Structural, electronic, and magnetic properties Of Co(n)Cu(m) nanoalloys (m + n = 12) from first principles calculations.

Using the generalized gradient approximation (GGA) to density functional theory (DFT), we compute the electronic structure and related magnetic properties of free-standing Co(12-x)Cu(x) clusters (x = 0-12) with structures resulting from the optimization of those of the low-lying energy isomers of pure Co(12) and Cu(12) in which Co(Cu) were replaced by Cu(Co) atoms. Structural transitions for the lowest energy homotop are obtained as a function of the concentration, but in all cases, a clear surface segregation of Cu is found in the low concentration regime x < 5. The binding energy decreases monotonically when x increases. The dipole moment changes abruptly from 0.06 D for x = 2 to 0.59 D for x = 3 in coincidence with a structural change. The electronegativity of the lowest energy homotop exhibits minimum (maximum) value for x = 11 (x = 9). The x = 5, 9 clusters show local maxima of the hardness, of the excess energy, and of the second difference in energy, clear indicators of specially stable stoichiometries. The magnetic behavior of Co(12-x)Cu(x) is a monotonous function of the Co concentration, decreasing by steps of ≥2 μ(B) for every Co atom that is replaced by Cu, although for certain concentrations, different spin isomers, sometimes accompanied by structural transitions, are found close to the ground state. Ferromagnetic-like order is obtained as the ground state in all cases, contrary with the trend found in binary clusters of the same elements by other authors who predicted antiferromagnetic order. We analyze in detail the possible spin excitations in Co(12)Cu to demonstrate that local antiferromagnetic couplings can only exist as metastable spin states.