fe2O3 or CoxFe3-xO4. The macroscopic magnetic properties of the products were measured by vibrating sample magnetometry (VSM) and revealed the hard and soft magnetic domains in the nanocomposites to be effectively exchange-coupled. An increase of approximately 70% in specific saturation magnetisation, remanence magnetisation, and coercivity compared to the parent CoFe2O4 material was achieved for the best sample.

Temperature-dependent heating efficiency of magnetic nanoparticles for applications in precision nanomedicine

The power released by magnetic nanoparticles submitted to an alternating driving field is temperature dependent owing to the variation of the fundamental magnetic properties. Therefore, the heating efficiency of magnetic nanoparticles for applications in precision nanomedicine (such as magnetic hyperthermia or heat-assisted drug delivery) can be significantly affected by the local instantaneous temperature of the host medium. A rate equation approach is used to determine the hysteretic properties and the power released by magnetite nanoparticles, and the heat transport equation is solved in a simple geometry with boundary conditions appropriate to both in-lab experiments and in vivo applications. Size plays a fundamental role in determining the heating efficiency of magnetic nanoparticles; above a critical size, nanoparticles remain inactive, although they can undergo secondary activation. The experimental conditions for optimal thermal efficiency are expressed by a thermal activity diagram for nanoparticles. In the light of the model's results, features, methods, advantages and dangers of magnetic-particle assisted precision nanomedicine ought to be reconsidered. In vivo antitumor applications should take into account the hazards arising from the heat generated by magnetic nanoparticles that diffuse into the neighboring healthy tissue.