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Extraordinary Responsive Rare Earth Magnetic Materials

Responsive systems, where a small change of an extrinsic thermodynamic variable, such as temperature, pressure, or magnetic field, triggers an intrinsic phase change, have both fundamental and technological significance.  Alloys and compounds that exhibit strong field-, temperature-, or pressure-controlled reactions, tunable by chemistry, crystallography, and processing, provide broad benefits to energy-related applications, such as sensors and smart materials, and materials for energy conversion, generation, and utilization devices.  Responsive materials, therefore, have the potential to make transformative changes that can be used to help meet our Nation’s future energy demands.  Conventional (and strong) stimuli are temperature and pressure, but a magnetic field is weak and often an underappreciated trigger, whose role in initiating strong changes in solids is much less understood.  Knowledge of the mechanisms delivering minor-stimulus driven phase change that is then followed by a major perturbation of properties is crucial for guiding the discovery of new materials, and is the focus of this research.  Our goal is to uncover the underlying electronic, atomic and microscopic interactions that result in an extraordinarily strong coupling of the magnetic and crystal lattices in chosen model systems representing rare earth-based intermetallic materials.  Development and validation of phenomenological models of transformations that range from magneto-volume to magnetic-martensitic is one of our prime objectives.

This research is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering.

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  • Cover featuring an image from Petit et al. The figure shows the 3D Fermi surface for a paramagnetic compound of gadolinium and magnesium.

    Theoretical modeling has led to a key development in our understanding of the deeply complex magnetic properties in a series of rare-earth intermetallic materials.  Rare-earth elements are unique in that their cores hold strongly localized electrons that underpin their novel magnetic properties.  When combined with transition metals, rare earths become technologically-useful intermetallic materials.

  • Scientists have discovered a fascinating secret about praseodymium aluminide. When PrAl2 is cooled, its crystal structure changes from high symmetry cubic to low symmetry tetragonal below -400 °F (32 K). However, when the cooling is done in a high magnetic field, the material retains the cubic structure. This change is not observed in other rare-earth aluminides. Furthermore, PrAl2 has an anomalous heat capacity per unit mass at low temperatures.  It is 10x higher than pure praseodymium.

  • The use of the highest purity starting materials in fundamental research seems to be an obvious choice and is a priori assumed in experimental science, including rare earth metallurgy. Yet an ambiguous “99.9%” purity reported by commercial vendors for the rare earths, in almost all cases refers to the purity with respect to only the other rare earth elements, and generally does not include other metals and more importantly the presence of the interstitial non-metallic elements oxygen, nitrogen, hydrogen, and carbon.

  • Researchers have shown that the same atom can have different roles in magnetism, depending on its location in the crystal structure. The complex metallic compound Gd5Ge4 has a crystal structure with three distinct sites for its gadolinium atoms.