T. M. Holden

Spin and phonon excitations in actinide systems

Abstract Recent Chalk River neutron scattering experiments on antiferromagnetic UN, on ferromagnetic UTe and US, and on non-magnetic UPd 3 , are reviewed. The spin excitations may be sharp or diffuse and the phonons exhibit anomalies reminiscent of mixed-valent compounds. A very detailed study of the dispersion of the sharp collective excitations seen in dhcp UPd 3 has shown that the uranium is in a stable f 2 , J = 4, ground state subject to a conventional crystal field and exchange. Thus, UPd 3 is the canonical 5f system, analogous to dhcp Pr in the 4f series. Although UN is cubic, large anisotropies are found in its critical scattering, and large anisotropy gaps are found in the spin wave spectra of UN and UTe.

Low temperature magnetic behaviour of CuFeS2 from neutron diffraction data

Abstract Measurements of neutron diffraction spectra were made at temperatures 4.2, 25, 45, 65, 85, 150 and 300 K on powdered polycrystalline synthetic CuFeS2. Standard Rietveld profile analysis using GSAS showed that at all temperatures, a good fit to the data could be obtained with both the chemical and magnetic space groups taken as I 4 2d, and an antiferromagnetic configuration of the Fe spins, as reported previously by Donnay et al. However, graphs of lattice parameters a and c against temperature showed a distinct change in slope close to 50 K, indicating the presence of a transition. Graphs of integrated intensity of diffraction lines against temperature showed corresponding discontinuities in the case of magnetic and mixed nuclear-magnetic lines but not for nuclear lines, indicating that the transition was magnetic. Detailed calculations of predicted magnetic intensities showed that the intensity variations could be explained by Cu spins, having a paramagnetic arrangement down to 50 K and then ordering to an antiferromagnetic form at lower temperatures. The analysis gave a value of ∼ 0.05 μB for the magnetic moment of the Cu ions.

Neutron diffraction studies of diluted magnetic semiconductors (invited)

The nature of the spin correlations in Zn1−cMncTe alloys for several values of c has been studied by neutron powder diffraction as a function of temperature T between 2 and 300 K. Results have been obtained for c=0.376, 0.565, 0.594, and 0.68. The correlations are weak at low c and high T, but intensify progressively as c increases and/or T decreases. For c as high as 0.68, the experiments do not indicate any phase transition to a fully ordered magnetic structure, but the appearance of broad, intense magnetic diffuse scattering peaks at particular positions suggests a strong tendency towards antiferromagnetic ordering of type III. The results are analyzed in terms of a theory of the magnetic scattering from magnetic ions surrounded by near‐neighbor shells of correlated spins. Comparison is also made with the correlations expected from an ordered type III structure modified by various exponential functions of a correlation length κ(c). Application of a magnetic field up to 2 Tesla to the c=0.594 specimen p...

Magnetic excitations and lattice vibrations in uranium rock‐salt structure compounds

The inelastic magnetic and lattice vibrational scattering of neutrons from several uranium pnictides (UN, UAs, and USb) and chalcogenides (US, UTe) have been measured in recent years. The character of the low‐temperature magnetic scattering depends on the lattice constant. The compounds with the largest lattice parameters (USb, UTe) exhibit several branches of well defined magnetic excitations. The other compounds exhibit only a broad, featureless frequency response at low temperature, which may be extremely weak, as in new data presented here for US. Nonetheless, intense paramagnetic scattering is observed above the ordering temperature, with a marked 2d anisotropy in all the pnictides. A force‐constant analysis of the phonon dispersion relations for these materials has been carried out. We show here that UTe, US, and UAs have negative values of the elastic constant C12, as has been found in certain mixed valence compounds. As expected from the increase in ionic size, the compressibility increases with t...

Characterization of residual stresses in bent incoloy-800 tubing by neutron diffraction

The circumferential distributions of axial, hoop, and radial residual strains in a bent INCOLOY-800* tube have been measured by neutron diffraction. High residual axial tensile strains are observed just above the neutral axis with corresponding compressive strains just below the neutral axis. There are marked and systematic differences in the amount of strain in grains with a [001] direction aligned along the axis of the tube and those with a [111] direction aligned along the tube axis. Model calculations show that these differences originate in the different yield points and elastic moduli for different crystallographic directions.

Spin and phonon response in uranium compounds

Recent progress in understanding the spin localization and moment formation in metallic uranium compounds is summarized. Measurements have been made of the neutron scattering from UN, US, UTe and UPd3. The phonon spectra are unusual e.g. the (111) TA branch of UTe lies below the (111) TA branch from ζ = 0.2 to the zone boundary. Force‐constant analysis of the UX series shows that when the U‐U forces are large the moment is small. In non‐ordering UPd3 the high‐frequency band of crystal‐field‐like excitations is well described by a localized spin model. The low‐frequency band (ν∼0.4 THz) is broadened, however, suggesting a delicate balance in the spin localization. In antiferromagnetic UN, where the U‐U overlap is greater, the spin response is entirely broad with a large anisotropy gap. Ferromagnetic UTe has a quadratically rising spin‐wave branch as well as a higher flat branch. Thus UPd3 and UTe provide clear evidence for the existence of crystalline electric fields in metallic actinide compounds.

Exploration of magnetic excitation behaviour of actinide compounds with neutron scattering

Abstract Neutron scattering has revealed that the magnetic response in many actinide rock-salt structure compounds forms a broad continuum that extends to frequencies much higher than kT 0 , where T 0 is the ordering temperature. In ferromagnetic uranium sulphide the continuum extends to 17 THz. Consistent with evidence from resonant photoemission, the neutron inelastic scattering and moment suppression suggest the presence of f-d fluctuations that are a function of interatomic spacing. Thus for compounds with large lattice constants such as UTe and USb the characteristic frequency of the f-d spin fluctuations decreases so as to allow collective spin-wave excitations to appear.

Ground‐ and excited‐state spin waves in PrAl2

The magnetic excitations in ferromagnetic PrAl2 have been studied by means of neutron inelastic scattering. Branches of excitations corresponding to four of the six allowed magnetic‐dipole transitions have been observed. At the magnetic zone boundary, where the excitation spectrum most closely approximates a single‐ion spectrum, the observed frequencies are 1.54±0.04, 2.05±0.05 and 3.76±0.06 THz. The results are analyzed in terms of exchange and crystal‐field parameters using the pseudoboson theory for all states of the J=4 manifold. The crystal‐field‐only ground state is found to be a non‐magnetic doublet but the exchange interaction is strong enough to induce almost the full moment at low temperatures. At elevated temperatures in the ordered phase, excited‐state spin waves have been observed at 0.55±0.10 THz. These correspond to a transition between the upper member of the exchange‐split ground doublet, and the second member of the exchange‐split triplet state.

Temperature dependence of critical neutron scattering in Cr+0.2 at. % V and chromium

The dynamic magnetic susceptibilities, χ(Q,ω), of Cr+0.2 at. % V and chromium were measured in the temperature range (TN−15 K)<(TN+20 K) by neutron scattering. Here, χ (Q,ω) of Cr+0.2 at. % V is clearly incommensurate, in contrast to the apparently commensurate inelastic scattering seen in pure chromium. The observed form of the scattering for each material does not qualitatively change in going from above TN (paramagnetic) to below it (antiferromagnetic). Since no discontinuity in elastic magnetic intensity was observed in the alloy crystal, analysis in terms of the critical phenomena of a second‐order phase transition can now be attempted.

Electronic damping of spin waves in ferromagnetic uranium chalcogenides: Uranium selenide

Abstract Neutron scattering measurements in ferromagnetic USe show peaks following a quadratic dispersion relation with a zone-centre frequency of (9.4±0.3) Thz. The damping of the spin excitations is intermediate between the sharp spin waves in UTe and the overdamped response in US.