1. Institut de Physique des Nanostructures, EPF-Lausanne, CH-1015 Lausanne, Switzerland
2. Institut für Festkörperforschung, Forschungszentrum Jülich, D-52425 Jülich, Germany
3. Berliner Elektronenspeicherring-Gesellschaft für Synchrotronstrahlung m.b.H., Albert-Einstein-Strae 15, 12489 Berlin, Germany
4. Max-Planck-Institut für Festkörperforschung, Heisenbergstr. 1, D-70569 Stuttgart, Germany
5. Istituto di Struttura della Materia, Consiglio Nazionale delle Richerche, Area Science Park, I-34012 Trieste, Italy

Correspondence to: P. Gambardella¹ Correspondence and requests for materials should be addressed to P.G. (e-mail: Email: pietro.gambardella@epfl.ch).

Abstract

Two-dimensional systems, such as ultrathin epitaxial films and superlattices, display magnetic properties distinct from bulk materials¹. A challenging aim of current research in magnetism is to explore structures of still lower dimensionality^{2, 3, 4, 5, 6}. As the dimensionality of a physical system is reduced, magnetic ordering tends to decrease as fluctuations become relatively more important⁷. Spin lattice models predict that an infinite one-dimensional linear chain with short-range magnetic interactions spontaneously breaks up into segments with different orientation of the magnetization, thereby prohibiting long-range ferromagnetic order at a finite temperature^{7, 8, 9}. These models, however, do not take into account kinetic barriers to reaching equilibrium or interactions with the substrates that support the one-dimensional nanostructures. Here we demonstrate the existence of both short- and long-range ferromagnetic order for one-dimensional monatomic chains of Co constructed on a Pt substrate. We find evidence that the monatomic chains consist of thermally fluctuating segments of ferromagnetically coupled atoms which, below a threshold temperature, evolve into a ferromagnetic long-range-ordered state owing to the presence of anisotropy barriers. The Co chains are characterized by large localized orbital moments and correspondingly large magnetic anisotropy energies compared to two-dimensional films and bulk Co.