Observing spin polarization in single atoms

Understanding the behavior of individual atomic spins in condensed-matter environments is important for the development of spintronics, quantum-information systems, and other novel applications. In a new advance toward that understanding, physicists from the University of California, Berkeley, and the US Naval Research Laboratory have now directly observed the spin polarization of individual magnetic iron and chromium atoms adsorbed on a surface. Such measurements have proved challenging in the past because of the low energy barrier for spin fluctuations. The team overcame that difficulty by sprinkling atoms on islands of ferromagnetic cobalt; the image here shows Fe adatoms as green protrusions on the triangular Co islands. The coupling between the adatoms and the Co kept the adatom spins stationary at the experiment’s low temperatures. The researchers probed the spin orientation of adatoms using a scanning tunneling microscope outfitted with a magnetic tip. When the tip was over an adatom, the tunneling current depended on the relative polarization directions of the Cr tip, adatom, and Co island. The Fe and Cr adatoms were found to exhibit opposite coupling behavior, in agreement with theoretical calculations: The Fe atoms aligned ferromagnetically (spins parallel) with the Co island, and the Cr atoms aligned anti-ferromagnetically (spins antiparallel). (Y. Yayon et al., Phys. Rev. Lett. 99 , 067202, 2007 http://dx.doi.org/10.1103/PhysRevLett.99.067202 .)