The isotropic magnetic moment of a free atom is shown to develop giant magnetic anisotropy energy due to symmetry reduction at an atomically ordered surface. Single cobalt atoms deposited onto platinum (111) are found to have a magnetic anisotropy energy of 9 millielectron volts per atom arising from the combination of unquenched orbital moments (1.1 Bohr magnetons) and strong spin-orbit coupling induced by the platinum substrate. By assembling cobalt nanoparticles containing up to 40 atoms, the magnetic anisotropy energy is further shown to be dependent on single-atom coordination changes. These results confirm theoretical predictions and are of fundamental value to understanding how magnetic anisotropy develops in finite-sized magnetic particles.

Maximizing atomic magnetic memory: A study of the magnetic response of cobalt atoms adsorbed on oxide surfaces may lead to much denser storage of data. In hard drives, data are stored as magnetic bits; the magnetic field pointing up or down corresponds to storing a zero or a one. The smallest bit possible would be a single atom, but the magnetism of a single atom —its spin—has to be stabilized by interactions with heavy elements or surfaces through an effect called spin-orbit coupling. Rau et al. (see the Perspective by Khajetoorians and Wiebe) built a model system in pursuit of single-atom bits—cobalt atoms adsorbed on magnesium oxide. At temperatures approaching absolute zero, the stabilization of the spin's magnetic direction reached the maximum that is theoretically possible. Science , this issue p. 988 ; see also p. 976

Stable magnets from single atoms: An important goal in molecular magnetism is to create a permanent magnet from a single atom. Metal atoms adsorbed on surfaces can develop strong magnetization in an applied field (paramagnetism). Donati et al. show that single holmium atoms adsorbed on a magnesium oxide film grown on a silver substrate show residual magnetism for temperatures up to 30 K and bistabilty that lasts for 1500 s at 10 K (see the Perspective by Khajetoorians and Heinrich). The atom avoids spin relaxation by a combination of quantum-state symmetry and by the oxide film preventing the spin from interacting with the underlying metal via tunneling. Science , this issue p. 318 ; see also p. 296

The morphology of the Ag(110) surface after ${\mathrm{Ar}}^{+}$ ion sputtering has been studied by a variable temperature scanning tunneling microscope. For ion energies greater than 800 eV and in the temperature range $270<\mathrm{}{T}_S<320\mathrm{}\mathrm{K}$ (where $T_S$ is the sputtering temperature), a peculiar ripple structure well aligned along the $〈1\overline{1}0〉$ direction and independent of the ion beam angle has been observed. A simple model of this effect, based on the anisotropy and on the temperature hierarchy of the Ag adatom and vacancy surface diffusion, explains the observed features.