Structural phase transformation of FePt nanoparticles by ion irradiation

Owing to its large magneto-crystalline anisotropy energy, L10 ordered tetragonal FePt is among the most intensively discussed materials when it comes to pushing the superparamagnetic limit towards minimum particle sizes for future ultra-high density magnetic data storage media [1]. Depending on the preparation technique, however, the formation of the L10 thermodynamic equilibrium phase is often impeded by either a lack of thermodynamic driving forces or a lack of diffusivity.
Recently it has been shown [2,3] that gas phase prepared FePt nanoparticles can exhibit a very narrow size distribution with a mean diameter of roughly 6 nm. Together with a packing density of 2.8 × 10^12 particles /cm^2 a potential data storage density of 18 Tbit/inch^2 could be achieved. Unfortunately these particles are superparamagnetic at room temperature due to their multiply twinned icosahedral structure. Therefore it is of essential importance to transform these nanoparticles into the favourable L10 phase in a post-deposition treatment. Since annealing usually results in sintering of adjacent particles, a possible alternative is the use of ion irradiation techniques to create vacancies within the particles and thereby enhance the bulk diffusion of the constituents which has been successfully demonstrated to promote the L10 ordering in the case of FePd films [4].
In the present study, post-deposition 5 keV He irradiation has been performed on such gas phase prepared FePt nanoparticles. Structural characterization of the samples was carried out by means of High Resolution Transmission Electron Microscopy (HRTEM). Fig. 1 shows the HRTEM micrographs of two typical Fe42Pt58 particles with a mean diameterof dP = 6.2 nm. The power spectra as obtained by Fourier transform of the original images (see insets in Fig 1) clearly evidence that the particles are multiply twinned and of icosahedral structure. In Fig. 2, two typical HRTEM micrographs of particles are presented which had been subjected to irradiation with 5 keV He ions at a fluence of f = 3×10^17 ions/cm2. The particles are no longer multiply twinned but rather single crystal fcc, and their mean diameter is reduced to dP_irr = 5.2 nm. This size reduction is ascribed to irradiation induced sputtering of the particles. Although the defect concentration apparently increases the atomic diffusivity temporarily to an extent that allows for structural transformations in the FePt nanoparticles at RT, no L10 ordered particles have been observed. This observation is in contrast to the bulk phase diagram [5], theoretical calculations [6], and thermodynamic investigations in thin films [7], based on which the L10 phase is expected to be energetically favoured over the fcc phase by some 0.1 eV/atom.
There are two possible origins for the experimental findings. Either the particles investigated are already smaller than a critical particle size below which the L10 phase is no longer the thermodynamic equilibrium phase in FePt, or kinetic aspects may be of increasing importance at these length scales. Since the sputtering process that comes along with the ion irradiation is element specific and tends to remove more iron than platinum atoms, the He ion irradiation results in a shift of the elemental composition of the FePt nanoparticles towards a more Pt-rich concentration. As a consequence, the remaining alloy approaches the borderline of the L10
stability region in the phase diagram. Concurrently, the thermodynamic ordering temperature is reduced and the chemical driving force for the L10 ordering will be significantly reduced.

References
[1] D. Weller, A. Moser, L. Folks, M.E. Best, W. Lee, M.F. Toney, M. Schwickert, J.-U. Thiele, and M.F. Doerner, IEEE Trans. Mag. 36, 10 (2000).
[2] S. Stappert, B. Rellinghaus, M. Acet, and E.F. Wassermann, J. Cryst. Growth 252, 440 (2003).
[3] B. Rellinghaus, S. Stappert, M. Acet, and E.F. Wassermann, J. Magn. Magn. Mater. 266, 142 (2003).
[4] H. Bernas, J.-Ph. ...