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Magnetismus Poster: Do., 13:00–15:30 D-P265
Lowering of the ordering temperature of FePt nanoparticles induced by
He + ion irradiation
Ulf Wiedwald 1 , Andreas Klimmer 1 , Birgit Kern 1 , Hans-Gerd Boyen 1 , Paul
Ziemann 1
1 Abteilung Festkörperphysik, Universität Ulm, Albert-Einstein-Allee 11, 89069 Ulm
Further reduction of the volume of magnetic nanostructures appears as a natural
route towards the enhancement of information density stored by magnetic devices.
To overcome thermal fluctuations an extremely large magnetic anisotropy energy density
(∼ 10 7 J/m 3 ) is required to stabilize the magnetization of a bit coding magnetic
sphere. Chemically ordered FePt particles fulfil this need. Self assembled monolayers
(SAM) of ligand-stabilized FePt nanoparticles can be prepared by means of colloidal
chemistry [1]. It turns out, that the as-prepared FePt nanoparticles exhibit the lowanisotropy
FCC phase, which has to be transformed into the FCT phase by annealing
at temperatures larger 600 ◦ C. Such a heat treatment, however, is likely to result in
the formation of larger agglomerates of the as-deposited particle ensembles thereby
destroying the order of the particle array. Thus, a significant decrease of the ordering
temperature would be desirable to maintain the quality of the SAM. One possibility to
reduce the transition temperature is the creation of vacancies and interstitials by He +
ion irradiation as observed for FePt films [2].
Here, we make use of a micellar method [3,4] allowing the preparation of arrays
of well separated and chemically pure FePt nanoparticles (diameter 3-10 nm) with
adjustable interparticle distances of 18-100 nm. In a first experiment, 9 nm FePt
nanoparticles were exposed to He + ions at a dose of 10 16 ions/cm 2 . SEM images reveal
that the irradiated particles are still located at the substrate surface as single units
like the untreated ones. The magnetic properties of the particles are investigated by
means of x-ray magnetic circular dichroism. The corresponding hysteresis loops of the
particles after annealing at elevated temperatures show that as-prepared particles need
to be annealed at 620 ◦ C for 30 min or higher to achieve room temperature hysteresis
while the phase transformation for particles exposed to He + ions already starts around
450 ◦ C.
[1] S. Sun et al., Science 287, 1989 (2000)
[2] H. Bernas et al., Phys. Rev. Lett. 91, 077203 (2003)
[3] G. Kästle et al., Adv. Funct. Mater. 13, 853 (2003)
[4] A. Ethirajan et al., submitted