Computer
hard drives store data using magnetized cells that each consist of up
to a million atoms; the direction of the atoms’ collective magnetization
determines whether the cell holds a 1 or 0. Scientists would love to
coax individual atoms to store information, but single atoms are
unpredictable. For one thing, they continually exchange electrons with
their surroundings, causing their magnetization to vary from one moment
to the next. Latest research shows that individual atoms can store information for minutes at a time, as researchers report in the Nov. 14
Nature. That’s more than a billion times as long as any previous experiment with single atoms.
Single magnetic atoms, and assemblies of such atoms, on non-magnetic
surfaces have recently attracted attention owing to their potential use
in high-density magnetic data storage and as a platform for quantum
computing
1, 2, 3, 4, 5, 6, 7, 8.
A fundamental problem resulting from their quantum mechanical nature is
that the localized magnetic moments of these atoms are easily
destabilized by interactions with electrons, nuclear spins and lattice
vibrations of the substrate
3, 4, 5. Even when large magnetic fields are applied to stabilize the magnetic moment, the observed lifetimes remain rather short
5, 6
(less than a microsecond). Several routes for stabilizing the magnetic
moment against fluctuations have been suggested, such as using thin
insulating layers between the magnetic atom and the substrate to
suppress the interactions with the substrate’s conduction electrons
2, 3, 5, or coupling several magnetic moments together to reduce their quantum mechanical fluctuations
7, 8.
Here we show that the magnetic moments of single holmium atoms on a
highly conductive metallic substrate can reach lifetimes of the order of
minutes. The necessary decoupling from the thermal bath of electrons,
nuclear spins and lattice vibrations is achieved by a remarkable
combination of several symmetries intrinsic to the system: time reversal
symmetry, the internal symmetries of the total angular momentum and the
point symmetry of the local environment of the magnetic atom.
Researchers including Wulf Wulfhekel, a physicist at
the Karlsruhe Institute of Technology in Germany, set out to minimize
atoms’ interaction. They placed atoms of the element holmium on a
platinum surface. Surveying the atoms one by one with a supersensitive
microscope, Wulfhekel’s team found that the holmium atoms got wedged
between atoms of platinum, making it difficult for holmium to gain or
lose electrons. The holmium atoms had consistent magnetic strength and
direction for an average of 10 minutes, besting the previous record of
200 billionths of a second.
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