Intrinsically localized mode in α-U as a precursor to a solid-state phase transition

The high-temperature behavior of an intrinsically localized mode (ILM) in α-U was measured using inelastic neutron scattering. The mode, which forms above 450 K on the [010] boundary, becomes undetectable at 675 K. Thermodynamic and transport anomalies that develop with the ILM persist to temperatures above 675 K, but mechanical and electronic anomalies show changes at both 450 and 675 K. Anisotropic thermal expansion shows that ILMs drive the structure toward hexagonal symmetry. On the [1 2 1 2 0] zone boundary, which becomes equivalent to [010] under a hexagonal distortion, a normal mode shows a softening coincident with the disappearance of the ILMs. We argue that the symmetry local to the ILMs becomes hexagonal above 600 K, causing ILMs to hop between equivalent orientations and putting the structure on a path toward the high-temperature γ phase (bcc). © 2008 The American Physical Society

Antiferromagnetism in the magnetoelectric effect single crystal LiMnPO₄

Elastic and inelastic neutron-scattering studies reveal details of the antiferromagnetic transition and intriguing spin dynamics in the magnetoelectric effect single crystal LiMnPO4. The elastic-scattering studies confirm that the system is antiferromagnetic below TN =33.75 K with local magnetic moments (Mn2+; S=5/2) that are aligned along the crystallographic a axis. The spin-wave dispersion curves propagating along the three principal axes, determined by inelastic scattering, are adequately modeled in the linear spin-wave framework assuming a spin Hamiltonian that is parametrized by inter- and in-plane nearest- and next-nearest-neighbor interactions and by easy-plane anisotropy. The temperature dependence of the spin dynamics makes this an excellent model many-body spin system to address the question of the relationship between spin-wave excitations and the order parameter

Search for magnetic order in superconducting Ru Sr2 Eu1.2 Ce0.8 Cu2 O10

Neutron diffraction, polarized neutron transmission, and small angle neutron scattering have been used to investigate the crystal structure and nature of the magnetic order in a polycrystalline sample of Ru Sr2 Eu1.2 Ce0.8 Cu2 O10. The sample was made with the Eu-153 (98.8%) isotope to reduce the high neutron absorption for the naturally occurring element. Full refinements of the crystal structure, space group I4mmm, are reported. At low temperatures only a single magnetic peak is clearly observed in a relatively wide angular range. A sharp spin reorientation transition (SRT) is observed around 35 K, close to the superconducting transition temperature (Tc ∼40 K). Between the spin reorientation temperature and the Neel temperature of 59 K, additional magnetic reflections are observed. However, none of these can be simply indexed on the chemical unit cell, either as commensurate peaks or simple incommensurate magnetism, and the paucity of reflections at low T compels the conclusion that these magnetic Bragg peaks arise from an impurity phase. X-ray and neutron diffraction on the pressed pellet both show that the sample does not appear to contain substantial impurity phases, but it turns out that the magnetic impurity peaks exhibit strong preferred orientation with respect to the pellet orientation, while the primary phase does not. We have been unable to observe any magnetic order that can be identified with the ruthenate-cuprate system. © 2007 The American Physical Society

Spin-wave dispersion in magnetostrictive Fe-Ga alloys: Inelastic neutron scattering measurements

Fe-Ga alloys of appropriate Ga concentration and heat treatment show a very large enhancement in the tetragonal magnetostriction over that of pure α-Fe [λ100 (Fe-Ga) ∼15 λ100 (Fe)]. In order to gain further understanding of the extraordinary magnetoelastic characteristics of this system, the spin dynamics of two of these alloys, Fe1-x Gax (x=0.160 and 0.225), were studied using inelastic neutron scattering techniques. The correlation of the spin-wave dispersion curve with the lattice constant and atomic radii of solute is examined for this and other Fe alloys. © 2007 The American Physical Society

Order and dynamics of intrinsic nanoscale inhomogeneities in manganites

Neutron elastic, inelastic, and high-energy x-ray scattering techniques are used to explore the nature of the polaron order and dynamics in the colossal magnetoresistive (CMR) system La0.7 Ca0.3 Mn O3. Polaron correlations are known to develop within a narrow temperature regime as the Curie temperature is approached from low temperatures, with a nanoscale correlation length that is only weakly temperature dependent. The static nature of these short-range polaron correlations indicates the presence of a glasslike state, very similar to the observations for the bilayer manganite in the metallic-ferromagnetic doping region. In addition to this elastic component, inelastic scattering measurements reveal dynamic correlations with a comparable correlation length and with an energy distribution that is quasielastic. The elastic component disappears at a higher temperature T*, above which the correlations are purely dynamic. These observations are identical to the polaron dynamics found in the bilayer manganite system in the CMR regime, demonstrating that they are a general phenomenon in the manganites. © 2007 The American Physical Society

Double-focusing thermal triple-axis spectrometer at the NCNR

The new thermal triple-axis spectrometer at the NIST Center for Neutron Research (NCNR) is located at the BT-7 beam port. The 165 mm diameter reactor beam is equipped with a selection of Söller collimators, beam-limiters, and a pyrolytic graphite (PG) filter to tailor the beam for the dual 20×20 cm2 double-focusing monochromator system that provides monochromatic fluxes exceeding 108 n/cm2/s onto the sample. The two monochromators installed are PG(002) and Cu(220), which provide incident energies from 5 meV to above 500 meV. The computer controlled analyzer system offers six standard modes of operation, including a diffraction detector, a position-sensitive detector (PSD) in diffraction mode, horizontal energy focusing analyzer with detector, a Q-E mode employing a flat analyzer and PSD, a constant-E mode with the analyzer crystal system and PSD, and a conventional mode with a selection of Söller collimators and detector. Additional configurations for specific measurement needs are also available. This paper discusses the capabilities and performance for this new state-of-the-art neutron spectrometer

Evolution of the commensurate and incommensurate magnetic phases of the S=3/2 kagome staircase Co3 V2 O8 in an applied field

Single crystal neutron diffraction studies have been performed on the S=3/2 kagome staircase compound Co 3 V 2 O 8 with a magnetic field applied along the magnetization easy-axis (). Previous zero-field measurements (Chen Y etal 2006 Phys. Rev. B 74 014430) reported a rich variety of magnetic phases, with a ferromagnetic ground state as well as incommensurate, transversely polarized spin density wave (SDW) phases (with a propagation vector of ) interspersed with multiple commensurate lock-in transitions. The magnetic phase diagram with adds further complexity. For small applied fields, 0 H0.05T, the commensurate lock-in phases are destabilized in favor of the incommensurate SDW ones, while slightly larger applied fields restore the commensurate lock-in phase with =1/2 and yield a new commensurate phase with =2/5. For measurements in an applied field, higher-order scattering is observed that corresponds to the second harmonic

Intrinsic nature of thermally activated dynamical modes in α-U: nonequilibrium mode creation by x-ray and neutron scattering

Inelastic x-ray and neutron scattering were used to measure two matching lattice excitations on the [01ζ] zone boundary in α -uranium. The excitations have the same polarization and reciprocal-space structure, but one has energy consistent with the thermal activation energy of the other, indicating that it creates the mode. The implied mechanism, where a mode is created by an amplitude fluctuation that mirrors the mode itself, is consistent with an intrinsically localized mode (ILM), and this is supported by thermodynamic data. The reciprocal-space structure, however, indicates a mode that is extended along its polarization direction, [010] , and yet fully localized along a perpendicular direction, [001]. An enhancement of the thermal but not electrical conductivity with mode activation also suggests that these modes are more mobile than conventional ILMs. The behavior is, however, qualitatively similar to that predicted for ILMs on two-dimensional hexagonal lattices, where in-plane localization has been shown to be extended over more than ten discrete units, and the modes can be highly mobile. © 2008 The American Physical Society

Polaron formation in the optimally doped ferromagnetic manganites La0.7Sr0.3MnO3and La0.7Ba0.3MnO3

The nature of the polarons in the optimally doped colossal magnetoresistive materials La0.7 Ba0.3 MnO3 (LBMO) and La0.7 Sr0.3 MnO3 (LSMO) is studied by elastic and inelastic neutron scattering. In both materials, dynamic nanoscale polaron correlations develop abruptly in the ferromagnetic state. However, the polarons are not able to lock-in to the lattice and order, in contrast to the behavior of La0.7 Ca0.3 MnO3. Therefore ferromagnetic order in LBMO and LSMO survives their formation, explaining the conventional second-order nature of the ferromagnetic-paramagnetic transition. Nevertheless, the results demonstrate that the fundamental mechanism of polaron formation is a universal feature of these ferromagnetic perovskite manganites. © 2008 The American Physical Society

Nitrogen contamination in elastic neutron scattering

Nitrogen gas accidentally sealed in a sample container produces various spurious effects in elastic neutron scattering measurements. These effects are systematically investigated and the details of the spurious scattering are presented