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Abstract

Two-dimensional systems, such as ultrathin epitaxial films and
superlattices, display magnetic properties distinct from bulk
materials(1). A challenging aim of current research in
magnetism is to explore structures of still lower
dimensionality(2-6). As the dimensionality of a physical system
is reduced, magnetic ordering tends to decrease as fluctuations
become relatively more important(7). 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(7-9). 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.