Dispersible and manipulable magnetic L10-FePt nanoparticles

Operating interfaces to synthesize L10-FePt@Bi-rich nanoparticles by modifying the heating process

A facile strategy to operate interfaces when synthesizing L1₀-FePt@Bi-rich nanoparticles (NPs) has been proposed. Two interfaces are indispensable to obtain the high ordering L1₀-FePt structure. One is the mismatched interfaces between the initial γ-PtBi₂ nuclei and the disordered fcc-FePt phase. The other is the in situ grown coherent interfaces between the L1₀-FePt and Bi-rich phases. Increasing the heating rates when the temperature rises from 120 °C to 310 °C benefits the formation of mismatched interfaces and improves the uniformity of phases and composition in NPs. Reducing the heating rate at higher temperature ensures sufficient time for Bi to diffuse across the coherent interface, which facilitates the disorder-order transition of L1₀-FePt NPs. This study provides a new perspective on operating interfaces during the wet-chemical synthesis process.

Effect of Ag evolution process on ordering transition for L10-FePt nanoparticles synthesized by Ag addition

As a typical element, Ag can effectively promote the ordering transition in the direct synthesis of L10-FePt nanoparticles (NPs). However, the role of Ag in the ordering process and the...

Oriented exchange-coupled L10-FePt/Co core-shell nanoparticles with variable Co thickness

Exchange-coupled core–shell nanoparticles are expected to be the new generation of permanent magnets, where the orientation of the hard magnetic phase is supposed to play a key role in improving their magnetic performance. In this work, L1₀-FePt/Co core–shell nanoparticles with Co thickness ranging from 0.6 to 2.2 nm have been synthesized by a seed-mediated growth method. The exchange coupling effect between the hard core and soft shell led to a 60% improvement of the maximum magnetic energy product ((BH)_max), compared with the pure L1₀-FePt core. By tuning the amount of precursor, nanoparticles with different Co shell thicknesses were synthesized. Furthermore, the L1₀-FePt/Co core–shell nanoparticles were dispersed in epoxy resin and oriented under an external magnetic field. The (BH)_max of the anisotropic nanocomposite magnet with a Co thickness of 1 nm is 7.1 MGOe, enhanced by 117% compared with the isotropic L1₀-FePt magnet, which paves the way for the development of high-performance permanent magnets for energy conversion applications.

With the increase of Co layer thickness, the outer layer Co and the core gradually decoupled.

Magnetic Nanoparticles: Synthesis, Anisotropy, and Applications

Anisotropy is an important and widely present characteristic of materials that provides desired direction-dependent properties. In particular, the introduction of anisotropy into magnetic nanoparticles (MNPs) has become an effective method to obtain new characteristics and functions that are critical for many applications. In this review, we first discuss anisotropy-dependent ferromagnetic properties, ranging from intrinsic magnetocrystalline anisotropy to extrinsic shape and surface anisotropy, and their effects on the magnetic properties. We further summarize the syntheses of monodisperse MNPs with the desired control over the NP dimensions, shapes, compositions, and structures. These controlled syntheses of MNPs allow their magnetism to be finely tuned for many applications. We discuss the potential applications of these MNPs in biomedicine, magnetic recording, magnetotransport, permanent magnets, and catalysis.

Synthesis of FePt Nanoparticles by Photoreduction and Chemical Reduction in Poly(ethyleneimine)

Using poly(ethyleneimine) (PEI) polymer substrates, spherical FePt nanoparticles (NPs) with diameter <10 nm have been synthesized by photoreduction and chemical reduction in aqueous media at room temperature. In the photoreduction approach, PEvI acts as both the template into which the metal ions are coordinated and as a reductant when irradiated by ultraviolet light. In the chemical reduction method, PEI acts as only a template, with NaBH₄ as the reductant. The as-prepared NPs were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), and vibrating sample magnetometry (VSM). Starting from the same precursor state and relative concentrations, the as-prepared NPs from both methods are spherical, crystalline solid solutions with a chemically disordered face-centered cubic (fcc) structure. The as-prepared NPs from both methods are superparamagnetic with some contribution from a ferromagnetic phase. The photoreduced NPs have broad size distribution of (5 ± 1.0 nm), an expanded lattice (3.913 Å), and relatively lower magnetic moment (0.02 emu/g) compared to the narrower size distribution (4 ± 0.7 nm), shortened lattice (3.890 Å), and a dominant moment (15 emu/g) of the chemically reduced NPs. The difference in the rate of particle formation apparently leads to a low efficiency of FePt NP formation via photoreduction compared to chemical reduction.

Annealing Condition Effects on the Structural Properties of FePt Nanoparticles Embedded in MgO via Pulsed Laser Deposition

FePt nanoparticles (NPs) were embedded into a single-crystal MgO host by pulsed laser deposition (PLD). It was found that its phase, microstructures and physical properties were strongly dependent on annealing conditions. Annealing induced a remarkable morphology variation in order to decrease its total free energy. H₂/Ar (95% Ar + 5% H₂) significantly improved the L1₀ ordering of FePt NPs, making magnetic coercivity reach 37 KOe at room temperature. However, the samples annealing at H₂/Ar, O₂, and vacuum all showed the presence of iron oxide even with the coverage of MgO. MgO matrix could restrain the particles' coalescence effectively but can hardly avoid the oxidation of Fe since it is extremely sensitive to oxygen under the high-temperature annealing process. This study demonstrated that it is essential to anneal FePt in a high-purity reducing or ultra-high vacuum atmosphere in order to eliminate the influence of oxygen.