Two-dimensional systems, such as ultrathin epitaxial films and superlattices, display magnetic properties distinct from bulk materials. A challenging aim of current research in magnetism is to explore structures of still lower dimensionality. As the dimensionality of a physical system is reduced, magnetic ordering tends to decrease as fluctuations become relatively more important. 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. 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.

The temperature dependence of the rare-earth valence bands has been regarded as a realization of the Stoner behavior. The exchange splitting of the electronic states appears to scale as the magnetic order parameter for T , T C and to vanish at T T C . We report here a spin-resolved photoemission study on the evolution of Gd bulk bands for 0.5 # T ͞T C # 1. The spin-polarized spectral line shapes display a complex temperature dependence, which clearly contrasts with the interpretation of previous experimental results. The spin-resolved photoemission data demonstrate the inadequacy of the Stoner model to the description of magnetism in rare earths. DOI: 10.1103/PhysRevLett.88.167205 PACS numbers: 75.70.Ak, 75.25. +z, 79.60.Bm Understanding the relation between electronic and magnetic properties at finite temperatures is a central question for many branches of solid state physics. While the electronic theory accurately accounts for the ground state properties of magnetic materials, it meets only limited success in describing their finite temperature behavior, such as the magnetic phase transitions, the corresponding ordering temperatures, and critical parameters.Two simple models are usually considered to schematically describe the finite temperature behavior of the electronic structure, depending on the degree of the electron localization. The Stoner model [1] predicts that the exchange splitting of delocalized states parallels the temperature dependence of the magnetization. In this case, the exchange-split electronic subbands gradually merge together and become degenerate at the Curie temperature, T C . The spin-mixing model [2], on the other hand, describes strongly localized systems where thermal fluctuations of the local magnetic moment directions reduce the magnetization, but leave the magnitude of the local moments and the local electronic structure unchanged. Thus, the exchange splitting of the electronic states does not vary according to this model, while the characteristic spin polarization of the states is gradually lost on approaching T C .Several experimental studies have examined the finite temperature magnetism in the elemental ferromagnets. The itinerant 3d-transition metals (Fe, Co, and Ni) show an intermediate behavior. The complexity of the temperature dependent effects on the electronic structure in these systems defies any simple description [3][4][5]. Rare earths, on the other hand, provide a different and apparently simpler scenario. The magnetic moment in these systems primarily originates from the unfilled 4f subshell, which displays a simple spin-mixing behavior due to its atomiclike character. The magnetic coupling between the 4f moments occurs via the highly delocalized (5d6s)-valence states [6]. Various investigations by spin-integrated photoemission spectroscopy (PES) and inverse photoemission spectroscopy (IPES) report that the exchange splitting of the bulk valence bands vanishes at T C [7][8][9][10][11]. These experimental results have been interpreted as a first evidence for t...

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One-dimensional Co atomic wires grown on Pt(997) have been investigated by x-ray magnetic circular dichroism. Strong changes of the magnetic properties are observed as the system evolves from 1D- to 2D-like. The easy axis of magnetization, the magnetic anisotropy energy, and the coercive field oscillate as a function of the transverse width of the wires, in agreement with theoretical predictions for 1D metal systems.

The dispersion of the Gd ͑5d6s͒-valence bands has been investigated by means of spin-and angleresolved photoemission. The spin analysis of various spectral features shows that their weak dispersion and unusual broadening is due to the photoelectron lifetime rather than to correlation induced band narrowing as previously proposed. These results resolve a long-standing discrepancy between theoretical and experimental descriptions of the rare earth band structure. DOI: 10.1103/PhysRevLett.86.2846 The unique electronic and magnetic properties of lanthanides result from the coexistence of both highly localized and delocalized levels in the valence region. While the 4f electrons maintain an atomic character in these metals, the (5d6s) states form the metallic bonding and mediate the indirect exchange interaction among the 4f moments.Photoemission spectroscopy gives direct access to the excited-and ground-state configurations of the 4f shell that are indeed well understood for the lanthanide metals [1]. On the contrary, no satisfactory picture of the (5d6s) states exists so far. Even fundamental questions such as the accuracy of the band structure methods for the ground-state description or the influence of many-body effects on the excitation spectra have not yet found an answer. In fact, band structure calculations based on the local spin density approximations (LSDA) predict a large bandwidth for the (5d6s) states in rare earths similar to that of the preceding elements (alkali and alkaline earth metals) in the Periodic Table. These latter cases [2-5] exhibit strongly dispersive photoemission features that are reasonably well interpreted within the paradigms based on effective single-particle approximations, although a quantitative description requires detailed consideration of many-body effects. In sharp contrast, angle-resolved photoemission from lanthanides does not display the expected dispersive behavior of the (5d6s)-bulk states. Various observations [6][7][8] seem to indicate a failure of the effective singleparticle descriptions to the (5d6s) photoemission and suggest the importance of correlation effects.The situation is well illustrated by the case of Gd, which is the most extensively studied lanthanide element for its importance as a prototypical Heisenberg ferromagnet (T C 293 K). Photoemission studies report the energy separations between the G 4 2 2 G 1 1 points [9] and G 4 2 2 A 1 points [8] to be by about a factor of 2 smaller than that predicted by the state-of-the-art calculations [10,11] [shown in Fig. 1(a) for the ferromagnetic ground state [12] ]. Correspondingly, only a significantly weak energy dispersion of the D 2 band is observed by varying the photon energy [8,9]. While this latter observation, separately taken, could possibly be explained by momentum broadening [2], the concomitant discrepancy between the experimental and theoretical bandwidth leads to attribute [8] both the effects to strong electron-electron interactions. Notably, such band narrowing in Gd appears to be much larger than ...