FePt nanoparticles are promising materials for high-density magnetic data storage media  and bio-medical applications such as drug-targeting and hyperthermia . To understand their magnetic properties  it is essential to get insights into the lattice structure of isolated nanoparticles which influence the magnetic behavior.
Typically, lattice fringes are observed with high-resolution transmission electron microscopy (HR-TEM). In this case delocalization effects disturb imaging of the lattice structure in particular if 2 to 6 nm small nanoparticles are involved. Therefore, FePt nanocrystals were investigated by reconstructing amplitude and phase of the scattered electron wave from a focal series of HRTEM images, which can produce delocalization free and direct images of the crystal structure . The formation of 5-fold twinned structures of 3 to 7 nm face-centered cubic FePt nanocrystals is investigated that were grown from a colloidal solution . The results are compared with abinitio density functional (DFT) calculations of FePt particles with a diameter of larger than 2 nm. Image simulations were performed with the Accelrys Cerius2 software package (Version 4.6). Good agreement between the ab-initio calculations and the experimental data is found.