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Scattering at the Ames Laboratory


A new material’s journey from discovery to ultimately improving everyday products often starts with scientists studying the material’s microscopic properties through neutron and X-ray scattering.

“The first thing that we need to know about a new material is its structure and how it behaves under external stimuli,” says Alan Goldman, Ames Laboratory physicist and member of the Lab’s scattering research group. “Minor changes in material composition, made to either reduce costs or weight or to enhance efficiency, can represent significant savings over current materials. So, the structure and behavior of materials is critical for understanding how to create the next generation of materials and products.”


The scattering research group at Ames Laboratory uses X-ray resonant
magnetic scattering, along with other types of scattering, to map out the
phase diagrams for new materials. Shown here is the phase diagram for
a cobalt-doped iron arsenide. The scattering team’s research indicates that
in this material, magnetic order (blue area) and superconductivity (yellow
area) can coexist (green area), which is unusual.

Take, for instance, the new type of iron-arsenic superconductor discovered several years ago. Scientists hope iron-arsenides may prove useful in applications, such as in high-strength superconducting magnets, but first the materials’ properties must be mapped out. In particular, Ames Laboratory’s scattering group — which includes Goldman, Rob McQueeney, David Vaknin and Andreas Kreyssig, all Ames Laboratory physicists, along with several post-doctoral researchers and graduate students — was interested in the magnetic structure of a type of iron-arsenide where a small percentage of the iron was replaced with cobalt in a process called “doping.”

The team studied the material using neutron scattering, which is best suited for studying magnetic structure as neutrons are sensitive to the magnetism in materials. In neutron scattering, a beam of neutrons is generated in a nuclear reactor or an accelerator-based spallation source and, using specialized instrumentation, is aimed at a sample of material. The neutrons in the beam encounter nuclei and atomic magnetic moments in the sample and bounce off. The way the neutrons “scatter” from a material gives scientists information

about where the nuclei and magnetic moments are located and how they are moving.

In studying cobalt-doped iron-arsenides, the Ames Laboratory scattering group used finely-tuned samples synthesized at Ames Lab by fellow physicist Paul Canfield. The scattering research group was able to gather enough information about the magnetic order and superconducting phase to map out a phase diagram of the material.

“Turns out this cobalt-doped iron-arsenide is an interesting case where the magnetic order and superconductivity can coexist in a finite doping range,” says Rob McQueeney. “Our group was the first to discover that.”

For other scientific questions, like studies of rare-earth materials, one of Ames Laboratory’s specialties, X-ray resonant magnetic scattering is better suited. In X-ray resonant magnetic scattering, a material sample is exposed to a beam of finely tuned X-ray energy that is specific to the probed element. The X-rays scatter from electrons in the material, revealing information about materials’ magnetic structure.

“This technique is only possible at high-brilliance X-ray sources like the Advanced Photon Source at Argonne National Laboratory,” says Goldman. “In fact, we spend a lot of time traveling because our research requires ‘beam time’ at the major scientific facilities in the United States and around the world.”

The scattering group also uses various scattering techniques in research on bio-inspired materials. It tries to understand the mechanisms by which organisms build functioning minerals such as bones, teeth and shells, and magnetic nanocrystals. The objective of the group is to employ similar biomineralization processes to design novel materials for various applications.

“In all these projects, we use the scattering technique that is appropriate to the scientific question, and often we use both neutron scattering and X-ray resonant magnetic scattering because the data gathered from each is complementary,” says Andreas Kreyssig. “I think that is a strength of our team here at Ames Laboratory.”

“From sample growth and the variety of very detailed kinds of scattering techniques we use, to theoretical interpretation, we have it all here at the Ames Laboratory,” adds McQueeney. “That’s what makes it possible for us to make such an impact in this field.”

~ by Breehan Gerleman Lucchesi