Structural and magnetic properties of $\mathrm{CoPt}$ mixed clusters

In this present work, we report a structural and magnetic study of mixed ${\mathrm{Co}}_{58}{\mathrm{Pt}}_{42}$ clusters. A $\mathrm{MgO}$, Nb, and Si matrix can be used to embed clusters, avoiding any magnetic interactions between particles. Transmission electron microscopy observations show that ${\mathrm{Co}}_{58}{\mathrm{Pt}}_{42}$ supported isolated clusters are about $2\mathrm{nm}$ in diameter and crystallized in the $A1$ fcc chemically disordered phase. Grazing incidence small-angle x-ray scattering (GISAXS) and grazing incidence wide-angle x-ray scattering (GIWAXS) reveal that buried clusters conserve these properties, interaction with matrix atoms being limited to their first atomic layers. Considering that 60% of particle atoms are located at surface, this interactions leads to a drastic change in magnetic properties that were investigated with conventional magnetometry and x-ray magnetic circular dichroism. Magnetization and blocking temperature are weaker for clusters embedded in Nb than in $\mathrm{MgO}$, and totally vanish in silicon as silicides are formed. Magnetic volume of clusters embedded in $\mathrm{MgO}$ is close to the crystallized volume determined by GIWAXS experiments. Cluster can be seen as a pure ferromagnetic $\mathrm{CoPt}$ crystallized core surrounded by a cluster-matrix mixed shell. The outer shell plays a predominant role in magnetic properties, especially for clusters embedded in niobium that have a blocking temperature three times smaller than clusters embedded in $\mathrm{MgO}$.

Figure 1

(a) Micrograph of ${\mathrm{Co}}_{58}{\mathrm{Pt}}_{42}$ clusters. Inset: ${\mathrm{Co}}_{58}{\mathrm{Pt}}_{42}$ size distribution [abscissa: Diameter (in nanometers), ordinate: Number of particles] and log-normal fit curve (mean diameter $D_m=2.1\mathrm{nm}$, dispersion $\sigma =0.35$). (b) Electron diffraction diagram of ${\mathrm{Co}}_{58}{\mathrm{Pt}}_{42}$ cluster film. The fcc $A1$ phase diffraction rings are indexed.

Figure 2

(a) HRTEM micrograph of ${\mathrm{Co}}_{58}{\mathrm{Pt}}_{42}$ cluster. (b) Fourier transform of the micrograph of the cluster shown in (a). Crystallographic planes are indexed. Interreticular distances and angles between planes correspond to $A1$ phase. (c) Representation of a truncated octahedron containing 201 atoms. The $\left[100\right]$, $\left[111\right]$, and $\left[1\overline{1}\overline{1}\right]$ directions (arrows) are reported.

Figure 3

GISAXS signal of the ${\mathrm{Co}}_{58}{\mathrm{Pt}}_{42}$clusters in $\mathrm{MgO}$ matrix.

Figure 4

Comparison of GISAXS simulation and measure: ${\mathrm{Co}}_{58}{\mathrm{Pt}}_{42}$ clusters buried in $\mathrm{MgO}$ matrix (a) or Si matrix (b).

Figure 5

Comparison of GISAXS simulation and measure: ${\mathrm{Co}}_{58}{\mathrm{Pt}}_{42}$ supported clusters with TEM parameters (a) or with a bimodal distribution (b).

Figure 6

GIWAXS measures and fits for ${\mathrm{Co}}_{58}{\mathrm{Pt}}_{42}$ clusters embedded in $\mathrm{MgO}$ (a) and Si matrix (b) respectively.

Figure 7

XMCD measure on $\mathrm{CoPt}$ clusters embedded in $\mathrm{MgO}$ matrix at the cobalt threshold. (a) Left (plain line) and right (dashed line) polarization spectra. (b) Dichroic signal (plain line) and integrated dichroic signal (dashed line).

Figure 8

XMCD measurements on $\mathrm{CoPt}$ clusters embedded in Nb matrix (a) and Si matrix (b) at the cobalt threshold.

Figure 9

Zero field cooled measurements of ${\mathrm{Co}}_{58}{\mathrm{Pt}}_{42}$ clusters embedded in $\mathrm{MgO}$ matrix $(T_{\mathrm{m}}\approx 40\mathrm{K})$ and Nb matrix $(T_{\mathrm{m}}\approx 14\mathrm{K})$. Lines are guides for the eye.