Structure and magnetic property changes of epitaxially grown L1-FePd isolated nanoparticles on annealing

Isolated 10-nm-sized FePd nanoparticles with the ${\text{L1}}_0$ -type ordered structure have been fabricated by electron-beam evaporation and postannealing above 773 K, and the structural details have been investigated by transmission electron microscopy. FePd particles were epitaxially grown on a cleaved NaCl(001) substrate and were two-dimensionally dispersed on the substrate. In FePd particles formation, Pd nanoparticles were first deposited as “seed” particles epitaxially on NaCl followed by a successive deposition of Fe particles. All the Fe particles were captured by Pd particles forming Fe/Pd nanocomplex particles with a mutual fixed orientation. Coalescence and growth of the particles were not prominent during annealing, indicating that the alloying and atomic ordering reactions proceeded mostly within each nanoparticle. The negligible coalescence can be attributed to an “anchoring effect” of the seed Pd to the coalescence growth. Moreover, both of these reactions are thought to proceed almost simultaneously during annealing at temperatures between 723 and 823 K. Most of the annealed particles were single crystal particles with c axes oriented both normal and parallel to the film plane. Large coercivities above 3 kOe were obtained after annealing at 873 K, though they were smaller than those expected from the theoretical model. The small coercivity value can be attributed to the low magnetocrystalline anisotropy. The magnetocrystalline anisotropy constant of the present FePd nanoparticles estimated was less than half of that of the bulk materials.