the memory inside electronic devices may often be
more reliable than that of humans, it, too, can worsen
Now a team of scientists from UW-Madison and Argonne
National Laboratory may understand why. The results
were published in a recent edition of the journal
Smart cards, buzzers inside watches and even ultrasound
machines all take advantage of ferroelectrics, a family
of materials that can retain information, as well
as transform electrical pulses into auditory or optical
signals, or vice versa.
“The neat thing about these materials is that they
have built-in electronic memory that doesn't require
any power,” explains Assistant Professor of Materials
Science and Engineering Paul Evans, a co-author of
the recent paper.
But there's a problem preventing many of these materials
from being used more widely in other technologies,
including computers. As Evans says, “Eventually they
The ability of ferroelectrics to store information
resides in their arrangement of atoms, with each structure
holding a bit of information. This information changes
every time the material receives a pulse of electricity,
basically switching the arrangement of atoms.
However, each electric pulse — and corresponding change
in structure — gradually diminishes the capability
of these materials to store and retrieve information
until they either forget the information or quit switching
“It could switch 10,000 or even millions of times
and then stop working,” says Evans.
Engineers call this problem fatigue. With little evidence
for what happens to the structure of ferroelectrics
as the material's memory fatigues, Evans and his colleagues
decided to look inside this material as its arrangement
of atoms, controlled by electrical pulses, switched
inside an operating device.
“We'd like to understand how it switches so we could
build something that switches faster and lasts longer
before it wears out,” says Evans.
To create a detailed picture of how the atoms rearrange
themselves inside an operating device during each
electrical pulse, the researchers used the Advanced
Photon Source — the country's most brilliant source
of X-rays for research, located at the Argonne National
Laboratory — to measure changes in the location of
atoms. By seeing how the atoms changed their positions,
the researchers could determine how well the material
switched, or remembered information.
“One advantage to working with X-rays is their ability
to penetrate deep into materials, which is why they
are so extensively used today in medical imaging,”
says Eric Isaacs, director of Argonne's Center for
Nanoscale Materials, and one of the paper's co-authors.
“Utilizing this property of X-rays, [we] were able
to peer through layers of metal electrodes in order
to study ferroelectric fatigue in a realistic operating
He adds that the very high brightness of the Advanced
Photon Source allowed the researchers to focus X-rays
to unprecedented small dimensions.
The X-rays showed that, as the researchers repeatedly
pulsed the device, progressively larger areas of the
device ceased working, suggesting that the atoms were
switching structures less and less.
“After 50,000 switches, the atoms were stuck — they
couldn't switch anymore,” says Evans, adding that
a stronger electrical charge did put the atoms back
When the researchers used a higher voltage of electricity
from the beginning, switching stopped 100 times later,
as reported in the paper. And, in this instance, applying
an even stronger pulse made no difference.
“With higher voltages, the material can't switch because
something has changed about the material itself,”
says Evans. “When you use bigger voltages, it's not
just the switching that stops working, but something
even more fundamental.”
Because previous researchers have not peeked inside
working ferroelectric materials to understand their
arrangement of atoms — key to the ability to recall
information — the reasons why switching eventually
stops had not been clearly identified.
“The electronic memory is stored in the structure
of atoms, and that's why it's so important to see
what the structure looks like,” explains Evans. By
looking inside these devices, he says engineers can
begin to understand why the atoms stop switching and
then manufacturers can start to design better devices.
With this promise, Evans asks, “Wouldn't it be nice
to have a computer that doesn't forget what it's doing
when you turn it off?”
Other researchers involved in the work include Professor
of Materials Science and Engineering Chang-Beom Eom,
Department of Materials Science and Engineering researcher
Dong Min Kim, and the paper's first author, Dal-Hyun
Do, also from the department; and Eric Dufres, from
the University of Michigan.
2004 The Board of Regents of the University of Wisconsin