Building Nanocomposite Magnets by Coating a Hard Magnetic Core with a Soft Magnetic Shell

Abstract

Controlling exchange coupling between hard magnetic and soft magnetic phases is the key to the fabrication of advanced magnets with tunable magnetism and high energy density. Using FePt as an example, control over the magnetism in exchange-coupled nanocomposites of hard magnetic facecentered tetragonal (fct) FePt and soft magnetic Co (or Ni, Fe 2 C) is shown. The dispersible hard magnetic fct-FePt nanoparticles are first prepared with their coercivity (H c ) reaching 33 kOe. Then core/shell fct-FePt/Co (or Ni, Fe 2 C) nanoparticles are synthesized by reductive thermal decomposition of the proper metal precursors in the presence of fct-FePt nanoparticles. These core/shell nanoparticles are strongly coupled by exchange interactions and their magnetic properties can be rationally tuned by the shell thickness of the soft phase. This work provides an ideal model system for the study of exchange coupling at the nanoscale, which will be essential for building superstrong magnets for various permanent magnet applications in the future.

Tuning crystal structure and magnetic property of dispersible FePt intermetallic nanoparticles

“…1b). MgO is coated on these NPs by the decomposition of Mg(acac) 2 ·2H 2 O in the presence of OA and OAm in benzyl ether, which is similar to our previous method [28]. Using A1-FePt NPs as seeds, the ratio of OAm to OA plays an essential role in the MgO coating.…”

“…A1-FePt NPs were synthesized following the method in our previous literature [28]. In a four-neck flask, a mixture of Pt(acac) 2 (0.10 g, 0.25 mmol), OA (0.7 g), OAm (0.7 g), and 1-octadecene (7.89 g) was stirred sufficiently and degassed at 80°C for 20 min.…”

“…FePt intermetallic NPs were synthesized according to our previous method [28]. Typically, A1-FePt NPs were treated with the mixture of Mg(acac) 2 ·2H 2 O, OA, OAm and dibenzyl ether at 300°C to coat MgO on the surface.…”

Exchange Springs in <inline-formula> <tex-math notation="LaTeX">${L} 1_{0}$ </tex-math></inline-formula>-FePt(110)/<inline-formula> <tex-math notation="LaTeX">${A}1$ </tex-math></inline-formula>-FePt Bilayer Films

“…wing to large uniaxial magnetocrystalline anisotropy (K u ≈ 7×10 7 erg/cm 3 with magnetization along the shortened c axis), L1 0 -FePt alloy with a crystalline-ordered face centered tetragonal structure has attracted extensive interest in recent years for potential applications in magnetic storage media with ultrahigh areal recording density [1,2]. The coercivity of granular L1 0 -FePt film with grain size at a scale of 10 nm can reach as high as ~70 kOe [3].…”

Exchange‐Coupled fct‐FePd/α‐Fe Nanocomposite Magnets Converted from Pd/Fe3O4 Core/Shell Nanoparticles

“…Exchange coupling of soft and hard magnetic phases has been considered to be one of the promising approaches to fabricate advanced permanent magnets for various magnetic applications. [1][2][3][4] In exchange-coupled nanocomposites, the soft and hard magnetic phases have intimate contact with each other, providing large coercivity (H c ), high magnetization (M s ), and enhanced energy product. [5][6][7][8] In such cases, well-controlled nanostructures with homogenous distributions of nanosized magnetic phases are desired.…”