Synthesis of Hexagonal Close-Packed Cobalt Nanoparticles From Thermolysis of Cobalt Carbonyl

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Magnetic materials with low magnetic loss are required to realize both a high-frequency support and a miniaturization of radio frequency components. Hexagonal close-packed cobalt (hcp-Co) nanoparticles are considered suitable for high frequencies due to their nanoparticle morphology and high magnetocrystalline anisotropy. However, the face-centered cubic (fcc) or the ϵ phase with low magnetocrystalline anisotropy is fabricated in the synthetization of Co nanoparticles with a size of less than a few hundred nanometers. In this letter, we investigate the synthesis of Co nanoparticles by the thermolysis of dicobalt octacarbonyl at various temperatures for obtaining Co particles with a single hcp phase. Although Co nanoparticles synthesized at 453 K exhibited a mixture of hcp and fcc phases with an hcp phase ratio of 25%, Co nanoparticles almost achieved the hcp phase ratio of 100% by decreasing the thermolysis temperature to 333 K or lower. Furthermore, we evaluated the permeability spectrum of the composite with Co particles of 10 vol% dispersed in polystyrene. Although the real part of the permeability in the composite containing Co nanoparticles with the mixed phase of fcc and hcp monotonously decreased with frequency, the composite contained Co nanoparticles with a single phase with a suitable constant value up to 3 GHz for high-frequency applications.