Gerard H. Lander

Magnetization Density in URhAl - Evidence For Hybridization Effects

URhAl is one of a large class of uranium compounds with the hexagonal ZrNiAl structure. It becomes ferromagnetic at 27 K and the ordered moment is found to be 0.94 mu B per formula unit. The authors have performed experiments with polarized neutrons in order to measure the magnetization density in the unit cell. A crystallographic study gives atomic positions in agreement with previous work, but they find that the crystals, which were grown by a Czochralski method, are highly perfect and exhibit a large amount of extinction. Corrections for these effects are discussed. The magnetization density shows that the total moment as measured by magnetization is made up of four contributions. The moments (in mu B) on uranium, RhI, RhII, and from the conduction electrons are, respectively 0.94(3), 0.28(2), 0.03(2), and -0.11(3). The observation of a large moment at the RhI site, which lies in the (001) plane with the U atoms shows that strong anisotropic hybridization occurs between the U and RhI. This is the cause of the very large bulk anisotropy. At the uranium site they have analysed the contributions from the spin and orbital moments and find - mu L/ mu S=1.81(7). The free ion 5f3 value for this ratio if 2.6. The reduction gives one a quantitative measure of hybridization in the system. The results are compared with those calculated from density functional theory.

The Valence-Fluctuating Ground State of Plutonium

Determination of plutonium’s fluctuating electronic and magnetic ground state resolves long-standing “missing” magnetism puzzle. A central issue in material science is to obtain understanding of the electronic correlations that control complex materials. Such electronic correlations frequently arise because of the competition of localized and itinerant electronic degrees of freedom. Although the respective limits of well-localized or entirely itinerant ground states are well understood, the intermediate regime that controls the functional properties of complex materials continues to challenge theoretical understanding. We have used neutron spectroscopy to investigate plutonium, which is a prototypical material at the brink between bonding and nonbonding configurations. Our study reveals that the ground state of plutonium is governed by valence fluctuations, that is, a quantum mechanical superposition of localized and itinerant electronic configurations as recently predicted by dynamical mean field theory. Our results not only resolve the long-standing controversy between experiment and theory on plutonium’s magnetism but also suggest an improved understanding of the effects of such electronic dichotomy in complex materials.

Evidence for Anisotropic Hybridization in Uranium Compounds

We have performed polarized-neutron experiments on a single crystal of paramagnetic URuAl with the ZrNiAl hexagonal structure. Comparisons with the magnetization density previously determined for ferromagnetic URhAl show that in both materials there is a large (~ 30% of the uranium signal) magnetic moment induced by hybridization between the 5f and 4d electrons at the 4d site. However, these effects are present only on the d sites in the (001) plane–an equally close d site out of the plane shows no such effects. These results show the importance of anisotropic 5f-ligand interactions.

Magnetization density in heavy-fermion UPd2Al3

The induced magnetization density has been measured at T = 36 K (above the antiferromagnetic ordering temperature of 14 K) in a single crystal of the heavy-fermion material UPd2Al3. A magnetic field of 5 T was applied along the easy axis of magnetization. The magnetization density as measured by the polarized-neutron technique is associated totally with the uranium atom; no spin transfer is found on the Pd atoms. The induced moment, as measured by neutrons, is some 12% below that measured by magnetization measurements on the same crystal. This indicates the presence of a significant positive (with respect to the uranium moment) conduction electron polarization. The ratio of the orbital to the spin moment of uranium found experimentally is -2.01(25), compared with the free-ion values of -2.56 (for U3+) and -3.29 (for U4+), and a recent theoretical prediction of-1.35.

Self-irradiation effects in plutonium alloys stabilized in the δ-phase

Plutonium aging leads to the creation of decay products, such as americium, uranium, helium, but also to self-irradiation defects such as vacancies and interstitials. These self-irradiation defects can affect the properties of plutonium; as an example, swelling of PuGa or PuAl alloys has been observed at room temperature. This work is devoted to the study of the effect of aging on the magnetic susceptibility of pure and alloyed plutonium. An anomaly has been revealed at low temperatures (T < 75K) for magnetic fields lower than 4.5 T. This anomaly decreases after heat treatment. A tentative explanation, based on results from molecular dynamics calculation in δ-plutonium, is given to explain the origin of the anomaly.

Interaction between U/UO2 bilayers and hydrogen studied by in-situ X-ray diffraction

This paper reports experiments investigating the reaction of H

Magnetic Studies of Transuranium Compounds

_{2}

Higher Order Speciation Effects on Plutonium L3 X-Ray Absorption near Edge Spectra.

with uranium metal-oxide bilayers. The bilayers consist of

Charge Distribution and Local Structure and Speciation in the UO2+x and PuO2+x Binary Oxides for x <= 0.25

leq

Structural and magnetization density studies of La2NiO4

100 nm of epitaxial