Scientists
have given us a plethora of new materials – all created by combining
individual elements under varying temperatures and other conditions. But
to tweak an intermetallic compound even more, in order to give it the
attributes you desire, you have to go deeper and re-arrange individual
atoms.
It’s
a process similar to what bioengineers employ when they add and delete
genes to create synthetic organisms, and it was the focus of a group of
researchers at the U.S. Department of Energy’s Ames Laboratory, when
they replaced key atoms in a gadolinium-germanium magnetic compound with
lutetium and lanthanum atoms.
The
group was led by Vitalij Pecharsky, Ames Lab senior scientist and
Distinguished Professor of Materials Science and Engineering at Iowa
State University, and included his Lab colleagues, Karl Gschneidner Jr.,
Ames Lab senior metallurgist and Distinguished Professor of MS&E at
ISU, and Gordon Miller, Ames Lab senior scientist and ISU professor of
chemistry, along with assistant scientists Yaroslav Mudryk and Durga
Paudyal. Also participating was Sumohan Misra, research associate at the
DOE’s SLAC National Accelerator in Menlo Park, Calif., formerly a Ph.D.
student of Miller’s.
Creating
materials by design is no easy task, especially in the case of the
complex gadolinium-germanium – Gd5Ge4 – compound. Making things even
more difficult, the compound’s structure is highly symmetrical, which is
common in intermetallics, so predicting which atoms are key to changing
the material’s characteristics would be difficult if not impossible
unless some methodology was available to help in the selection process.
The
Gd5Ge4 compound’s uniformity results from the fact that like nearly all
metallic solids’ atoms are arranged in a highly symmetrical crystal
structure called a lattice. The more complex the material, the more
intricate its lattice. And while the individual elements making up the
lattice influence its characteristics, in some cases the location of
specific atoms within the lattice can also have a profound influence on
such things as its melting point, mechanical strength or – in the case
of magnets – ferromagnetic properties.
“Individuality
doesn’t happen often among the atoms of metallic crystals,” Pecharsky
explained, “But atoms still are able to ‘cooperate’ with one another in
areas such as magnetic ordering and superconductivity.”
By
discovering these cooperative relationships, scientists can determine
what will happen if they replace one or more of the atoms with those of
another element, which is precisely what the team accomplished.
“We
revealed that a single site occupied by the Gd atoms is much more
active than all of the other Gd sites when it comes to bringing the
ferromagnetic order in a complex crystal structure of gadolinium
germanide,” Pecharsky said.
Pecharsky,
Gschneidner and other researchers at the Ames Lab have spent years
working with gadolinium alloys, because of the magnetic compound’s use
in the green, energy-saving field of magnetic refrigeration. However,
that was not the main reason the Ames Lab researchers chose Gd5Ge4 for
their work.
As
it turns out, “the metal exhibits an impressive combination of
intriguing and potentially important properties, the researchers
explained in their paper, “Controlling Magnetism of a Complex Metallic
System Using Atomic Individualism,” published in the August 10, 2010
Physical Review Letters. “The extraordinary responsiveness to relatively
weak external stimuli makes Gd5Ge4 and related compounds a phenomenal
playground for condensed matter science.”
Besides
being unusually responsive, Gd5Ge4 was an ideal alloy for the work,
because any changes in its magnetic properties resulting from the
group’s manipulations could be easily measured.
In
2008, Pecharsky and members of the same research team had already
discovered that adding silicon to the alloy resulted in a
magnetostructural transition that occurred without the application of a
magnetic field. Chemical pressure alone was able to enhance the
material’s ferromagnetism.
That
earlier finding led the team to experiment with other additions to the
alloy. To ferret out precisely which atoms in the lattice were the best
candidates for manipulation, the researchers called upon density
functional theory, which is a means of studying the electronic structure
of solids developed by Nobel Prize winning physicist Walter Kohn.
Kohn’s
methodology enabled the group to model the effects substituting small
amounts of gadolinium atoms within the Gd5Ge4 solid with the elements
lutetium and lanthanum. With the modeled results in hand, the group’s
next step was to create the actual alloys in the lab, in order to test
the accuracy of their computer-based predictions.
In
fact, the complex fabrication process confirmed the modeling results.
The researchers found if they replaced just a few gadolinium atoms with
lutetium, the result would be a severe loss in the alloy’s
ferromagnetism. By contrast, substituting an equal number of lanthanum
atoms had no significant effect; though substituting greater amounts of
lanthanum might have a more pronounced impact on the resulting alloy’s
ferromagnetism, the researchers speculated.
Going
forward, the lessons learned in this experiment could have important
far-reaching implications, as materials scientists search for new exotic
substances to be used in this and future generations of high-tech
products. “Knowing how to identify key atomic positions is similar to
understanding the roles specific genes play in an organism’s DNA
sequence,” Pecharsky said. “And that knowledge could ultimately lead to
materials by design.”
