Structure and magnetism of cobalt clusters

Abstract

The lowest-energy geometric structures and isomers of freestanding Co N clusters (4рNр60) and their corresponding magnetic moments are calculated using an evolutive algorithm based on a many-body Gupta potential and a self-consistent spd tight-binding method, respectively. We found an icosahedral growth pattern for the global minimum with some hcp and fcc structures for some particular sizes, whereas for the second isomer, distorted icosahedral structures are obtained in general. With the aim to study the possible coexistence of isomers within the experimental cluster beam we assumed an equilibrium distribution and calculated for each cluster size the different coexistent structures and the relative populations at room temperature. Our results show that the coexistence is present only at some particular sizes, in agreement with chemicaladsorption and photoionization experiments. Our self-consistent tight-binding calculations considering 3d, 4s, and 4p valence electrons for the magnetic properties show that the magnetic moments for the global minima and the second isomers are in general very similar except in a small region of 20ϽNϽ40 atoms where the magnetic moment of the global minimum is smaller than that of the second isomer. We compare our results for the magnetic behavior of the global minimum with theoretical calculations available in the literature as well as with experimental results.

Structure, bonding, and magnetism of cobalt clusters from first-principles calculations

“…In the present calculation, the optimal pentagonal pyramid lies much higher (1.7 eV) in energy compared to the ground state and also has 12 µ B magnetic moment. Present result is in agreement with previous theoretical studies, 12,16,30,34 and the octahedral structure is generally accepted as the most stable structure for Co 6 cluster. However, photoelectron spectroscopic study 33 predicted a pentagonal pyramid with bond distances ∼ 2.75 ± 0.1Å to be the most probable structure for Co − 6 anion cluster, i.e., the geometrical structure might strongly be correlated with the charged state of the cluster.…”

“…However, the size of the clusters were limited only up to 5 atoms. Recently, Lopez et al 16 studied Co n clusters (4 n 60), where minimization was done using an evolutive algorithm based on a many-body Gupta potential 17 and magnetic properties have been studied by a spd tight-binding method. As compared to abinitio methods, the parameterized tight-binding Hamiltonian reduces the computational cost drastically, but its main problem is the lack of transferability of its parameters.…”

Dynamic stabilities of icosahedral-like clusters and their ability to form quasicrystals

“…The Gupta potential is a semi-empirical potential based on the second-moment approximation of the tight-binding model. It has been widely applied in studying the geometrical structures 31,32 and dynamic behavior 33 The latter has the same number of atoms as that of the closed-shell structure shown in Fig. 1.…”

Molecular dynamical simulations of melting behaviors of metal clusters

“…The Gupta potential is a semi-empirical potential based on the second approximation of tight binding model. It has been widely applied to study the geometrical structures 25,26 and dynamical behaviors 27 of metal and alloy clusters. The formula of the Gupta potential can be written as below: 28…”