Evidence of $L{1}_0$ chemical order in CoPt nanoclusters: Direct observation and magnetic signature

We report the synthesis and characterization of well-defined CoPt clusters with a mean diameter of 3 nm, produced in ultrahigh vacuum conditions following a physical route. Samples made of diluted layers of CoPt clusters embedded in amorphous carbon have been studied by transmission electron microscopy. High-resolution observations have revealed the appearance of $L{1}_0$ chemical order upon annealing, even for clusters with a 2 nm diameter, without cluster coalescence. The magnetic properties of both chemically disordered and ordered CoPt clusters embedded in amorphous carbon have then been measured by x-ray magnetic circular dichroism and superconducting quantum interference device magnetometry. Despite a striking change of the Co magnetic moment, the magnetic anisotropy of chemically ordered nanoparticles increases, with respect to the chemically disordered $A1$ phase, in much lower proportions than what is observed for the bulk.

Figure 1

TEM images of a 2D film of CoPt clusters (equivalent thickness $\simeq 0.5\text{Å}$) embedded in amorphous carbon, as prepared (a) and after annealing (b). The size histograms deduced by TEM images analysis, together with the best fits corresponding to a log-normal size distribution, are shown in the inset. The fit parameters [see Eq. (1)] are $d_m=2.58\text{nm}$, $w=0.42$ and $d_m=2.67\text{nm}$, $w=0.47$, respectively, for as-prepared and annealed clusters.

Figure 2

HRTEM images of CoPt clusters of 2D films on amorphous carbon. (a) As-prepared clusters in the chemically disordered fcc $A1$ phase. (b,c) Annealed clusters: Some of them are in an orientation where $L{1}_0$ chemical order is visible, whereas some others appear as chemically disordered crystals (c).

Figure 3

(a) Fourier transform corresponding to the HRTEM image of the annealed CoPt cluster shown in (b). The (001) peak is the signature of $L{1}_0$ chemical order corresponding to the strong contrast modulation visible in (b). (c) HRTEM image simulation obtained with a perfectly $L{1}_0$ chemically ordered CoPt cluster, observed along the [010] direction, with a slight tilt.

Figure 4

Small annealed CoPt clusters, around 2 nm in diameter, showing a $L{1}_0$ chemical order contrast: (110) in the case of image (a) and (001) for images (b,c,d). Samples corresponding to images (c) and (d) are capped with an a-C layer.

Figure 5

(Color online) (a) XAS spectra at the Co $L_3$ and $L_2$ edges for the as-prepared and annealed samples. (b) XAS signals (${\mu }_{-}$ and ${\mu }_{+}$ corresponding to the two different circular polarizations) for the as-prepared sample, and the corresponding XMCD curve $\left({\mu }_{-}-{\mu }_{+}\right)$.

Figure 6

Hysteresis loops of the as-prepared (a) and annealed (b) sample, at 300 K $(○)$ and 10 K $(●)$. The solid line corresponds to a fit using a Langevin function, with a log-normal clusters size distribution (parameters given in Table ). ZFC-FC magnetization curves of the as-prepared (c) and annealed (d) sample. The solid line corresponds to a fit using a theoretical semianalytical model (see text), with the same clusters size distribution as for the high temperature cycles fit.