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He Ion Irradiation Induced Phase Transformation of Icosahedral FePt Nanoparticles
Dmitrieva, O.; Acet, M.; Liedke, M. O.; Rellinghaus, B.; Fassbender, J.ORC
One potential material system for high density magnetic recording consists of monodisperse, highly anisotropic FePt nanoparticles. In order to exhibit high magnetic anisotropy the fct-L10 phase is required.
FePt nanoparticles (dP = 6 nm) with concentration close to the stoichiometric composition have been prepared by DC magnetron sputtering in Ar and subsequent thermal annealing in the gas phase [1]. Recently, we have shown that under appropriate preparation conditions (pAr = 0.5 mbar, rAr = 100 sccm, Tanneal = 600 – 1000 °C), the obtained particles are monodisperse but unfortunately of icosahedral structure [1,2]. Strikingly, these icosahedra are very stable and even post-deposition annealing of these FePt icosahedra for 30 min at T = 1000 °C did not result in a structural phase transition of the particles, although L10 phase is known to be the thermodynamic equilibrium structure of (bulk) FePt.
In order to investigate both the stability of the icosahedral structure and the potential of He ion irradiation for inducing a structural phase transformation, likewise prepared FePt nanoparticles were deposited onto amorphous carbon films (TEM grids) and irradiated with He ions (5 keV) at room temperature. The ion fluence was varied in the range between 1•1016 and 3•1017 ions/cm2. HRTEM investigations on the irradiated particles show that with increasing fluence, the icosahedral particles become increasingly destabilized at the expense of fcc polycrystalline and single crystal particles. Adjacent particles are observed to coalesce (sinter) under the impact of the ion beam thereby indicating an enhanced diffusivity due to (temporarily) increased defect concentrations. The influence of the modified particle structure (with lower symmetry) on the result of a subsequent post-deposition annealing of the irradiated FePt nanoparticles is discussed.

Part of this work is supported by the Deutsche Forschungsgemeinschaft within SFB 445.

[1] S. Stappert, B. Rellinghaus, M. Acet, and E.F. Wassermann, J. Cryst. Growth 252 (2003) 440-450.
[2] S. Stappert, B. Rellinghaus, M. Acet, and E.F. Wassermann, Proc. Mat. Res. Soc. Vol. 704, pp. 73-78 (2002).

Keywords: magnetism, nanoparticles, FePt, hard magnetic materials, He ion irradiation, structural phase transformation
  • Poster
    Conference on Magnetism and Magnetic Materials, Jacksonville, USA, 07.-11.11.2004, Abstract DR-5

Publ.-Id: 6400 - Permalink