Anomalous magnetic enhancement by doping Mn in Co clusters

Magnetic moments of Co$_N$Mn$_M$ and Co$_N$V$_M$ clusters (with Mn and V concentration less than 30\%) are measured using Stern-Gerlach deflection method in molecular beams. Their average moments per atom as a function of impurity concentrations are studied and compared with bulk. Both Co$_N$Mn$_M$ and Co$_N$V$_M$ clusters have higher average moments than their bulk counterpart due to reduced sizes. The average moments for Co$_N$Mn$_M$ clusters are found to increase with Mn concentration, opposite to that of CoMn bulk. The enhancement effect is found to be independent of cluster size and composition. Meanwhile, Co$_N$V$_M$ clusters show reduction of average moments with V doping, consistent with what are expected in CoV bulk. The results are discussed within the virtual bound states model.