Antiferromagnetic order in an iron atom array revealed by spin-polarized imaging with a scanning tunneling microscope. |
Scientists
from IBM and the German Center for Free-Electron Laser Science (CFEL)
have built the world’s smallest magnetic data storage unit. It uses just
twelve atoms per bit, the basic unit of information, and squeezes a
whole byte (8 bit) into as few as 96 atoms. A modern hard drive, for
comparison, still needs more than half a billion atoms per byte. The
team recently presented their work in the weekly journal Science. CFEL
is a joint venture of the research centre Deutsches
Elektronen-Synchrotron DESY in Hamburg, the Max-Planck-Society (MPG) and
the University of Hamburg.
“With
CFEL the partners have established an innovative institution on the
DESY campus, delivering top-level research across a broad spectrum of
disciplines,” says DESY research director Edgar Weckert.
The
nanometer data storage unit was built atom by atom with the help of a
scanning tunneling microscope (STM) at IBM’s Almaden Research Center in
San Jose, Calif. The researchers constructed regular patterns of iron
atoms, aligning them in rows of six atoms each. Two rows are sufficient
to store one bit. A byte correspondingly consists of eight pairs of atom
rows. It uses only an area of 4 by 16 nm.
“This
corresponds to a storage density that is a hundred times higher
compared to a modern hard drive,” explains Sebastian Loth of CFEL, lead
author of the Science paper.
Data
are written into and read out from the nano storage unit with the help
of an STM. The pairs of atom rows have two possible magnetic states,
representing the two values ‘0’ and ‘1’ of a classical bit. An electric
pulse from the STM tip flips the magnetic configuration from one to the
other. A weaker pulse allows to read out the configuration, although the
nano magnets are currently only stable at a frosty temperature of minus
268 C (5 K).
“Our
work goes far beyond current data storage technology,” says Loth. The
researchers expect arrays of some 200 atoms to be stable at room
temperature. Still it will take some time before atomic magnets can be
used in data storage.
For
the first time, the researchers have managed to employ a special form
of magnetism for data storage purposes, called antiferromagnetism.
Different from ferromagnetism, which is used in conventional hard
drives, the spins of neighbouring atoms within antiferromagnetic
material are oppositely aligned, rendering the material magnetically
neutral on a bulk level. This means that antiferromagnetic atom rows can
be spaced much more closely without magnetically interfering with each
other. Thus, the scientist managed to pack bits only 1 nm
apart.
“Looking
at the shrinking of electronics components we wanted to know if this
can be driven into the realm of single atoms,” explains Loth. But
instead of shrinking existing components the team chose the opposite
approach: “Starting with the smallest thing—single atoms—we built data
storage devices one atom at a time,” says IBM research staff member
Andreas Heinrich. The required precision is only mastered by few
research groups worldwide.
“We
tested how large we have to build our unit to reach the realm of
classical physics,” explains Loth, who moved from IBM to CFEL four
months ago. Twelve atoms emerged as the minimum with the elements used.
“Beneath this threshold quantum effects blur the stored information.”
If
these quantum effects can somehow be employed for an even denser data
storage is currently a topic of intense research.
With
their experiments the team have not only built the smallest magnetic
data storage unit ever, but have also created an ideal testbed for the
transition from classical to quantum physics. “We have learned to
control quantum effects through form and size of the iron atom rows,”
explains Loth, leader of the Max Planck research group ‘dynamics of
nanoelectric systems’ at CFEL in Hamburg and the Max-Planck-Institute
for Solid State Research at Stuttgart, Germany. “We can now use this
ability to investigate how quantum mechanics kicks in. What separates
quantum magnets from classical magnets? How does a magnet behave at the
frontier between both worlds? These are exciting questions that soon
could be answered.”
A
new CFEL laboratory offering ideal conditions for this research will
enable Loth to follow up these questions. “With Sebastian Loth, one of
the world’s leading scientists in the field of time-resolved scanning
tunneling microscopy has joined CFEL,” stresses CFEL research
coordinator Ralf Köhn. “This perfectly complements our existing
expertise for the investigation of the dynamics in atomic and molecular
systems.”
