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10 pages, 7 figures.-- PACS nrs.: 57.10.Pq; 75.30.-m; 75.30.Gw; 78.70.Dm.-- Published online Oct 22, 2005., We present a combined experimental and theoretical study of the magnetization of one-dimensional atomic cobalt chains deposited on a platinum surface. We discuss the intrinsic magnetization parameters derived by X-ray magnetic circular dichroism measurements and the observation of ferromagnetic order in one dimension in connection with the presence of strong, dimensionality-dependent anisotropy energy barriers of magnetocrystalline origin. An explicit transfer matrix formalism is developed to treat atomic chains of finite length within the anisotropic Heisenberg model. This model allows us to fit the experimental magnetization curves of cobalt monatomic chains, measured parallel to the easy and hard axes, and provides values of the exchange coupling parameter and the magnetic anisotropy energy consistent with those reported in the literature. The analysis of the spin–spin correlation as a function of temperature provides further insight into the tendency to magnetic order in finite-sized one-dimensional systems., We would like to acknowledge the contribution to the experiments reported in this work by A. Dallmeyer, K. Maiti, M.C. Malagoli, and W. Eberhardt (Forschungszentrum Jülich), P. Ohresser, S.S. Dhesi, N.B. Brookes, and K. Larsson (beamline ID12B at the ESRF), and K. Kern (EPF Lausanne and MPI Stuttgart). A. Vindigni, A. Rettori, and M.G. Pini acknowledge financial support from the Italian MIUR (projects FISR ‘Nanotecnologie per dispositivi di memoria’ and FIRB ‘Microsistemi e nanomateriali magnetici’).

7 pags. ; 7 figs. ; 1 tab., Structured nanocomposite films consisting of five Fe layers embedded in an amorphous Al2O3 matrix (Fe:Al₂O₃) have been grown by sequential pulsed laser deposition. The formation of well isolated quasispherical nanocrystals is observed for samples with Fe content per layer close to 6.5×10¹⁵ atoms/cm². Increasing the Fe content leads first to the formation of elongated nanocrystals and then to quasicontinuous layers. The evolution in the shape and size of the nanocrystals is reflected in the magnetic behavior of these systems. A crossover from a low temperature ferromagnetic regime to a high temperature superparamagnetic regime is observed at a temperature of 23 K in the samples containing isolated quasi-spherical nanocrystals. In this case, a reduced moment per Fe atom (1.4 μβ/atom) with respect to the value for α-Fe (2.2 μβ/atom) is estimated. This behavior is attributed to the presence of a Fe-oxide surface shell on the nanocrystals. The large values of the estimated effective magnetic anisotropy (1.4×10⁶ J/m³) and the low temperature coercivity in these samples are attributed to a strong surface contribution to anisotropy, whereas the temperature dependence of coercivity is attributed to thermal activation., This work has been partially supported by the European Union (BRITE Project No. 98-0616) and the Spanish CICYT~TIC Project No. 99-0866. One of the authors (N. M. D.) had a Marie Curie fellowship from the European Union. The authors acknowledge the support of the French and Spanish governments (HF1998-0055) and the British Council-Spanish government (HB1998-0182) Integrated Actions scheme for travel.