R.A. Robinson

Heavy fermion behavior of U2T2X compounds

Magnetic and specific-heat studies of U[sub 2][ital T][sub 2][ital X] compounds show a frequent occurrence of the [gamma] enhancement in conjunction with the onset of antiferromagnetic ordering. The largest value of 830 mJ/mol K[sup 2] was observed in U[sub 2]Pt[sub 2]In, which is nonmagnetic down to 1.2 K. Variations of electronic structure are documented by optimized relativistic LCAO calculation.Magnetic and specific‐heat studies of U2T2X compounds show a frequent occurrence of the γ enhancement in conjunction with the onset of antiferromagnetic ordering. The largest value of 830 mJ/mol K2 was observed in U2Pt2In, which is nonmagnetic down to 1.2 K. Variations of electronic structure are documented by optimized relativistic LCAO calculation.

Magnetic and crystallographic properties of UNiSn

Abstract We have performed neutron diffraction measurements on the compound UNiSn above and below its magnetic transition temperature. Below the transition, the magnetic structure is FCC type 1 antiferromagnetism with a uranium moment of (1.42±0.03)μ B parallel to the [100] axis. A small amount of second phase U 3 Ni 3 Sn 4 was present in the sample and its structure was also analysed as the Y 3 Au 3 Sb 4 structure, with space group 1 43d.

Neutron scattering study of the magnetic excitations in ferromagnetic iron at high energy transfers

We have measured the spin‐wave spectrum of iron by neutron inelastic scattering using the Low‐Resolution Medium‐Energy Chopper Spectrometer at the Intense Pulsed Neutron Source (IPNS) of the Argonne National Laboratory. Interest focuses on the magnetic excitations at high energy transfers where the spin‐wave dispersion relation has not previously been determined. In measurements performed at 10 K using a 23‐g single crystal of pure iron, we observed magnetic scattering around the (110) reciprocal lattice point with spin‐wave energies from 40 to 160 meV. The spin waves over the entire range of energy are found to be consistent with an isotropic spin‐wave dispersion relation. With the present experimental sensitivity we were unable to observe any band structure effects such as Stoner excitations or optical magnons in this range of wave vector and energy.

Internal magnetic structure of Mn12 acetate by polarized neutron diffraction

The internal magnetic structure of [Mn12 O12 (CD3 COO)16 (H2 O)4 ]2CD3 COOH 4H2 O as determined by polarized-beam single-crystal neutron diffraction is reported. The standard picture, in which the inner tetrahedron of (S = 3/2) Mn4+ ions is polarized antiparallel to an outer ring of eight (S = 2) Mn3+ ions, is confirmed directly. While the total magnetization for the molecule is in good agreement with bulk measurements, the individual moment components on each of the three symmetry-independent Mn sites are less than predicted by the standard picture. There is no evidence for net moments on the oxygen atoms, but overlap of positive and negative magnetization on the oxygen sites cannot be ruled out.

Inelastic neutron scattering study of Mn12–acetate

We report zero-field inelastic neutron scattering experiments on a deuterated powder sample of Mn{sub 12}{endash}acetate consisting of a large number of nominally identical spin-10 magnetic clusters. Our resolution enables us to see a series of peaks corresponding to transitions between the anisotropy levels within the spin-10 manifold. A fit to the spin Hamiltonian H={minus}DS{sub z}{sup 2}{minus}{mu}{sub B}{bold B{center_dot}g{center_dot}S}{minus}AS{sub z}{sup 4}+C(S{sub +}{sup 4}+S{sub {minus}}{sup 4}) yields an anisotropy constant D=(0.54{plus_minus}0.02)K and a fourth-order diagonal anisotropy coefficient A=(1.2{plus_minus}0.1){times}10{sup {minus}3}K (the other terms being negligible). Performed in the absence of a magnetic field, our experiments do not involve the {ital g} values as fitting parameters, thereby yielding particularly reliable values of {ital D} and {ital A}. {copyright} {ital 1999 American Institute of Physics.}

Magnetism in U2T2X compounds

Abstract Magnetic and other electronic properties are presented for the U 2 T 2 X compounds (T = late transition metal, X = Sn or In), that crystallize in the tetragonal U 3 Si 2 -type structure. They show the formation of 5f magnetic moments and antiferromagnetic ordering in compounds where weak 5f-d hybridization is expected. Near the onset of magnetic ordering strongly enhanced γ-values are found. Neutron-diffraction experiments on U 2 Pd 2 Sn and U 2 Pd 2 In point to a connection between the mutual coordination of U atoms and the type of magnetic anisotropy.

Magnetic structures of actinide materials by pulsed neutron diffraction (invited)

We describe some attempts to observe magnetic structure in various actinide (5f‐electron) materials. Our experimental technique is neutron powder diffraction as practiced at a spallation (pulsed) neutron source. We will discuss our investigations of α‐Pu, δ‐Pu, α‐UD3, and β‐UD3. β‐UD3 is a simple ferromagnet: Surprisingly, the moments on the two nonequivalent uranium atoms are the same within experimental error. α‐UD3, α‐Pu, and δ‐Pu are nonmagnetic, within the limits of our observations. Our work with pulsed neutron diffraction shows that it is a useful technique for research on magnetic materials.

Temperature dependence of magnetic order in single-crystalline UPdSn

The noncollinear hexagonal antiferromagnet UPdSn exhibits two magnetic phase transitions, at 35.5 and 23 K. The first transition is from a hexagonal paramagnetic state to a noncollinear antiferromagnetic state with a doubled unit cell (phase I). The second 23‐K transition is to a monoclinic magnetic structure (phase II). Ever since these transitions were discovered, the question has been whether the moments simply rotate at 23 K, or whether the y and z components of the moment order at 35.5 K while the x component orders out of incipient fluctuations at the lower 23‐K transition. While previous powder studies were rather inconclusive on this point, in this study new single‐crystal neutron‐diffraction results are presented that show the second picture to be correct. In addition, the structural distortions that accompany the change in symmetry are discussed and show that there is phase‐II type magnetic short‐range order between 23 and 35.5 K.

Magnetic phases in UNiGe

Original measurements of 4.2 K magnetization curves and temperature dependence of magnetic susceptibility were completed by an extended study of magnetization, specific heat, and electric resistivity as a function of temperature and magnetic field. Neutron diffraction experiments were also performed on powder and single crystal. The specific heat shows a sharp peak at 41.5 K, corresponding to the first-order phase transition, and a broad anomaly around 50 K. The magnetic structures in UNiGe confirm that the strong bonding of 5f orbitals along the a axis leads to a huge magnetic anisotropy with U magnetic moments perpendicular to this direction.

Magneto-structural distortions in the noncollinear hexagonal antiferromagnet UPdSn

Abstract The intermetallic compound UPdSn has been studied by means of high-resolution neutron powder diffraction at low temperature, and both orthorhombic and monoclinic structural distortions have been observed. In addition to confirming the previously published noncollinear antiferromagnetic structures with these symmetries, full Rietveld refinements including the magnetic cross sections have been made on the new data, jointly with older low-resolution data. The structure is monoclinic below 25 K with space group P2 1 and magnetic symmetry P c 2 1 , orthorhombic between 25 and 40 K with structural space group Cmc2 1 and magnetic space group P C m′c2 1 , and paramagnetic above 40 K with space group P6 3 mc. The temperature variations of the orthorhombicity ( b /√3− a ) and monoclinicity (γ−90) parameters have been extracted and the monoclinicity is linearly coupled to the x -component of the uranium magnetic moment μ x , the ‘magnetic monoclinicity’ order parameter. In contrast, the orthorhombicity seems to be coupled to the total uranium magnetic moment. The sign of the coupling between μ x and γ−90 has also been determined. It is positive, that is the projected moments prefer to point across the shorter diagonal of the monoclinic basal plane, rather that the longer diagonal.