Magnetic Anisotropy Energy of Transition Metal Alloy Clusters

The binary clusters of transition metal atoms form an interesting platform for studying the effects of shape, size, chemical compositions, and ordering on its magnetic properties. Notably, mixed clusters often show higher magnetic moments compared to pure elemental clusters. Due to the reduced dimension of the clusters, they tend to behave as single domain particles. One important parameter of their magnetic behavior is the magnetic anisotropy energy. In this chapter we review previous works on the magnetic anisotropy energy of binary alloy clusters, along with a density functional theory based method to compute the anisotropy energy with applications to binary metal clusters. The clusters of transition metal atoms often show high spin moments but generally are also reactive with the environment. Passivation of the surface atoms can lead to more stable clusters. We have explored one such avenue for passivation in this work. We consider the As@Ni12@As20 cluster which in the neutral state has a magnetic moment of 3 μB. We dope this cluster by substituting various numbers of Ni atoms by Mn atoms. The substitutional doping leads to spin moments located mostly on the Mn atoms. The doping also leads to symmetry breaking and as a consequence the number of structural isomers and spin ordered states for each isomer becomes very large. We have investigated all possible ferromagnetic isomers for a given number of dopants and subsequently all the possible anti-ferromagnetic states for the lowest energy structure were examined. The results show that the encapsulation within the As20 cage stabilizes the clusters and the atomization energy of the clusters increases as the number of dopant increases. These clusters have small energy barrier for reversal of magnetization and also have rich variation in configuration and spin states with many low-lying spin states.