Structural evolution of free Co cluster magnetism J. Guevara a,,² , A.M. Llois a , F. Aguilera-Granja b , J.M. Montejano-Carrizales b a Departamento de Fı ´sica, Comisio ´n Nacional de Energı ´a Ato ´mica, Avda. del Libertador 8250, 1429 Buenos Aires, Argentina b Instituto de Fı ´sica, “Manuel Sandoval Vallarta”, Universidad Auto ´noma de San Luis Potosı ´, 78000 San Luis Potosı ´, S.L.P., Mexico City, Mexico Received 4 December 1998; accepted 31 March 1999 by C.E.T. Gonc ¸alves de Silva Abstract We present a systematic study of the average magnetic moments of free Co N clusters having different geometries: hexahedral, octahedral and decahedral. The spin-polarized electronic structure is calculated with a parameterized Hubbard Hamiltonian with spd electrons within the unrestricted Hartree–Fock approximation, and spillover effects are considered. We compare our calculations with other theoretical results found in the literature and we comment on the existing experimental results.  1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Nanostructures 1. Introduction Since the early 1980s the electronic and structural properties of small atomic clusters have been studied with a main goal in mind: namely to understand the evolution from atomic to bulk-like behavior as a func- tion of size. The two outputs result from the understand- ing of this key point; one is the fundamental knowledge of specific size-dependent cluster properties and the other is the possibility of technological applications. Both of them converge in one point, which is the novel and unexpected properties shown by small atomic clusters that can be considered of fundamental impor- tance for the design of the next generation high-tech electronic components. This has motivated the investi- gation of the structural, electronic and magnetic proper- ties of low-dimensional systems by many experimental and theoretical groups [1,2–22]. In the case of the magnetic properties most of the experimental attention has focused on 3d ferromagnets: Fe, Co and Ni. Current Stern–Gerlach techniques based on the deflection of a cluster beam due to its interaction with an external inho- mogeneous magnetic field allow one to determine the average magnetization. Pioneering work along this line has been performed by Billas et al. for Fe, Ni and Co clusters [1,2], and more recently by Apsel et al. for Ni clusters only [3]. For these 3d ferromagnets, a non- monotonic decrease of the average magnetization with increasing cluster size was observed (up to 700 atoms). This decrease comes along with oscillations displaying a strong reduction of the magnetization for certain sizes. These minima are in general related to geometrical closed-shell structures [1,2–4]. However, their structure depend on the growth techniques and the experimental setup, and thus a mix of several structures can be expected [5,9–11]. Experimental information coming, e.g. from magnetic measurements, chemical reactivity [8,9– 11], photo-ionization experiments [12,13], and Solid State Communications 111 (1999) 335–340 0038-1098/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S0038-1098(99)00189-1 PERGAMON * Corresponding author. Fax: +0541-754-7121. ² Present address: Department of Chemistry, Boston University, Boston, MA 02215, USA (guevara@bu.edu). E-mail address: guevara@cnea.gov.ar (J. Guevara)