STUDY FINDS NEW PROPERTIES IN NON-MAGNETIC MATERIALS
Posted on Wednesday, June 11, 2008 @ 22:22:16 GMT by vlad
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A team of Penn State researchers has shown for the first time that the entire class of non-magnetic materials, such as those used in some computer components, could have considerably more uses than scientists had thought. The findings are important because they reveal previously unknown information about the structure of these materials, expanding the number of properties that they potentially could have. A material's properties, such as electrical conductivity and mechanical strength, are what determine its usefulness. The research will be published in the journal Physical Review Letters.
A non-magnetic lattice, shown in Figure A, can have the same symmetry
as a magnetic lattice, shown in Figure B. Both lattices, in this case,
are described as having the point group symmetry that the scientists
call 4'mm'. Figure A (non-magnetic material): The non-magnetic crystal
structure of strontium titanate, SrTiO3, is composed of strontium (blue
balls), titanium (red balls), and oxygen (yellow balls). The grey
motifs are oxygen octahedra structures that twist counter to the
neighboring octahedra, as shown by the green arrows. Credit: Sava
Denev, Penn State
A material's properties are
determined by its structure, explained Venkatraman Gopalan, a
researcher in Penn State's Center for Nanoscale Science, a professor of
materials science and engineering, and the project's leader. "If I was
out hiking and I found a rock that contained a quartz crystal, I could
tell you what properties the crystal can and cannot have just based on
what we call its symmetry--the number and arrangement of crystal planes
it has. Symmetry results from the way the atoms are arranged in the
quartz," he said. "It is an extremely powerful way of understanding our
world."
The non-magnetic materials that
Gopalan and his colleagues studied were thought to have one of the 32
different crystal symmetries--called point group symmetries--known to
exist in nature. On the other hand, magnetic materials have 90
different point group symmetries because their atomic particles have
magnetic spins, which can be imagined as tiny loops of current. "Motion
is an extremely important aspect of magnetism," said Gopalan.
"Magnetism develops in nature as soon as charged particles start moving
or spinning."
A magnetic lattice, shown in Figure B, can have the same symmetry as a non-magnetic lattice, shown in Figure A. Both lattices, in this case, are described as having the point group symmetry that the scientists call 4'mm'. Figure B (magnetic material): This magnetic lattice, equivalent to that shown in Figure A, shows the analogy between the magnetic spins (green arrows in Figure B) and the loops of current (green arrows in Figure A). Credit: Sava Denev, Penn State
Scientists long have believed that symmetry allows magnetic
materials to have more properties than non-magnetic materials because
flipping the direction of spin creates an additional symmetry. But
Gopalan's team has shown that non-magnetic materials, theoretically,
can have just as many properties as magnetic materials. According to
Gopalan, some non-magnetic materials have groups of atoms that distort
by twisting or rotating. This slight movement is equivalent to a tiny
loop of current and is enough to give the material some additional
properties that previously were thought to belong only to magnetic
materials.
The researchers tested their theory experimentally using strontium
titanate, which is a non-magnetic material. They cooled the material
and found that its oxygen atoms responded by twisting into a tighter
postion to save energy and space. "The oxygen atoms don't rotate all
the way around like a loop of current does in magnetic materials, but
theoretical analyses show that they do twist and, therefore, it is
possible that these materials could have previously unknown
properties," said Gopalan.
Next, the team investigated whether the twisting movement
translated into the expression of additional properties. In particular,
they predicted and tested for an optical property that they call roto
second harmonic generation, which is analogous to a well-known property
called magnetic second harmonic generation. Second harmonic generation
is found, for example, in the crystals that are used in green laser
pointers to convert infrared laser light into green laser light. The
group found that the strontium titanate material does have a small
amount of roto second harmonic generation.
"Nobody has thought of
relating magnetic symmetries to a non-magnetic material like strontium
titanate, but that's precisely what our paper does," said Gopalan. "We
first did a theoretical analysis in which we applied the symmetry
framework that traditionally is used to describe magnetic materials to
this vast class of non-magnetic materials. Then we did a laboratory
experiment with a particular non-magnetic material and we found that it
has a property that previously was thought to belong only to magnetic
materials. We suggest that it is possible for the entire class of
non-magnetic materials to have more symmetries and more properties than
previously have been thought possible."
The team's findings could lead to an explosion of research into new
properties of non-magnetic materials and to possible applications of
these properties. "These materials are used in hundreds of
applications," said Peter Schiffer, associate vice president for
research and a professor of physics at Penn State, "but this new work
holds great promise for finding many more uses."
Source: Penn State
Via: http://www.physorg.com/news132418941.html
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