Multi-$L{1}_0$ Domain CoPt and FePt Nanoparticles Revealed by Electron Microscopy

The atomic structure of CoPt and FePt nanoparticles (with a diameter between 2 and 5 nm) has been studied by transmission electron microscopy. The particles have been produced by a laser vaporization cluster source and annealed under vacuum in order to promote chemical ordering. For both alloys, we observe a coexistence of crystalline and multiply twinned particles with decahedral or icosahedral shapes. In addition to particles corresponding to a single $L{1}_0$ ordered domain, we put into evidence that even small particles can display several $L{1}_0$ domains. In particular, the chemical order can be preserved across twin boundaries which can give rise to spectacular chemically ordered decahedral particles made of five $L{1}_0$ domains. The stability of such structures, which had been recently predicted from theoretical simulations, is thus unambiguously experimentally confirmed.

Figure 1

$C_s$-corrected HRTEM images of CoPt (left) and FePt (right) nanoparticles. The three different types of structure are observed. The arrows indicate (111) twins, which are also highlighted by dotted lines in the case of decahedral particles, in order to emphasize the fivefold symmetry.

Figure 2

$C_s$-corrected HRTEM images of a FePt cluster [(a) and (b)], corresponding to two different defocus values) and of a CoPt cluster (d). (c) Fast Fourier transform corresponding to image (a), with clear [001] and [110] peaks which are the signature of $L{1}_0$ order. In (a), the chemical order is evident from the difference in brightness and apparent size between two successive planes of atoms along the [001] direction, which is related to the difference in the scattering factors of Fe and Pt atoms.

Figure 3

(a) $C_s$-corrected HRTEM image of a CoPt particle displaying a continuous $L{1}_0$ chemical order over two (111) twins [indicated by arrows in (b)]. As a guide to the eyes, in (b) the atomic columns are colored according to their apparent size in the original image (a): the chemical order follows a “zig-zag” pattern (stressed by the green dashed line). (c) STEM-HAADF image (also called $Z$-contrast image) of a CoPt particle: the bright dots correspond to Pt-rich atomic columns. Three different $L{1}_0$ domains can be distinguished, as schematized in (d): the dots represent Pt-rich atomic columns, and are colored (in red, blue or green) according to their domain (intermediate colors correspond to dots common to two domains). The different $c$ axis orientations are also indicated.

Figure 4

(a), (b) HRTEM images of FePt particles with two $L{1}_0$ domains joined by a (111) twin. (c) $C_s$-corrected HRTEM image of a FePt particle with a (001) periodicity visible only on one side. This contrast corresponds to a chemically ordered decahedral particle [see Fig. 5b] viewed from the side, and is well reproduced by multislice HRTEM simulations (d) [53].

Figure 5

(a) HRTEM image of a FePt particle displaying simultaneously a fivefold symmetry and $L{1}_0$ ordered domains. (b) schematic view of the corresponding chemically ordered decahedral structure, predicted by theoretical calculations.

Figure 6

(a), (c) $C_s$-corrected experimental HRTEM images of two different chemically ordered decahedral FePt particles, for two defocus values. (b) Multislice image simulation, using the geometry shown in Fig. 5b with Fe atoms at the center, to be compared to images (a). Note the reverse contrast of the central atomic column which is made of Fe in (a) and of Pt in (c).