****NON-TECHNICAL ABSTRACT**** Many uranium compounds are known to exhibit unusual magnetic properties, and the reason for that is still heavily disputed within the scientific community. This project attempts to shed some light onto the underlying mechanisms, and this investigator will utilize a combination of neutron-diffraction and high-magnetic-field studies. Neutron diffraction has been the technique of choice when it comes to testing structural and magnetic properties of a material at a microscopic level, and studies in high magnetic fields measure the macroscopic response. This project is intended to provide important insight into the correlations between magnetism and atomic configuration in such compounds. It is widely accepted that both the cohesive and the magnetic properties of uranium compounds are strongly influenced by interactions of electrons belonging to uranium atoms with the electrons of neighboring atoms. The community has phrased the term ?hybridization? in order to describe such interactions, but it has not been adequately included into existing theories. The results of this project are intended to pinpoint exactly that mechanism. This project will contribute to the development of a strong Materials Science program, a fast-growing field with many rewarding career opportunities. The program at NMSU is expected to increase scientific capacity in the predominantly hispanic communities in Southern New Mexico and Western Texas.

Technical Abstract

There is little doubt that hybridization effects between the uranium 5f states and the electronic states of the ligands play a prominent role for the development of magnetic moments and long-range magnetic order in uranium intermetallics. Even so, to date, there is no fully satisfactory theoretical model of the magnetism in these materials. This single investigator proposes a systematic in-depth study of the role of hybridization for the evolution of magnetic order and its anisotropy in uranium intermetallics. A vigorous study on families of isostructural uranium compounds with different stoichiometries will be performed. The properties of these compounds will be tuned to critical values by chemical substitutions and/or modifying external variables, such as magnetic fields or external hydrostatic pressure. Using a combination of neutron-diffraction and high-magnetic-field techniques, the structural and magnetic parameters of the individual compounds will be determined. Particular emphasis is put onto testing the correlation between magnetism and interatomic distances, which is believed to be intimately related to the strength of different hybridization effects. The research will provide hands-on training for students to become qualified users at neutron-scattering and high-magnetic-field facilities.

Project Report

Principal Investigator (PI): Heinz Nakotte, Physics, New Mexico State University Scientists have established a good understanding of solid materials in terms of rigid chemical bonds of elements from the periodic table. Electrons of each element occupy well-defined electron orbitals. The exact type of each electron orbital and its interactions with neighboring electron orbitals determines the nature of the chemical bonds (for example: covalent, ionic or metallic) that form in a material. Interactions of the electron orbitals from one element with other electron orbitals (e.g. of neighboring elements) is commonly referred to as hybridization. The strength of such hybridization effects and the type of bonding will ultimately determine all of the physical properties of a material. Combining two or more elements leads to the formation of compounds, many of which exhibit metallic properties. Intermetallic compounds that contain uranium are known to exhibit peculiar and fascinating magnetic properties at temperatures far below room temperature. For example, uranium compounds exhibit a huge magnetic anisotropy, which is a measure of the ease of aligning the magnetic moments in a material with an applied magnetic field. To date, there is no theoretical model that fully explains the mechanisms responsible for the magnetic behavior and anisotropy in uranium compounds. The main intellectual merit of this project has been that it resulted in a much improved understanding of the all-important parameters responsible for magnetic properties in uranium compounds. This was achieved by studying the magnetism of uranium compounds with different compositions and atomic arrangements. The project focused on so-called ternary uranium compounds, consisting of uranium, transition metals (for example: copper, cobalt, iron, nickel, palladium, iridium) and elements from the ‘p-blocks’ in the periodic table (for example: silicon, germanium, gallium, aluminum, tin). The project focused on systematic studies of various families of uranium compounds with different stoichiometries (for example: 1:1:1, 1:1:2, 2:2:1, 1:4:8). The experimental studies utilized complementary bulk and diffraction studies at extreme conditions. In particular, bulk studies provided measurements of the transport, thermal and magnetic properties as a function of temperature, high magnetic field and/or high pressure. The results of such bulk studies provide a measure of the strength of interactions of the ions embedded in the material. X-ray and neutron diffraction studies, on the other hand, were done to determine the structural arrangement of atoms in a material at an atomic level. In addition, unlike X-rays, neutrons carry magnetic moments and these interact with the moments of the magnetic ions of the material, thus providing information about the distribution, arrangement and coupling of the magnetic-ion moments. In-depth analysis and comparison of the magnetic properties and the development of magnetism for the different families of uranium compounds led to a new model of competing hybridizations. This model correctly predicts the formation of magnetic order and the behavior in the proximity of a magnetic instability for the different families of ternary uranium compounds studied. Project findings have been disseminated as papers in scientific journals and/or presentations at scientific conferences and meetings. The main contributions of this project for broader impacts are in the fields of human resource development, infrastructure enhancement and educational activities. Much of the research utilized the two user facilities at Los Alamos Laboratory, the Los Alamos Neutron Science Center (LANSCE) and the Pulse Field Facility of the National High Magnetic Field Laboratory (NHMFL). Student participants had the opportunity to use state-of-the-art instruments at these facilities for their research, and they received world-class training in forefront research. Research ties with LANSCE were particularly strong since the project PI was appointed as instrument scientist of LANSCE’s single-crystal diffractometer, SCD. Two of the involved students finalized their respective PhD theses during the project period, and both were able to secure rewarding post-doctoral fellowships soon thereafter. This project (directly or indirectly) supported several upgrades and improvements of the research infrastructure, particularly for SCD at LANSCE, which will provide future benefit to a variety of research projects. Many of the project’s educational efforts were geared toward more general long-term goals, such as course and curriculum development for the materials science and engineering physics programs at New Mexico State University (NMSU). Materials Science and Engineering Physics are fast-growing fields with many rewarding career opportunities. Further development and continuous improvements of such programs at NMSU are expected to increase scientific capacity in the predominantly hispanic communities in New Mexico and West Texas.

National Science Foundation (NSF)
Division of Materials Research (DMR)
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Daniele Finotello
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New Mexico State University
Las Cruces
United States
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