Polyol-process-derived CoPt nanoparticles: Structural and magnetic properties

We report the synthesis and magnetic properties of CoPt nanoparticles by using the polyol process. Since the reduction potential of Pt is more positive than Co, Pt is easily reduced compared to Co. Hence, CoPt nanoparticles were realized by coreducing cobalt and platinum acetylacetonate in the presence of an appropriate amount of OH ions in trimethylene glycol. X-ray diffraction and transmission electron microscopy studies showed that the as-synthesized CoPt nanoparticles had fcc structure and about 5 nm in diameter. Composition analysis reveals that the as-synthesized particles are almost equiatomic ${\mathrm{Co}}_{50}{\mathrm{Pt}}_{50}.$ Magnetic characterization revealed that these nanoparticles are ferromagnetic at room temperature, and that the magnetization and coercivity values were 8 emu/g and 380 Oe, respectively. Differential scanning calorimetry studies showed that the ordering temperature of the as-synthesized particles was only $\text{550\hspace{0.05em}°}\mathrm{C}$ (peak temperature), against $\text{825\hspace{0.05em}°}\mathrm{C}$ of the bulk. Annealing the CoPt nanoparticles above $\text{550\hspace{0.05em}°}\mathrm{C}$ induced ordering with enhanced magnetic properties.