Observing Magnetic Polarization in Single Atoms

Physicists from UC Berkeley and the Naval Research Lab have measured the spin properties of individual atoms added to a metal surface. They do this by first forming nm-sized triangular islands of cobalt on top of a copper crystal. The cobalt is ferromagnetic, which means that the spins of the cobalt atoms in the islands all line up together (half of the islands have their collective spins pointing up, while the other half point down).

Additional magnetic atoms sprinkled on top of the islands (adatoms) have spins that interact magnetically with the underlying cobalt, causing the adatom spins to either align or anti-align with the underlying island spins. Thus when a small amount of iron atoms (chromium atoms were also used) are dropped onto the islands they immediately become oriented (polarized) by contact with a cobalt island.

In this way isolated atoms (up to 5 nm apart) were prepared with a definite spin polarization state (see figure at http://www.aip.org/png/2007/285.htm). Next the quantum energy levels of the magnetic adatoms were studied using the tip of a scanning tunneling microscope (STM) which itself had been magnetized.

The quantum energy levels of the iron and chromium adatoms were sampled by observing currents flowing from the adatoms into the STM tip. Current measured in this way will be larger or smaller depending on whether the spin polarization of the tip is aligned with or against the polarization of the individual magnetic adatoms being probed. The adatom energy states are seen to differ for spin-up and spin-down states, indicating that iron and chromium atoms couple magnetically to cobalt with opposite polarity.

One of the researchers, Michael Crommie of UCB (crommie@berkeley.edu, 510-642-9392), says that it is still too early to try to store data in the form of individual polarized atoms. Rather they are seeking to understand how the spin of a single atom is influenced by its environment, with an eye toward future spintronics and quantum information applications. (Yayon et al., Physical Review Letters, 10 August 2007; lab website, http://physics.berkeley.edu/research/crommie/)