Magnetic Enhancement in Cobalt-Manganese Alloy Clusters

Magnetic moments of ${\mathrm{Co}}_N{\mathrm{Mn}}_M$ and ${\mathrm{Co}}_N{\mathrm{V}}_M$ clusters ($N\le 60$; $M\le N/3$) are measured in molecular beams using the Stern-Gerlach deflection method. Surprisingly, the per atom average moments of ${\mathrm{Co}}_N{\mathrm{Mn}}_M$ clusters are found to increase with Mn concentration, in contrast to bulk CoMn. The enhancement with Mn doping is found to be independent of cluster size and composition in the size range studied. Meanwhile, ${\mathrm{Co}}_N{\mathrm{V}}_M$ clusters show reduction of average moments with increasing V doping, consistent with what is expected in bulk CoV. The results are discussed within the virtual bound states model.

Figure 1

Part of the position-sensitive mass spectrum of ${\mathrm{Co}}_N{\mathrm{Mn}}_M$ clusters. Dashed line is the spectrum without magnetic field. Solid line is with magnetic field (0.98 T). Magnetic deflections cause the cluster mass peaks to shift to the right when the magnet is activated. The magnetization is determined from the deflections of the mass peaks. The compositions of the ${\mathrm{Co}}_N{\mathrm{Mn}}_M$ clusters as determined from the masses are denoted as ($N$, $M$).

Figure 2

Total magnetic moment of ${\mathrm{Co}}_N{\mathrm{Mn}}_M$ clusters as a function of $N$. Each series represents clusters with the same number of Mn atoms $N$. The experimental uncertainty is about $1.5{\mu }_B$.

Figure 3

Magnetic moments per atom of ${\mathrm{Co}}_N{\mathrm{Mn}}_M$ (a) and ${\mathrm{Co}}_N{\mathrm{Mn}}_M$ (b) clusters. Representative cluster sizes ($N+M$) are shown,; other clusters have similar behavior. The data for bulk CoMn are from 20. Dotted line is the expected magnetic moments assuming the VBS are above the Fermi level.

Figure 4

Normalized average moment for ${\mathrm{Co}}_N{\mathrm{Mn}}_M$ clusters. The contribution due to the electron deficiency of Mn compared to Co (i.e., 1.7 electrons per substituted atom) is subtracted. The normalized average moments only depends on the total number of atoms, which indicates the presence of the VBS below $E_F$. If the spin-up band of the VBS is completely below $E_F$ , then the strong magnetism picture applies and the enhancement is $2{\mu }_B$. We observe that the enhancement is $1.7{\mu }_B$, which is reasonably close to this prediction.