Evolution of spin correlations in SrDy2O4 in an applied magnetic field

The development of short- and long-range magnetic order induced in a frustrated zig-zag ladder compound SrDy2O4 by an applied field is studied using neutron diffraction techniques. In zero field, SrDy2O4 lacks long-range magnetic order down to temperatures as low as 60 mK, and the observed powder neutron diffraction (PND) patterns are dominated by very broad diffuse scattering peaks. Single crystal neutron diffraction reveals that the zero-field magnetic structure consists of a collection of antiferromagnetic chains running along the c axis and that there is very little correlation between the chains in the ab plane. In an applied magnetic field, the broad diffuse scattering features in PND are gradually replaced by much sharper peaks, however, the pattern remains rather complex, reflecting the highly anisotropic nature of SrDy2O4. Single crystal neutron diffraction shows that a moderate field applied along the b axis induces an up-up-down magnetic order associated with a 1/3-magnetisation plateau, in which magnetic correlation length in the ab plane is significantly increased, but it nevertheless remains finite. The resolution limited k = 0 peaks associated with a ferromagnetic arrangement appear in powder and single crystal neutron diffraction patterns in fields of 2.5 T and above.Comment: 10 pages, 11 figure

Direct evidence for the magnetic ordering of Nd ions in NdMn2_2Si2_2 and NdMn2_2Ge2_2 by high resolution inelastic neutron scattering

We have investigated the low energy nuclear spin excitations in NdMn$_2$Si$_2$ and NdMn$_2$Ge$_2$ by high resolution inelastic neutron scattering. Previous neutron diffraction investigations gave ambiguous results about Nd magnetic ordering at low temperatures. The present element-specific technique gave direct evidence for the magnetic ordering of Nd ions. We found considerable difference in the process of the Nd magnetic ordering at low temperature in NdMn$_2$Si$_2$ and NdMn$_2$Ge$_2$. Our results are consistent with those of magnetization and recent neutron diffraction measurements

Anapole Correlations in Sr2IrO4 Defy the jeff = 1/2 Model

Zel'dovich (spin) anapole correlations in Sr2IrO4 unveiled by magnetic neutron diffraction contravene the spin-orbit coupled ground state used by the jeff = 1/2 (pseudo-spin) model. Specifically, spin and space know inextricable knots which bind each to the other in the iridate. The diffraction property studied in the Letter is enforced by strict requirements from quantum mechanics and magnetic symmetry. It has not been exploited in the past, whereas neutron diffraction by anapole moments is established. Entanglement of the electronic degrees of freedom is captured by binary correlations of the anapole and position operators, and hallmarked in the diffraction amplitude by axial atomic multipoles with an even rank

Phonons from neutron powder diffraction

The spherically averaged structure function \soq obtained from pulsed neutron powder diffraction contains both elastic and inelastic scattering via an integral over energy. The Fourier transformation of \soq to real space, as is done in the pair density function (PDF) analysis, regularizes the data, i.e. it accentuates the diffuse scattering. We present a technique which enables the extraction of off-center phonon information from powder diffraction experiments by comparing the experimental PDF with theoretical calculations based on standard interatomic potentials and the crystal symmetry. This procedure (dynamics from powder diffraction(DPD)) has been successfully implemented for two systems, a simple metal, fcc Ni, and an ionic crystal, CaF$_{2}$. Although computationally intensive, this data analysis allows for a phonon based modeling of the PDF, and additionally provides off-center phonon information from powder neutron diffraction

Crystal structure and phonon softening in Ca3Ir4Sn13

We investigated the crystal structure and lattice excitations of the ternary intermetallic stannide Ca3Ir4Sn13 using neutron and x-ray scattering techniques. For T > T* ~ 38 K the x-ray diffraction data can be satisfactorily refined using the space group Pm-3n. Below T* the crystal structure is modulated with a propagation vector of q = (1/2, 1/2, 0). This may arise from a merohedral twinning in which three tetragonal domains overlap to mimic a higher symmetry, or from a doubling of the cubic unit cell. Neutron diffraction and neutron spectroscopy results show that the structural transition at T* is of a second-order, and that it is well described by mean-field theory. Inelastic neutron scattering data point towards a displacive structural transition at T* arising from the softening of a low-energy phonon mode with an energy gap of Delta(120 K) = 1.05 meV. Using density functional theory the soft phonon mode is identified as a 'breathing' mode of the Sn12 icosahedra and is consistent with the thermal ellipsoids of the Sn2 atoms found by single crystal diffraction data

Neutron diffraction studies on liquids

The above examples serve to illustrate the extent to which neutron diffraction isotopic substitution methods have been used to determine interatomic structure in a wide range of liquid and amorphous systems. The direct determination of pair radial functions not only offers a means of characterising the different structures in liquids, but also provides theorists with information to construct more realistic model potentials which can be used to explore properties in regimes not currently accessible to experiment.\ud \ud It is anticipated that the NDIS methods will continue to be developed and applied to a wider range of systems. The construction and commissioning of new diffractometers with higher count rates, such as D20 and D4C at ILL, and GEM at ISIS with an optimised sample environment for work at non-ambient conditions will enable new and more extensive research to be undertaken. Besides the many problems of immediate interest suggested at the end of some sections, there are several investigations which will become feasible in the longer term as the technology develops. These include: (i) the use of isotopes such as 12C and 13C which will enable detailed and extensive structural studies to be carried out on a wide range of biologically significant materials, and (ii) the exploitation of higher count rates to investigate changes of structure as a chemical reaction occurs

Neutron optical beam splitter from holographically structured nanoparticle-polymer composites

We report a breakthrough in the search for versatile diffractive elements for cold neutrons. Nanoparticles are spatially arranged by holographical means in a photopolymer. These grating structures show remarkably efficient diffraction of cold neutrons up to about 50% for effective thicknesses of only 200 micron. They open up a profound perspective for next generation neutron-optical devices with the capability to tune or modulate the neutron diffraction efficiency.Comment: 4 pages, 2 figure

Carbon Concentration Dependence of the Superconducting Transition Temperature and Structure of MgCxNi3

The crystal structure of the superconductor MgCxNi3 is reported as a function of carbon concentration determined by powder neutron diffraction. The single-phase perovskite structure was found in only a narrow range of carbon content, 0.88 < x < 1.0. The superconducting transition temperature was found to decrease systematically with decreasing carbon concentration. The introduction of carbon vacancies has a significant effect on the positions of the Ni atoms. No evidence for long range magnetic ordering was seen by neutron diffraction for carbon stoichiometries within the perovskite phase stability range.Comment: 4 figure