Structural characteristics and magnetic properties of chemically synthesized CoPt nanoparticles

${\mathrm{Co}}_x{\mathrm{Pt}}_{\text{100−x}}$ nanoparticles with dimensions from approximately 2 to 5 nm were synthesized using the reverse micelle method. High-resolution electron microscopy revealed single- and poly-crystalline nanoparticle structures. Twin boundary is a common feature in the polycrystals. As-grown nanoparticles did not show any coercivity at room temperature. However, the nanoparticles became ferromagnetic after annealing at 550 °C for 4 h. Face-centered-cubic to face-centered-tetragonal phase transformation of the nanoparticles that occurred at annealing temperatures above 550 °C was confirmed by electron diffraction patterns and x-ray diffractometry. Coercivity of the annealed nanoparticles were found depending on the nanoparticle chemical compositions. Composition atomic ratio of Co to Pt at around unity gives the highest coercivity of 5500 Oe at room temperature.