HRTEM Studies of nm-Size FePd Particles Embedded in MgO after Annealing over 920 K

Abstract

FePd particles were fabricated in MgO crystalline films by using an ultra-high vacuum (UHV) co-deposition method. As-deposited samples and samples annealed at 973 K and 1073 K were studied by high resolution electron microscopy (HRTEM). The as-deposited FePd particles were in a disordered fcc phase with a=0.3837 nm and have a certain epitaxial orientations i.e. [100] (001)_FePd||[100] (001)_MgO. The size of particles was about 2-3 nm and separated distance of particles was about 3-4 nm. After the as-deposited sample was annealed at 973 K for 24 hours, there were neither change in particle size and separated distance, nor phase transformation to take place from disordered fcc to L1₀ order structure. Only change was that lattice fringes in the FePd particles became more straight and regularly spaced than those of the as-deposited sample, and faults within the FePd particles became less than those of the as-deposited sample. After the as-deposited sample was annealed at 1073 K for 4 hours, the size of particles became almost about 3-4 nm and separated distance of particles was about 2-3 nm. Some coalescence took place among the particles. There was, however, no transformation to take place from disordered fcc to L1₀ order structure. The reason why L1₀ order structure of FePd can not be obtained at such higher annealing temperature than the transformation temperature for the bulk is suggested to be that the system energy of small particle was subjected to strain inside, which is caused by large lattice mismatch between FePd and MgO.