Structural phase transformation of FePt nanoparticles by ion irradiation

Owing to its large magneto-crystalline anisotropy energy, L1_0 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. 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 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 x 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.
In the present study, post-deposition 5 keV He irradiation has been employed in order to perform the phase transformation of such gas phase prepared FePt nanoparticles. Structural characterization of the samples was carried out by means of High Resolution Transmission Electron Microscopy (HRTEM). After ion irradiation the previously multiply twinned icosahedral particles have transformed mostly into the single crystalline fcc structure. However, no indication for the chemically ordered L10 phase is found. This might be due to the fact that 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.