Magnetism and chemical ordering in binary transition metal clusters

Binary XY clusters, with X,Y∊{Mn,Fe,Co,Ni}, are investigated within first-principles simulations in the framework of density-functional theory. In the case of Co-Mn, the increase in magnetic moment with Mn concentration, which is in contrast to the bulk behavior and was also observed experimentally, is explained on the basis of the electronic structure of the clusters. For Co-Fe, Ni-Mn, Ni-Fe, and Ni-Co, due to the ferromagnetic couplings between the local moments, the same dependence is found, i.e., moments increasing with the concentration of the element with the lower value of atomic number. For Fe-Mn (as well as Co-Mn and Ni-Mn clusters containing many Mn atoms), the situation appears to be more complex with antiferromagnetic tendencies resulting in a nonmonotonic change of the cluster moment with Mn concentration.