HR-TEM Studies of FePt Nanoparticles by Exit Wave Reconstruction
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0998-J01-06
HR-TEM Studies of FePt Nanoparticles by Exit Wave Reconstruction Daniela Sudfeld1, Olga Dmitrieva1, Nina Friedenberger1, Guenter Dumpich1, Michael Farle1, ChengYu Song2, Christian Kisielowski2, Markus E. Gruner1, and Peter Entel1 1 Fachbereich Physik and Center for Nanointegration, Universit‰t Duisburg-Essen, Lotharstr. 1, Duisburg, 47048, Germany 2 National Center for Electron Microscopy, Ernest Orlando Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley, CA, 94720
ABSTRACT FePt nanoparticles are promising materials for high-density magnetic data storage media [1] and bio-medical applications such as drug-targeting and hyperthermia [2]. To understand their magnetic properties [3] 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 [4]. 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 [1]. 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. INTRODUCTION To meet the requirements of the next generation nano/biotechnologies it is essential to get insights into the microstructure and surface termination of isolated nanoparticles which influence the magnetic behavior. Small diameter FePt nanocrystals which are produced at an early stage in the wet-chemical synthesis were investigated carrying out a direct reconstruction of the phase and the amplitude of the scattered electron wave from a focal series of HR-TEM images. This exit wave reconstruction (EWR) is suitable to extend the interpretable resolution down to the information limit of the One-≈ngstrom Microscope (O≈M) which is a modified Philips CM300FEG/UT [6]. Besides the removal of delocalization effects and the correction of the spherical aberration Cs, the Exit Wave Reconstruction (EWR) allows the retrieval of the phase and the amplitude of the complex electron wave exiting the sample by recording a series of HRTEM images under slightly varying defocus conditions [8,9]. Note that separate amplitude and phase images can be determined, which are not directly accessible in conventional HR-TEM where the intensity, i.e. the squared amplitude (interaction probability) is recorded only. The amount of defocus in a HRTEM image can be determined from its
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