Chapter 4 - Electron Microscopy Studies on Magnetic L1 0-Type FePd Nanoparticles

Abstract

Abstract We studied the atomic ordering and structures of hard magnetic L10 FePd nanoparticles using modern electron microscopy techniques, including nanobeam electron diffraction (NBD), aberration-corrected high-resolution transmission electron microscopy (HRTEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), & three-dimensional (3D) electron tomography. The key issue throughout this study is the atomic ordering in the alloy nanoparticles in view of excellent hard magnetic properties due to the high magnetocrystalline anisotropy energy, which is dependent on the long-range order. The kinetics of atomic ordering plays a crucial role here—that is, annealing below a reduced order-disorder transformation temperature is essential in obtaining the L10 ordered phase in small nanoparticles. We proposed a novel technique to measure the degree of order in individual particles using NBD and diffraction intensity calculations, & quantitatively proved the size-dependent degradation of long-range atomic order in nanoparticles. Imaging using aberration-corrected HRTEM unambiguously revealed the atomic structures of the alloy nanoparticles 2–10 nm in diameter. Electron tomography revealed 3D shapes and spatial distribution of nanoparticles. We thus have demonstrated that a method based on a simple extrapolation of a raw dataset (the tilt-series extrapolation [TSE] method) is reliable and useful in examining the accuracy and resolution of 3D reconstructed information provided by different algorithms. All these techniques developed rapidly over the past 10 years and are now available for the structural analysis of alloy nanoparticles.