L1₀型FePd規則合金ナノ粒子の長範囲規則度

抄録

Hard magnetic properties of FePd nanoparticles originate from their L1₀-type tetragonal ordered structure with high magnetocrystalline anisotropy energy. In order to know correlations among the long-range order (LRO) parameter (S), the particle size and the anisotropy energy, we employed a new technique to determine LRO parameter of single L1₀-FePd nanoparticle using nanobeam electron diffraction (NBD) . The LRO parameter was determined by quantitative analysis of NBD intensities recorded on imaging plates together with intensity calculations considering the multiple scattering of electrons. In taking NBD patterns, hh0 systematic reflections were excited using a transmission electron microscope with a field emission gun operated at 300 kV. Specimen thickness was evaluated by electron holography. The obtained order parameters of individual nanoparticles larger than 8 nm are almost around the mean LRO parameter (S=0.79) determined by selected area electron diffraction. In contrast, the parameters gradually decrease as the particle size decreases below 8 nm (S=0.60-0.73) . The low degree of order in very small particles is responsible for the coercivity (anisotropy energy) decrease of the L1₀ nanoparticles in smaller sized region. Experimental conditions required for NBD analysis are presented in detail and the possible experimental errors of the determined LRO parameters are discussed.