Clusters of magnetic elements [1] exhibit very exciting properties in the frontier field of nanostructures. Of particular interest is the distinction between interior and interface contributions of large agglomerates embedded into a matrix. In order to confine small Co clusters in a silver matrix, cobalt was implanted at 50 keV in an Ag layer 50 nm thick, grown by molecular beam epitaxy (MBE), with a homogeneous concentration of 0.1 at%. Other methods were also used such as MBE co-evaporation of Ag and Co, and Co implantation during the silver layer growth, for dilutions up to 6 at%. The samples were observed at several temperatures (77, 125, 175, 225 K) by the XAFS technique at BM8, the GILDA CRG beamline, to investigate the local configuration around a Co absorber. The spectra, analysed by standard methods, clearly showed first, second, and third coordination shells around Co, with Co and Ag features very well resolved in each shell. From the coordination number and distance of each contribution we can easily deduce several important conclusions about the local aggregation of cobalt. Depending on the dilution and on the thermal treatment different scenarios can be drawn: the first is concerned with the presence of Co dimers slightly contracted in the matrix in quasi substitution configuration in the fcc silver lattice; they appear in a chainlike order with each dimer at 90° from each other, along opposite square faces of the silver lattice, as shown in Figure 117. This result was obtained for the as-prepared sample at low dilution. At higher concentration we detected small cobalt clusters as Co8 at 6 at% (co-evaporated sample) and Co20 at 1.9 at% (implanted sample). After suitable thermal treatment larger clusters can grow: we observed in fact Co80 and Co136 agglomerates. Here, the separation of the interface bonds from the homoatomic interactions is particularly valuable. The evolution of the raw absorption spectra in the near edge region demonstrates another interesting feature shown in Figure 118. A clear phase transition is visible from the comparison of the shape and features of a hcp Co foil with respect to the nanoaggregates and to the dimers. Whereas the spectrum of the largest Co cluster looks very similar to the Co foil spectrum, the dimers, in the fcc configuration of the silver lattice, present strongly shifted features, showing evidence of a fcc hcp phase transition as the number of atoms/cluster is larger than about 100. Furthermore, the gradual evolution, as the size of the clusters increases, towards the bulk cobalt structure could indicate the presence of fcc/hcp stacking faults and/or a combination of hcp clusters with a more diluted cobalt in Ag precursor stage. Finally, we point out that the vibrational behaviour of each configuration vs. temperature permitted the determination of the Debye temperature; we mention only the higher hardness of the Co-Co dimer bond with respect to Co-Ag, and the Debye temperature (200 K) of the interface bonds Co-Ag for the largest clusters.


Fig. 117: A possible configuration of 4 Co dimers in quasi substitutional positions in the silver fcc lattice. The dimers, contracted and disposed orthogonally to each other, form a kind of rotating chain on parallel square faces.

Fig. 118: Near-edge structure of the spectra showing dramatic modifications in the threshold features, clearly shifted with respect to the reference Co foil.

Reference
[1] J. Kortright et al., J. Magn. Magn. Mater. 207, 7 (1999).

Principal Publications and Authors
G. Faraci (a), A.R. Pennisi (a), A. Balerna (b), H.Pattyn (c), G.E.J. Koops (c) and G. Zhang (d), Phys. Rev. Lett. 86, 3566 (2001); Physics of Low Dimensional Systems, J.L. Morán-Lopez (Ed.), Kluwer Academic/Plenum Publishers, New York, 33-46, (2001).
(a) Dipartimento di Fisica, Università and Istituto Nazionale per la Fisica della Materia, Catania (Italy)
(b) Laboratori Nazionali Frascati, INFN, Frascati, (Italy)
(c) Instituut voor Kern- en Stralingsfysica, Phys. Dept., Leuven, (Belgium)
(d) Institute of Nuclear Research, Academy of Science, Shanghai, (China)