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.