"It's fascinating that we can still find surprises in a material like
magnetite that has been studied for thousands of years," said lead
researcher Doug Natelson, associate professor of physics and astronomy.
"This kind of finding is really a testament to what's possible now
that we can fabricate electronic devices to study materials at the
nanoscale."
The magnetic properties of lodestone, also known as magnetite, were
documented in China more than 2,000 years ago, and Chinese sailors
were navigating with lodestone compasses as early as 900 years ago.
Magnetite is a particular mineral of iron oxide. Its atoms are
arranged in a crystal structure with four oxygen atoms for every three
of iron, and their arrangement gives the mineral its characteristic
magnetic and electrical properties. Physicists have known for more
than 60 years that the electronic properties of magnetite change
radically and quickly at cold temperatures. As the material cools
below a critical temperature near minus 250 degrees Fahrenheit, it
changes from an electrical conductor to an electrical insulator - a
electrical transformation that's akin to the physical change water
undergoes when it freezes into ice.
"When we applied a sufficiently large voltage across our
nanostructures we found that we could kick the cooled magnetite out of
its insulating phase and cause it to become a conductor again,"
Natelson said. "The transition is very sharp, and when the voltage is
then lowered back below a lower critical value the magnetite snaps
back into its insulating phase. We don't know exactly why this
switching occurs, but we think further experiments will shed light on
this and the nature of the insulating state."
With engineers looking to exploit novel electronic materials for
next-generation computers and hard drives, phase transitions between
insulating and conducting states have become an increasingly hot
research topic in physics and materials science in recent years.
The debate about the causes and specifics of magnetite's
temperature-driven phase change has simmered much longer. Natelson
said physicists have long sparred about the possible underlying
physical and electronic causes of the phase transition. The discovery
of this new voltage-driven switching provides new clues, but more
research is still needed, he said.
"The effect we discovered probably wasn't noticed in the past because
nanotechnology is only now making it possible to prepare the
electrodes, nanoparticles, and thin films required for study with the
precision necessary to document the effect," he said.
Natelson's team experimented on two kinds of magnetite. One, called
nanorust, consists of tiny particles of magnetite developed in the
laboratory of Rice chemist Vicki Colvin, director of Rice's Center for
Biological and Environmental Nanotechnology. The second, thin films of
single-crystal magnetite, were produced by Igor Shvets' research group
at the University of Dublin's Trinity College. These high quality
materials with precise compositions were essential to the study, said
Natelson.
The research was funded by the Department of Energy.
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