J. M. Hastings

Magnetic structures of 3d transition metal double fluorides, KMeF3

The antiferromagnetic structures of KCrF3, KMnF3, eF3, KCoF3, KNiF3, and KCuF3 were obtained by neutron fraction. The manganese, iron, cobalt, and nickel doufluonides exhibited an order in which the divalent 3d ion as coupled antiferromagnetically to its six nearest neighbors. In the chromium double fluonide, the magnetic structure consisted of ferromagnetic (001) sheets, coupled antiferromagnetically along the STA001! direction. No magnetic ning was observed in the neutron d fraction pattern copper double fluoride. The magnetic structures and served crystallographic distortions are discussed in of an indirect exchange mechanism and in terms of the crystal field theory, respectively. (auth)

Magnetic Structure of Cr2O3

Pratt and Bailey have recently explained the optical and anomalous magnetic properties of Cr2O3 by means of a model consisting of the basic Cr2O3 antiferromagnetic structure with moments canted away from the c axis and forming some kind of spiral. Our neutron powder‐diffraction data confirm the original Brockhouse moment configuration but indicate that the c‐axis component of the Cr moment is 8% lower than the spin‐only value. This result can be interpreted as supporting the canted model of Pratt and Bailey. A careful search has been made, using single crystals, to find evidence for an ordered or spiraling perpendicular component. Polarization studies establish that there is no magnetic contribution to the fundamental reflections other than that arising from the c‐axis components. A survey of reciprocal space revealed extra spots at forbidden positions, but these could be accounted for semiquantitatively in terms of double‐Bragg scattering. We conclude, therefore, that either the Cr moment is intrinsicall...

Investigation of the Magnetic Structure of ErMn2, TmMn2, TbNi2 by Neutron Diffraction

Magnetic measurements by H. J. Williams (ErMn2 and TmMn2), and W. E. Wallace (TbNi2) have shown that these compounds are magnetically ordered at low temperature with a net moment per formula unit lower than that of the free rare‐earth ion. Neutron diffraction powder patterns have been used to determine the actual magnetic structure. ErMn2 and TmMn2 crystallize with the hexagonal Laves phase structure (C‐14 type). ErMn2 is ferromagnetic, with a moment of 7.72 μB/Er atom at 4.2°K aligned along the c axis. No detectable moment on Mn is observed. TmMn2 is almost completely ferromagnetic, with a moment of 4.95 μB/TM atom at 2.1°K aligned along the c axis, and essentially zero moment for Mn. A faint additional line at low angle seems to indicate a weak antiferromagnetic contribution. TbNi2 crystallizes with the cubic Laves phase structure (C‐15 type). At 4.2°K, ferromagnetic lines as well as additional peaks appear in the diffraction pattern, but the magnetic scattering is rather sensitive to sample preparation.

Strain field and scattered intensity profiling with energy dispersive x-ray scattering

Two powerful synchrotron x-ray scattering techniques for residual strain depth-profiling and tomography-like scatter-intensity profiling of materials are presented. The techniques utilize energy dispersive x-ray scattering, from a fixed microvolume, with microscanning of the specimen being used to profile its interior. The tomography-like profiles exploit scattering-cross-section variations, and can be contrast enhanced by separately monitoring scattering from different crystal structures. The strain profiling technique is shown to finely chronicle the internal strain variation over several mm of steel. Detailed strain profiling for a cantilever spring demonstrates the interplay of residual and external stresses in elastic/plastic deformation. Since surface compression, by shot peening, is a classic method to fortify against fatigue failure, the strain profile for a shot-peened, surface-toughened material is determined and discussed in terms of a simple elastic–plastic stress/strain model. Finally the lattice strains in a WC/Co coated steel composite material are discussed.

Neutron Diffraction Study of the Pseudobinary System ErCo2–ErAl2

Results obtained by neutron diffraction indicate that the magnetic moment of the ferromagnetic sublattice in the Laves phase structures, C15 (ErCo2, ErAl2) and C14 (ErCoAl), varies with composition while the ordering temperatures stay approximately constant. Low values of the aligned moment, however, are accompanied by high values of the disorder scattering at low temperatures. Co carries no moment from 30 mole % ErAl2 on. Results are discussed in terms of tendencies toward antiferromagnetism at certain values for the valence electron concentration.

An order-disorder transition in Sr2IrD5: Evidence for square pyramidal IrD5 units from powder neutron diffraction data

Abstract Neutron diffraction data have been collected on a powdered sample of Sr2IrD5 over a range of temperatures. The compound, which is cubic at room temperature, has been found to exhibit a gradual transformation to a tetragonal phase in the temperature range 200-140 K. As a result of the transition, deuterium atoms which randomly occupy sixfold positions in the cubic phase, become tetragonally ordered. A small fraction of the cubic phase remained untransformed at 4.2 K. Both the cubic and tetragonal structures are consistent with square pyramidal IrD5 units with average IrD distances of 1.714 and 1.718 A, respectively. Agreement factors, R1, for the two structural analyses are 3.44 and 4.94%.

Rotational tunneling of methane on MgO surfaces : a neutron scattering study

High‐resolution inelastic neutron scattering was used to investigate the rotational tunneling of methane molecules in a ■×■R45° commensurate, square‐lattice–solid monolayer adsorbed on MgO (100) surfaces. Good matches to the observed transition energies were obtained using potential functions with C2v symmetry, suggesting that the preferred orientation of the molecule is the dipod‐down configuration with two opposite edges of the H‐atom tetrahedron parallel to the surface plane.

The structure of ce-doped Bi2(MoO4)3 as determined by neutron profile refinement

Abstract The structure of Bi 1.8 Ce 0.2 (MoO 4 ) 3 has been refined with powder neutron diffraction data by the Rietveld method. The structure can be derived by severely distorting the scheelite structure ( AM O 4 ) and is perhaps better written A 2 3 O 1 3 M O 4 , where O = cation vacancy. Of the two bismuth atom sites, cerium preferentially occupies the more symmetric of the two (Bi(2) in the structure) with some cerium found in the scheelite subcell vacancies also. This site preference is understood by examining the symmetries of the two Bi sites. Crystal data: monoclinic, space group P2 1 c , Z = 4, a = 7.697(2), b = 11.535(3), c = 11.944(3) , β = 115.19.

Isotope Effect on Bond Rupture by Electron Impact on Hydrogen, Deuterium, and Tritium

The relative yields by electron impact of atomic and molecular ions, x+/x2+ ratios, from hydrogen, deuterium, and tritium have been measured with a 60° sector type mass spectrometer. It is shown that the major part of the previous disagreement between calculated and observed values can be understood on the basis of voltage discrimination by the ion source. By using magnetic scanning and a high ion accelerating voltage the experimental ratios are: H+/H2+=0.013, D+/D2+=0.0070, T+/T2+=0.0036 for 30 volt electrons. These are to be compared with the calculated ratios: H+/H2+=0.016, D+/D2+=0.0072, T+/T2+=0.0038.

Magnetic Structure and Metamagnetism of HgCr2S4

HgCr2S4 is a normal cubic spinel whose metamagnetic properties were first reported by Baltzer et al. Our neutron diffraction studies show that the magnetic structure at 4.2°K is a simple spiral, identical with that proposed by Plumier for ZnCr2Se4. The propagation vector τ is parallel to the symmetry axis of the spiral and directed along a particular cube edge in a given domain. The moment of Cr3+ is 2.73 μB, in agreement with the magnetization measurements, and the wavelength of the modulation is ∼42 A. The wavelength increases with temperature, reaching a value of ∼90 A at 30°K, and shows very little further variation up to the Neel point (∼60°K). Application of a magnetic field along a cube edge produces no magnetization in that direction, but rather a growth of domains for which τ ‖ H. This process is complete at ∼4 kOe and is followed by a rapid collapse of the spiral into the field direction which saturates at ∼10 kOe. The growth of favorably oriented domains decreases progressively as the direction...