cific number of atoms aligned antifer-romagnetically, the atoms’ tendency
to exhibit quantum superpositions
diminished greatly. The second was
that they could calculate the number
of atoms it would likely take to create
a stable storage mechanism at room
The fundamental property the antiferromagnets bring to scaling storage
down so small is that their defining
property—in which the atomic spin
of each atom is opposite to instead of
aligned with its next-door neighbor—
can eliminate the problem inherent
with trying to pack traditional ferromagnetic bits, whose magnetic spins
are all aligned, closer together. The
problem is that, as the magnetically
aligned atoms get closer together, the
magnetic field of each is more likely to
affect or be affected by its neighbor. Interestingly enough, Heinrich says the
antiferromagnetic nature of the structure was serendipitous.
“We didn’t set out saying ‘Let’s use
antiferromagnets because that would
be so cool,’” he says. “It was more like,
as quite often happens in this kind of
exploratory science, you try things out
and go with whatever nature wants
you to do with it. At least, in principle,
if you have this kind of control that we
have here, you can pack magnetic bits
much more densely together without
them interfering with each other. The
antiferromagnetism allows you to cancel out the interaction between the
bits. It wasn’t designed from the beginning that way, but that’s in the end
what is probably the most interesting
technological application of this.”
Silicon Plays nice
While the IBM project demonstrated a realistic possibility of much
denser storage capabilities at room
temperature, the UNSW project offered proof of concept of not only the
ability to control the placement of a
single-atom transistor with scientifically significant accuracy, but also the
ability to use industry-standard silicon, instead of a less universally applicable or economically viable material,
as the device’s substrate. However,
the UNSW researchers also discovered they could not employ atom relocation through STM manipulation
alone. They needed to find a way to
the Wales and
provide a bridge to
and a simultaneous
the likelihood of
place atoms upon the silicon and then
etch away all but the single atom they
needed to stabilize.
Michelle Simmons, the project’s
leader and director of UNSW’s ARC
Center for Quantum Computation and
Communication, says the idea of just
moving atoms as Eigler did in his pioneering STM experiment, which employed xenon on nickel, would be impossible on silicon.
“You cannot just pick up atoms, like
IBM did with its original demonstra-
tion, because silicon is too strongly
bound,” Simmons says. “So you basi-
cally destroy the substrate if you try to
do the same kind of experiment.”
The UNSW team used a combina-
tion of hydrogen-resist lithography
and STM microscopy to create the
stable phosphorus atom that served as
the transistor, employing phosphine
as the dopant precursor. The phos-
phine molecules, placed within three
adjacent pairs of surface silicon at-
oms, were successively dosed at room
temperature and at 350 Celsius. This
resulted in progressive breaking up of
the component phosphorus and hy-
drogen atoms, ultimately resulting in
the single phosphorus atom being in-
corporated into the silicon, substitut-
ing for one of the original six silicon
atoms. Nanowires were then attached
via silicon guides, and the entire struc-
ture was covered with a 180-nanometer
layer of silicon.
Fuechsle, M., et al.
A single-atom transistor, Nature
Nanotechnology 7, 4, April 2012.
IBM Research Almaden
IBM researchers store one bit of magnetic
information in just 12 atoms, http://www.
Loth, S., Baumann, S., Lutz, C.P.,
Eigler, D.M., and Heinrich, A.J.
Bistability in atomic-scale antiferromagnets,
Science 335, 6065, Jan. 13, 2012.
Morton, J. J. L., McCamey, D. R.,
Eriksson, M. A., and Lyon, S. A.
Embracing the quantum limit in silicon
computing, Nature 479, 7373, nov. 17, 2011.
Weber, B., et al.
Ohm’s law survives to the atomic scale,
Science 335, 6064, Jan. 6, 2012.
Gregory Goth is an oakville, Ct-based writer who
specializes in science and technology.
© 2012 aCM 0001-0782/12/09 $15.00