Magnetism in binary and encapsulated Co-Mn clusters

Two types of Co-Mn clusters are investigated in calculations within the framework of density-functional theory. First, we consider Co clusters with 19 and 38 atoms and study the effect on the magnetization of single and double substitution by Mn atoms. In all cases the preferred configurations have the Mn aligning parallel with the Co moments resulting in an enhancement of the cluster magnetization that is consistent with experimental observation. Second, we consider Co clusters encapsulated in a shell of Mn and again examine the effect of the Mn on the Co magnetization. The Co moment at the center of the core is somewhat smaller than that of bulk Co while the net moment on the Co atoms at the interface with the Mn is significantly reduced. A reduction in the Co moment is also observed in experimental measurements. In order to match the level of reduction seen experimentally, it is necessary to postulate some degree of alloying.