  The ordered structure of binary alloy nanoparticles determines their magnetic
and catalytic characteristics. In the alloys after annealing, one of the
components preferentially segregates on the surface to reduce surface energy.
This surface segregation has been known as a factor in the construction of an
ordered phase near the surface. However, the segregation-induced ordering has
not been observed for nanoparticles. Here, platinum@cobalt (Pt@Co) core-shell
nanoparticles were synthesized, and their structural changes after annealing at
600{\deg}C, 700{\deg}C, and 800{\deg}C for 3 hours were observed by a scanning
transmission electron microscope. We discovered an L10-PtCo structure near the
surface at 700{\deg}C, which was unexpected given the initial Pt:Co ratio of
about 4:1. The L10-PtCo structure was considered to form due to surface
segregation of Pt atoms and diffusion insufficient to mix Pt and Co atoms in
the particle overall because the structure did not form at 600{\deg}C and
800{\deg}C.
