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Magnetic studies of actinides: evidence for localized magnetism

Description: The position of the 5f elements in the periodic table suggests that the electronic properties of these elements (and their compounds) will resemble those of the lanthanide series. However, the extended nature of the 5f wave functions leads to fundamental differences between 4f and 5f systems. Evidence for localized magnetism is reviewed. Results of magnetization, Moessbauer, neutron, and low-temperature x-ray experiments on Np, Pu, and Am compounds are used to illustrate both the similarities to and differences from lanthanide magnetism. A determination of the ground-state 5f electron wave functions is, in principle, possible by measuring the neutron magnetic cross section. The interpretation of such experiments on UO$sub 2$, USb, and PuP requires a knowledge of the radial extent of the 5f electrons, which is obtained from relativistic Dirac-Fock calculations, and the use of the tensor-operator formalism to treat the spin- orbit interaction. This interaction in Pu$sup 3+$ (a 5f$sup 5$ configuration) results in a magnetic form factor that initially increases with increasing scattering angle. For USb the experimental magnetic scattering is used to determine the ground-state wave function of the U$sup 3+$ ion. In addition the temperature dependence of the quadrupole moment in USb has been measured and yields information both on the size of the crystal-field and exchange interactions, and on the interplay between them. (auth)
Date: January 1, 1975
Creator: Lander, G.H.
Partner: UNT Libraries Government Documents Department

Magnetism in the actinides: the role of neutron scattering

Description: Neutron scattering has played a crucial and unique role of elucidating the magnetism in actinide compounds. Examples are given of elastic scattering to determine magnetic structures, measure spatial correlations in the critical regime, and magnetic form factors, and of inelastic scattering to measure the (often elusive) spin excitations. Some future directions will be discussed.
Date: January 1, 1982
Creator: Lander, G.H.
Partner: UNT Libraries Government Documents Department

Magnetic response functions in actinide compounds

Description: A brief review is given of neutron inelastic scattering experiments on actinide (5f) systems. Only a few compounds exhibit sharp crystal-field levels so that analogies with 4f compounds are rarely straight forward. In the systems that order magnetically, we do find well-defined magnetic excitations, although in many cases the strong interaction with the conduction electrons gives rise to overdamped modes. Finally, in the strongly enhanced paramagnets and heavy-fermion systems the response is essentially featureless in ..-->..Q space and can be characterized by a single Lorentzian with full-width at half-maximum of GAMMA. Attempts to relate this quantity GAMMA to other physical properties, particularly the coefficient of the electronic specific heat ..gamma.., have not been too successful. 22 refs., 5 figs.
Date: January 1, 1986
Creator: Lander, G.H.
Partner: UNT Libraries Government Documents Department

Neutron scattering of transuranium materials

Description: A number of neutron experiments on transuranium materials are reviewed. Purpose of these experiments, which range from studies of crystal fields in the oxides to excitations in PuSb, is to increase our understanding of the 5f electron behavior across the first half of the actinide series. Comparisons are made with the more familiar uranium analogues.
Date: January 1, 1986
Creator: Lander, G.H.
Partner: UNT Libraries Government Documents Department

Neutron scattering studies of the actinides

Description: The electronic structure of actinide materials presents a unique example of the interplay between localized and band electrons. Together with a variety of other techniques, especially magnetization and the Mossbauer effect, neutron studies have helped us to understand the systematics of many actinide compounds that order magnetically. A direct consequence of the localization of 5f electrons is the spin-orbit coupling and subsequent spin-lattice interaction that often leads to strongly anisotropic behavior. The unusual phase transition in UO/sub 2/, for example, arises from interactions between quadrupole moments. On the other hand, in the monopnictides and monochalcogenides, the anisotropy is more difficult to understand, but probably involves an interaction between actinide and anion wave functions. A variety of neutron experiments, including form-factor studies, critical scattering and measurements of the elementary excitations have now been performed, and the conceptual picture emerging from these studies will be discussed.
Date: January 1, 1979
Creator: Lander, G.H.
Partner: UNT Libraries Government Documents Department

Actinide 5f systems: experimental determination of the magnetic response function

Description: Understanding of metallic actinide systems is in confusion. Searches for crystal-field levels with neutron spectroscopy have, for the most part, been unsuccessful, despite the acknowledged importance of the 5f electrons in determining the magnetic behavior. In systems such as UAl/sub 2/, USn/sub 3/ and UN a broad response function, S(Q vector,..omega..), reminiscent of that found in intermediate valent compounds, exists. Neutron inelastic scattering experiments on single crystals have shown the small influence of the crystal field. Instead we find an unusual response function dominated by the longitudinal susceptibility chi/sup zz/(Q vector,,..omega..) such that transverse excitations - conventional spin waves - do not exist at low energies. As yet a detailed theoretical interpretation of the measurements does not exist. Indeed, the small, although not necessarily negligible, role of the crystal field presents conceptual difficulties if we anticipate behavior analogous to that found in many lanthanide 4f systems. Some alternate approaches are discussed.
Date: January 1, 1979
Creator: Lander, G.H.
Partner: UNT Libraries Government Documents Department

Orbital effects in actinide systems

Description: Actinide magnetism presents a number of important challenges; in particular, the proximity of 5f band to the Fermi energy gives rise to strong interaction with both d and s like conduction electrons, and the extended nature of the 5f electrons means that they can interact with electron orbitals from neighboring atoms. Theory has recently addressed these problems. Often neglected, however, is the overwhelming evidence for large orbital contributions to the magnetic properties of actinides. Some experimental evidence for these effects are presented briefly in this paper. They point, clearly incorrectly, to a very localized picture for the 5f electrons. This dichotomy only enhances the nature of the challenge.
Date: January 1, 1983
Creator: Lander, G.H.
Partner: UNT Libraries Government Documents Department

Pulsed neutrons: one year of experience with the new source at Argonne National Laboratory

Description: The Intense Pulsed Neutron Source (IPNS) at Argonne National Laboratory is a spallation neutron source based on a 500-MeV proton accelerator operating at 30 Hz and with an average proton current of approx. 10 ..mu..A. Neutron-scattering instruments for elastic scattering include two powder diffractometers, a single-crystal diffractometer based on the Laue method and employing a large (30 x 30 cm) position-sensitive scintillation detector, a small-angle diffractometer using a position-sensitive detector, and a polarized-neutron diffractometer which will utilize the spin-refrigerator device to obtain a beam of white polarized neutrons. For inelastic scattering, we presently have the crystal-analyzer spectrometer and two chopper spectrometers capable of providing monoenergetic incident neutron beams of between 100 and 600 MeV. From its inception IPNS has been operating in a user mode and the selection of experiments is made by a Program Committee twice a year on the basis of the scientific merit of submitted proposals.
Date: January 1, 1982
Creator: Lander, G.H.
Partner: UNT Libraries Government Documents Department

Scientific opportunities with advanced facilities for neutron scattering

Description: The present report documents deliberations of a large group of experts in neutron scattering and fundamental physics on the need for new neutron sources of greater intensity and more sophisticated instrumentation than those currently available. An additional aspect of the Workshop was a comparison between steady-state (reactor) and pulsed (spallation) sources. The main conclusions were: (1) the case for a new higher flux neutron source is extremely strong and such a facility will lead to qualitatively new advances in condensed matter science and fundamental physics; (2) to a large extent the future needs of the scientific community could be met with either a 5 x 10/sup 15/ n cm/sup -2/s/sup -1/ steady state source or a 10/sup 17/ n cm/sup -2/s/sup -1/ peak flux spallation source; and (3) the findings of this Workshop are consistent with the recommendations of the Major Materials Facilities Committee.
Date: January 1, 1984
Creator: Lander, G.H. & Emery, V.J. (eds.)
Partner: UNT Libraries Government Documents Department

Small-angle neutron scattering of DyMo/sub 6/S/sub 8/ at low temperature

Description: The Chevrel phase compound DyMo/sub 6/S/sub 8/ becomes superconducting at 2.05 K and is antiferromagnetic (AFM) T/sub M/ = 0.4 K. Experiments with an applied magnetic field have shown that the AFM state is gradually suppressed and a ferromagnetic component seems to be induced before H/sub c2/ (1200 0e) is reached. The present experiments have been performed at the small-angle scattering facility D11 at the Institut Laue Langevin with a dilution refrigerator and applied fields of up to 2000 0e. Neutron wavelengths of 6.25 and 10A have been used allowing a minimum wavevector Q/sub min/ approx. = 0.005 A/sup -1/ to be achieved. For increasing H we find a sudden increase in the small-angle scattering at H = 180 0e, which we identify as H/sub c1/. With a position sensitive detector we have also investigated the scattering as a function of the relative orientation of Q Vector and H Vector. For a simple paramagnet we would expect more magnetic intensity with Q Vector perpendicular H Vector than with Q Vector parallel H Vector, however, we find the exact reverse with somewhat more intensity for Q Vector parallel H Vector. This is not understood, but we propose that the small-angle scattering is predominantly inelastic. Similar anisotropic effects have been found in other Chevrel phase systems. 7 references.
Date: January 1, 1983
Creator: Lander, G.H.; Pynn, R. & Ishikawa, M.
Partner: UNT Libraries Government Documents Department

Magnetoelastic interactions in UO$sub 2$

Description: Neutron diffraction measurements of the elastic magnetic scattering cross section from antiferromagnetic UO$sub 2$ show additional nuclear intensity below T/sub N/ = 30.8$sup 0$K. An examination was made of the possibility of analyzing the additional scattering in terms of homogeneous distortions, which involve shifts of the oxygen atoms from their fluorite lattice sites. The behavior arising from the presence of these homogeneous distortion modes formed the basis for Allen's theory of a cooperative Jahn--Teller effect in UO$sub 2$. However, an analysis in terms of these homogeneous distortions cannot explain the neutron data. But, by extending Allen's concepts to include inhomogeneous deformations, corresponding to a zone boundary q = ($pi$/a) (1,0,0) phonon, excellent agreement is obtained between theory and experiment. The oxygen displacement is 0.014(1) A from the fluorite lattice positions and, in addition, the inhomogeneous deformation (T/sub 2g/(Q$sup 1$)--T/sub 1g/) does not require a reduction in the overall symmetry of the unit cell. The essential features of Allen's theory for UO$sub 2$ can still be maintained. (auth)
Date: December 1975
Creator: Faber, J., Jr.; Lander, G. H. & Cooper, B. R.
Partner: UNT Libraries Government Documents Department

High-energy neutron spectroscopy on f-electron oxides

Description: Neutron inelastic scattering with the chopper spectrometers at IPNS has been used to measure, for the first time, the ground to excited crystal-field transitions in PrO/sub 2/ (130 meV), BaPrO/sub 3/ (255 meV), and UO/sub 2/ (approx.160 meV). From these measurements we deduce the values of V/sub 4/ = A/sub 4/ (r/sup 4/) and compare both the absolute values and their variation with theoretical estimates. In the case of UO/sub 2/ two peaks are seen, one at 155 and the other at 172 meV. This structure exists both below and above T/sub N/ (30.8K) and we believe is caused by a dynamic Jahn-Teller effect that splits the excited GAMMA/sub 3/ state. 10 refs., 2 figs., 1 tab.
Date: January 1, 1985
Creator: Kern, S.; Loong, C.K. & Lander, G.H.
Partner: UNT Libraries Government Documents Department

Induced magnetization density in the 5f system UA1/sub 2/

Description: A polarized-neutron diffractometer was used to study the induced magnetization density in the unit cell of the 5f system UA1/sub 2/. Measurements have been made on single crystals at 4.2/sup 0/K with an applied magnetic field of 42.5 KOe (induced magnetic moment 0.0344 ..mu../sub B/ per mole). Neutron wavelengths of 1.067 and 0.785 A and different crystal thicknesses have been used to correct for and eliminate any effects from extinction. It is found that the major part of the magnetization density appears as a contribution localized at the U site, exhibiting a form factor similar to that of the f/sup 3/ free ion, and extrapolating in the forward direction to the value given by the bulk susceptibility. However, there are two interesting aspects of the density. First, the form factor points differ significantly from a smooth curve. This indicates asphericity in the 5f electron distribution arising from the crystalline environment of the U atoms. Second, and more unusual, finite magnetic scattering amplitudes have been observed for a series of reflections to which the centrosymmetric density at the U site cannot contribute. We consider this as strong evidence of noncentrosymmetric bonding involving the 5f-6d electrons.
Date: September 1, 1980
Creator: Rakhecha, V.C.; Lander, G.H. & Moon, R.M.
Partner: UNT Libraries Government Documents Department

Neutron studies of the magnetic phase transition in UAs

Description: Uranium arsenide undergoes a first order phase transition to a type I antiferromagnetic (AF-I) structure at T/sub N/ approx. equal to 124 K. Above T/sub N/, UAs exhibits strongly anisotropic critical scattering suggestive of pseudo two-dimensional behavior. The temperature dependence suggests a tendency to order with an incommensurate wave vector but then suddenly orders with the AF-I structure. A mean-field theory is proposed based on an anisotropic Hamiltonian and provides a reasonable description of the results.
Date: January 1, 1980
Creator: Shapiro, S M; Sinha, S K; Lander, G H & Vogt, O
Partner: UNT Libraries Government Documents Department

Magnetically induced lattice distortions in the neptunium monopnictides

Description: From 19th conference on magnetism and magnetic materials; Boston, Massachusetts, USA (13 Nov 1973). The lattice parameters of the neptunium monopnictides (NaCl crystal structure) were measured between 5 and 300 deg K by x- ray diffraction. NpN becomes ferromagnetic at 87 deg K. At T/sub c/ a rhombohedral distortion, which indicates that the (111) is the easy axis, is observed. At 5 deg K, the rhombohedral angle is 60.46 plus or minus 0.02 deg . NpP exhibits a tetragonal distortion at 74 deg K, at which temperature the antiferromagnetic 3+, 3- structure becomes commensurate with the lattice. At 5 deg K, c/a = 0.9958 plus or minus 2. NpAs becomes tetragonal at T/sub N/ = 175 deg K when the material has a 4+, 4- structure. However, NpAs returns to being a cubic at 142 deg K at which temperature the magnetic structure is the simple plus or minus (type I) configuration. The return'' to cubic is first order; the volume of the unit cell expands by 0.23%. Np,Sb, which orders antiferromagnetically at 207 deg K with the type I structure, remains cubic in the ordered regime. (auth)
Date: January 1, 1973
Creator: Mueller, M.H.; Lander, G.H.; Knott, H.W. & Reddy, J.F.
Partner: UNT Libraries Government Documents Department

Uranium nitride: a cubic antiferromagnet with anisotropic critical behavior

Description: Highly anisotropic critical scattering associated with the transition at T/sub N/ = 49.5 K to the type-I antiferromagnetic structure has been observed in uranium nitride. The transverse susceptibility is found to be unobservably small. The longitudinal susceptibility diverges at T/sub N/ and its anisotropy shows that the spins within the (001) ferromagnetic sheets of the (001) domain are much more highly correlated than they are with the spins lying in adjacent (001) sheets. The correlation range within the sheets is much greater than that expected for a Heisenberg system with the same T/sub N/. The rod-like scattering extended along the spin and domain direction is reminiscent of two-dimensional behavior. The results are inconsistent with a simple localized model and may reflect the itinerant nature of the 5f electrons.
Date: November 1, 1977
Creator: Buyers, W.J.L.; Holden, T.M.; Svensson, E.C. & Lander, G.H.
Partner: UNT Libraries Government Documents Department

Crystal-field spectroscopy of UCl/sub 4/ by neutron inelastic scattering

Description: The crystal-field level scheme of uranium tetrachloride has been the subject of a number of investigations by both optical and magnetic measurements. Agreement does not exist. To attempt to resolve this problem we have performed experiments with the pulsed neutron source at ANL which allows electronic spectroscopy to be extended to approx.2000 cm/sup -1/. Initially using incident energies of 400 cm/sup -1/ we examined the GAMMA/sub 4/ (ground state) to GAMMA/sub 5/ (excited state) level with the sample at 10K. A very strong line is seen at 92 +- 1 cm/sup -1/ which is clearly the GAMMA/sub 4/ ..-->.. GAMMA/sub 5/' transition. This is in good agreement with a previous neutron study but in disagreement with the 110 to 120 cm/sup -1/ assignment by other techniques. On warming to 50K the peak position shifts to lower energy by approx.4%, broadens, and decreases dramatically in magnitude to approx.1/3 of its value at 10K. Boltzmann statistics would predict a decrease of only 20% on warming from 10 to 50K. We assume from this that a strong coupling occurs between the crystal-field and vibronic states. We have also used an incident energy of 2000 cm/sup -1/ to search for the higher GAMMA/sub 5/' state. This was found at 1125 +- 3 cm/sup -1/, in good agreement with previous work. On warming the sample this GAMMA/sub 4/ ..-->.. GAMMA/sub 5/ transition strength rapidly diminishes. Since the phonon spectrum does not extend beyond approx.300 cm/sup -1/, this behavior is most unusual. Because of this strong relaxational process, we have not been able to determine the positions of the other crystal-field levels. 6 refs.
Date: January 1, 1985
Creator: Delamoye, P.; Krupa, J.C.; Kern, S.; Loong, C.K. & Lander, G.H.
Partner: UNT Libraries Government Documents Department

Recent highlights of X-ray magnetic scattering studies from surfaces

Description: In this work, recent studies of surface magnetism, as observed by x-ray scattering techniques, are described. The experiments were concerned with uranium dioxide crystals for which x-ray resonance effects enhance the small magnetic signal from the surface. The main result is that, in contrast to the bulk which exhibits a discontinuous magnetic ordering transition, both the (001) and (111) surface layers order continuously. This is reminiscent of the general phenomenon of surface wetting, but had not been previously observed for magnetic materials. Magnetic reflectivity studies show further that the near-surface magnetic layers are more disordered than layers deep in the bulk, even at low temperatures.
Date: December 31, 1998
Creator: Watson, G.M.; Gibbs, D. & Lander, G.H.
Partner: UNT Libraries Government Documents Department

Phonon dispersion relation of uranium nitrate above and below the Neel temperature

Description: Neutron coherent inelastic scattering measurements have been made of the phonon dispersion relation of uranium nitride both above and below the Neel temperature T/sub N/ = 50 K. Within the precision of the measurements, about 1% in frequency and 10% in line width and in scattered neutron intensity, no significant changes in these phonon properties were observed as a function of temperature other than those arising from population factor changes and a small stiffening of the lattice as the temperature decreases. At 4.2 K, two acoustic and two optic branches have been determined for each of the (001), (110) and (111) directions. The optic mode measurements revealed (a) a 20% variation in frequency across the Brillouin zone and (b) an interesting disposition of the LO and TO modes, such that nu/sub LO/ > nu/sub TO/ along (001) and (110), while the reverse is true along the (111) directions. Within the experimental resolution, the LO and TO modes are degenerate near q = 0. We have been unable to obtain any satisfactory description of these results on the basis of conventional theoretical treatments (e.g. rigid-ion or shell models). Other possible interpretations of the results are discussed.
Date: January 1, 1977
Creator: Dolling, G; Holden, T M; Evensson, E C; Buyers, W J.L. & Lander, G H
Partner: UNT Libraries Government Documents Department