Low temperature epitaxy and magnetic property of ferromagnetic alloy nanoparticles with atomic order

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Low temperature epitaxial growth of L1_0-type FePt and FePd nanoparticles have been studied by transmission electron microscopy and electron diffraction. For FePt nanoparticles fabricated by successive deposition of Pt and Fe onto NaCl single crystal substrate cleaved in air, it was found that substrate temperatures higher than 623 K and post-deposition annealing at the temperatures higher than 873 K are necessary for obtaining well-isolated L1_0-FePt nanoparticles with large coercivity. Addition of a few atomic percent of Ag into FePt nanoparticles resulted in a decrease of the ordering temperature by 100 K. Also Cu addition into FePd nanoparticles lead to an enhancement of perpendicular anisotropy as well as a formation of the L1_0 structure at the reduced annealing temperature around 823 K. FePtCu nanoparticles fabricated by RF magnetron sputtering formed a well-oriented, well-isolated and densely dispersed L1_0 nanoparticles under the substrate temperature as low as 613 K without any post-deposition annealing. The presently observed decrease of the annealing temperature necessary for the L1_0 ordering was 260 K. We also fabricated Fe/FePd nanocomposite particles and observed an increase of saturation magnetization due to the exchange coupling between Fe and FePd.