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Theory Drives Magnetic Modification


In the typical research setting, the experimentalist comes up with a set of circumstances and relies on a theorist to explain what is taking place.

But on a recent project, a group of Ames researchers used data calculated by a theorist to modify a complex intermetallic compound at the atomic level to change the material’s fundamental magnetic characteristics, not unlike the process bioengineers use when splicing genes to create synthetic organisms.

“In general, the predictive power of theory is somewhat limited,” says Ames Laboratory senior scientist Vitalij Pecharsky, who led the research group. “But in this case we were working with a family of materials that we are very familiar with – we and many other groups of researchers all over the world have been studying them for more than 10 years – so the theory (assistant scientist) Durga Paudyal developed here was very specific and precise.”

 Still, 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 explains, “But atoms still are able to ‘cooperate’ with one another in areas, such as magnetic ordering and superconductivity.”

By analyzing 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.  In the case of Gd5Ge4, the gadolinium atoms occupy three distinct sites within the crystal lattice.
“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, senior metallurgist Karl 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 Aug. 10, 2010 issue of 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 Paudyal to model the effects of 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.

“We’ve successfully enhanced its anti-ferromagnetism and suppressed the ferromagnetism,” Pecharsky says. “Now, the question is can we enhance the ferromagnetism even further?”

That may present a challenge because gadolinium has the highest spin magnet moment of the 17 rare-earth elements. But such challenges are precisely what brings Pecharsky and his team back to the Lab day after day.

~ by Kerry Gibson