Fluctuation of long-range order in Co-Pt alloy nanoparticles revealed by time-resolved electron microscopy.

The development of long-range order in disordered Co-Pt alloy nanoparticles has been atomically resolved in situ with an ultra-high voltage electron microscope equipped with a direct electron detection camera. Electron-irradiation-enhanced ordering occurred at 573K with 1MeV electrons at a dose rate of 8.9X10²⁴ e/m²s. High-speed (400 frames/s) imaging revealed fluctuations of the c-axis orientation of the L1₀-type ordered structure. Specifically, the c-axis orientation changes occurred at 2.5-ms intervals. Thus, the atomic ordering rate at 573K is deduced to be 3×10_-17 m²/s, which is 1013 times higher than that estimated for interdiffusion in a bulk Co-Pt alloy. The observed kinetic ordering temperature of 573K is significantly lower than that reported previously (>800 K). The low-temperature ordering may be the result of enhanced atom migration via excess vacancies, 10⁶ times higher than that at thermal equilibrium, introduced by the high-energy electron irradiation. [ABSTRACT FROM AUTHOR]