Giant Magnetocaloric Materials Could Have Large Impact on the Environment

JUNE 15, 2007

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Spin density contour plots for Gd5Si2Ge2 show dramatic changes when Ge2 covalent bonds break at the magnetostructural transition responsible for the giant magnetocaloric effect in this material. (Calculations by Y. Lee and B. Harmon)

Amid the growing awareness of climate change as a concern in need of solutions, research at the Argonne Advanced Photon Source (APS) may help reduce the levels of hydrofluorocarbons (HFCs) in our atmosphere. Scientists carrying out x-ray experimentation at the APS are learning new information about the functioning of magnetocaloric materials that have potential for many applications, including environmentally friendly magnetic refrigeration systems that are not dependent on environmentally unfriendly HFCs.

Magnetic refrigeration is a clean technology that manipulates the degree of ordering (or entropy) of electronic or nuclear magnetic dipoles (or spins) in order to reduce a material’s temperature and allow the material to serve as a refrigerant. This manipulation involves sequential application and removal of strong magnetic fields, and is used in research laboratories worldwide to achieve ultra-cold temperatures in the milli- and micro-Kelvin (K) ranges. The magnetocaloric effect (a change in temperature accompanying a change in a material’s magnetization) is largest near a material’s intrinsic magnetic ordering temperature. In the case of rare-earth gadolinium (Gd), this ordering occurs near room temperature and results in a better than 3-4-K refrigeration-per-Tesla change in applied magnetic field, making Gd the current material of choice for magnetic refrigeration near room temperature.

The prospects for a viable magnetic refrigeration technology recently became brighter with the report of a giant magnetocaloric effect in gadolinium-silicon-germanium [Gd5(SixGe1-x)4] alloys. The addition of non-magnetic silicon and germanium ions brings about a giant magnetic entropy change whenGe(Si) chemical bonds connecting the magnetism-carrying gadolinium ions are quickly formed (or broken) by the application (or removal) of a magnetic field. As an added bonus, the magnetic ordering temperature can be tuned by changing the Ge(Si) ratio.

Understanding the delicate interplay between structure and magnetism at the core of the giant magnetocaloric effect displayed by these promising materials is essential for further advances in moving this technology from the laboratory to the household.

A collaboration between researchers from the U.S. Department of Energy’s Argonne and Ames national laboratories has now revealed key atomic-level information that makes clear the role played by the nominally non-magnetic Ge(Si) ions in the giant magnetocaloric effect of these materials. In an article published in Physical Review Letters, the researchers describe how they used high-brilliance, circularly-polarized x-ray beams at X-ray Operations and Research beamline 4-ID-D at the APS to probe the magnetism of Gd and Ge ions as the material underwent its bond-breaking magneto-structural transition. In addition to the expected strong magnetization of Gd ions, the researchers found significant magnetization attached to the Ge ions.

“This is surprising and important,” said Argonne physicist Daniel Haskel, who led the research team. “Because Ge has a closed-shell 3d10 electronic configuration, it was expected to be non-magnetic. Its magnetization is induced by the hybridization, or mixing, of otherwise non-magnetic Ge 4p atomic orbitals with the magnetic Gd 5d orbitals. This hybridization dramatically changes at the Ge(Si) bond-breaking transition, causing the destruction of magnetic ordering and leading to the giant magnetocaloric effect of these materials.”

By combining the novel experimental results with detailed numerical calculations of the electronic structure carried out at Ames Laboratory, the researchers were able to conclude that the magnetized Ge orbitals act as “magnetic bridges” in mediating the magnetic interactions across the distant Gd ions.

"As a result of this work we now have a better understanding of the role of nonmagnetic elements, such as germanium, in enhancing magnetic interactions between the rare-earth metals in these materials,” said co-author Vitalij Pecharsky of Ames. “This discovery is counterintuitive, yet it opens up a range of exciting new opportunities towards the engineering of novel magnetic materials with predictable properties."

Other authors in the paper are Y. Lee, B. Harmon, Y. Mudryk, K. Gschneidner (AMES) and Z. Islam, J. Lang, and G. Srajer (APS-ANL).

Contact: D. Haskel, haskel@aps.anl.gov

See: D. Haskel, Y.B. Lee, B.N. Harmon, Z. Islam, J.C. Lang, G. Srajer, Ya. Mudryk, K.A. Gschneidner, Jr., and V.K. Pecharsky, “Role of Ge in Bridging Ferromagnetism in the Giant Magnetocaloric Gd5(Ge1-xSix)4 Alloys,” Phys. Rev. Lett. 98, 247205 (2007). DOI: 10.1103/PhysRevLett.98.247205

This work was supported by the U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357 and No. DE-AC020-7CH11358, respectively. Use of the Advanced Photon Source was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

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