Chemical Synthesis of Magnetically Hard and Strong Rare Earth Metal Based Nanomagnets

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.

Stabilizing Hard Magnetic SmCo5 Nanoparticles by N-Doped Graphitic Carbon Layer

We report a chemical method to synthesize size-controllable SmCo₅ nanoparticles (NPs) and to stabilize the NPs against air oxidation by coating a layer of N-doped graphitic carbon (NGC). First 10 nm CoO and 5 nm Sm₂O₃ NPs were synthesized and aggregated in reverse micelles of oleylamine to form SmCo-oxide NPs with a controlled size (110, 150, or 200 nm). The SmCo-O NPs were then coated with polydopamine and thermally annealed to form SmCo-O/NGC NPs, which were further embedded in CaO matrix and reduced with Ca at 850 °C to give SmCo₅/NGC NPs of 80, 120, or 180 nm, respectively. The 10 nm NGC coating efficiently stabilized the SmCo₅ NPs against air oxidation at room temperature or at 100 °C. The magnetization value of the 180 nm SmCo₅/NGC NPs was stabilized at 86.1 emu/g 5 days after air exposure at room temperature and dropped only 1.7% 48 h after air exposure at 100 °C. The stable SmCo₅/NGC NPs were aligned magnetically in an epoxy resin, showing a square-like hysteresis behavior with their H_c reaching 51.1 kOe at 150 K and 21.9 kOe at 330 K and their M_r stabilized at around 84.8 emu/g. Our study demonstrates a new strategy for synthesizing and stabilizing SmCo₅ NPs for high-performance nanomagnet applications in a broad temperature range.

Dispersible SmCo5 nanoparticles with huge coercivity

It is difficult to obtain dispersed particles of SmCo5 by calciothermic reduction because of sintering during the high-temperature reaction. This study presents a new strategy to synthesize dispersible SmCo5 particles by co-precipitating a precursor containing amorphous Sm(OH)3 and coherent nanoscale Co(OH)2 and Ca(OH)2 crystallites. The Ca(OH)2 dehydrates into CaO which forms an isolation shell around the SmCo5 particles that prevents them sintering during the reaction at 860 °C. A magnetization of 90 Am2 kg-1, a remanence ratio of 0.96 and a huge coercivity of 6.6-7.2 T were achieved at room temperature after dissolving the CaO and orienting a dispersion of the particles in epoxy in a 0.8 T external field. Based on its scan-rate dependence in high quasi-static and pulsed magnetic fields, the coercivity mechanism is identified as nucleation and growth of 88 nm3 nucleation volumes in a low-anisotropy surface region about 15 nm thick. The coercivity is the highest yet reported for nanoparticles of any permanent magnet and it opens the prospect of new high-temperature magnet composites.

Chemically synthesized anisotropic SmCo5 nanomagnets with a large energy product

In this communication, we report a facile strategy to chemically synthesize anisotropic SmCo₅ nanomagnets with a large magnetic energy product (BH). First, we designed a Co₃O₄@Sm₂O₃-CaO precursor by a one-pot method, which could be further reduced into uniform single-crystal SmCo₅ particles under the stabilization of CaO coating. Following that, CaO was removed under an oxygen-free environment to impede oxidation. Finally, 130 ± 10 nm SmCo₅ particles were aligned to be nanomagnet, exhibiting a large (BH)_max value of 18.1 MGOe, which is the highest value reported by chemical methods.