Magnetic properties of small 3d and 4d transition metal clusters:: The role of a noncompact growth

Within the framework of a self-consistent spd tight-binding Hamiltonian and ab initio pseudopotential techniques we study the magnetic properties of small Co-N, Ni-N, Rh-N, and Pd-N (N=4-21) clusters having noncompact structures. Following the recently reported existence of a nonicosahedral growth for small Rh clusters, as well as the predicted high stability of open (biplanar) structures for the late 3d and 4d 13-atom transition metal clusters, we consider different growth sequences based in the atomic capping of small half-octahedral and tetrahedral cluster units, which can be considered in general as precursors of the lowest energy configurations reported by Bae et al. [Phys. Rev. B 70, 195413 (2004)] and Chang and Chou [Phys. Rev. Lett. 93, 133401 (2004)]. On the one hand, for noncompact unrelaxed Co-N and Ni-N clusters, which are elements characterized by a large exchange regime, we obtain a slowly decreasing average magnetization <mu > with increasing the size of the clusters. In addition, for a given cluster size, the value of <mu > for different isomers has been found to be determined in general by the average coordination number of the structures, i.e., less (more) coordinated structures have high (low) magnetization states due to d band narrowing effects. On the other hand, for unrelaxed Rh-N and Pd-N structures, which are defined as weakly ferromagnetic systems, a complex decreasing oscillatory behavior is obtained with average magnetic moments per atom which are in general in reasonable agreement with the measured data. Interestingly, in the case of Rh-N clusters for N >= 15, the suggested biplanarlike structures are characterized by values of <mu > of the order of 0.05 mu(B)/atom, in clear disagreement with experiment, a result that could be interpreted as the beginning of a different structural growth sequence where clusters need to rearrange themselves towards different structures with higher magnetization states. As expected, fully relaxed clusters leads to the existence of atomic configurations with a complex nonuniform distribution of interatomic distances that reduces in general the value of <mu >. We conclude that the normally observed oscillatory magnetization data in cluster systems could be induced by a growth behavior in which cluster geometries flip-flop between low and high symmetry atomic arrays.