Atomic structure imaging of L 10 -type FePd nanoparticles by spherical aberration corrected high-resolution transmission electron microscopy

The atomic structure of FePd nanoparticles has been studied by spherical aberration (C_s) corrected high-resolution transmission electron microscopy. The periodic arrangement of atoms arising from chemical order is clearly seen as bright contrast due to the small negative value of corrected C_s. The amount of optimal defocus (Scherzer defocus) is markedly reduced by the small C_s value. The interface between crystalline particles and the amorphous matrix can also be observed, free of imaging artifacts, at a small defocus value. The reconstructed phase image directly shows the projected potential distribution within the specimen and reveals the elemental differences due to chemical order. The clear-cut long-range order is lost when particle size is smaller than about 5 nm, at which locally ordered mixed-phase particles begin to dominate.