Neutrons reveal unexpected magnetism in rare-earth alloy

Researchers at the Department of Energy's Oak Ridge National Laboratory and their collaborators used neutron scattering to uncover magnetic excitations in the metallic compound ytterbium-platinum-lead (Yb2Pt2Pb). Surprisingly, this three-dimensional material exhibits magnetic properties that one would conventionally expect if the connectivity between magnetic ions was only one-dimensional. Their research is discussed in a paper published in the journal Science.

An electron can theoretically be understood as a bound state of three quasiparticles, which collectively carry its identity: spin, charge and orbit. It has been known that the spinon, the entity that carries information about electron spin, can “separate” itself from the others under certain conditions in one-dimensional chains of magnetic ions such as copper (Cu2+) in an insulating host. Now, the new study reveals that spinons are also present in metallic Yb2Pt2Pb.

The experimental team included ORNL postdoctoral researcher and lead author Liusuo Wu, Georg Ehlers, and Andrey Podlesnyak, instrument scientists at ORNL's Spallation Neutron Source (SNS), a DOE Office of Science User Facility. The team made use of the neutrons' sensitivity to magnetic fluctuations at the atomic scale and the world-leading capabilities of the SNS Cold Neutron Chopper Spectrometer (CNCS) instrument.

Placing a sample of Yb2Pt2Pb in the neutron beam and carefully mapping the dependence of the scattering intensity on angle and time-of-flight revealed characteristic signatures of the magnetic collective excitations in the material.

“An electron's ability to exhibit quantum magnetic behavior like this depends on how many pairings it can make with its nearest neighbors,” Wu said. “In a one-dimensional chain, each has only two neighbors, making quantum fluctuations much more dramatic.”

With contributions from collaborators at Brookhaven, Stony Brook University, and the University of Amsterdam, the research team developed a picture of spinons propagating in a particular direction, and how a magnetic excitation spectrum was calculated, based on that model, and compared it to the experimental data.

“The element ytterbium, found in the 4f group of the periodic table, often makes an interesting ingredient in a material. Here, the leading magnetic interactions conspire with the crystal structure and the local anisotropy to create an effect we call geometric frustration,” Ehlers said. “In materials research, this gives us a handle on the properties of a particular system and allows one, ultimately, to design materials with specific desirable properties.”

As in electrons, the magnetic moments in neutrons originate from their spin-1/2, the smallest possible magnetic moment in an ion or electron shell. Combined with their high penetrating power–due to the fact they carry no charge–neutrons are an ideal probe to explore magnetism in atomic systems. Here, the interesting phenomena occur at low energies and low temperatures, which is why the cold neutrons at CNCS are well suited to study them.

“At higher temperatures, the thermal effects will blur what we're able to see, because the ions start moving about more if the material is warmer, hindering our ability to see the excitations clearly,” Ehlers said.

Another advantage to the CNCS instrument is that it can detect neutron scattering patterns in all three spatial dimensions simultaneously, which, according to Ehlers, proved to be crucial in the team's investigation.

“At first we were really puzzled by our results,” Wu said. “But once we started to understand and look at it from the right perspective, we could see how the electrons hop between overlapping orbitals on nearest Yb neighbors.

“Now that we have a better understanding of how this happens, we can search for other materials that demonstrate similar properties, which will hopefully lead to even bigger discoveries and better technologies.”

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Co-authors of the paper, titled “Orbital-Exchange and Fractional Quantum Number Excitations in an f-electron Metal, Yb2Pt2Pb” include lead author Liusuo Wu, William Gannon, Michael Brockmann, Jean-Sebastien Caux, Moosung Kim, Yiming Qiu, John R. Copley, Georg Ehlers, Andrey Podlesnyak, Alexei Tsvelik, Igor Zaliznyak, and Meigan Aronson.

Additional contributions and complementary measurements were obtained from DOE's Brookhaven National Laboratory, the National Institute of Standards and Technology, Stony Brook University, and the University of Amsterdam.

The research conducted at ORNL's Spallation Neutron Source was supported by DOE's Office of Science. The paper is available at http://science.sciencemag.org/content/352/6290/1206.

UT-Battelle manages ORNL for the DOE's Office of Science. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit http://science.energy.gov/.