Dynamic crystallography reveals spontaneous anisotropy in cubic GeTe.

Cubic energy materials such as thermoelectrics or hybrid perovskite materials are often understood to be highly disordered. In GeTe and related IV-VI compounds, this is thought to provide the low thermal conductivities needed for thermoelectric applications. Since conventional crystallography cannot distinguish between static disorder and atomic motions, we develop the energy-resolved variable-shutter pair distribution function technique.

A neutron tomography study: probing the spontaneous crystallization of randomly packed granular assemblies.

We study the spontaneous crystallization of an assembly of highly monodisperse steel spheres under shaking, as it evolves from localized icosahedral ordering towards a packing reaching crystalline ordering. Towards this end, real space neutron tomography measurements on the granular assembly are carried out, as it is systematically subjected to a variation of frequency and amplitude. As expected, we see a presence of localized icosahedral ordering in the disordered initial state (packing fraction ≈ 0.

Hyperfine interaction in cobalt by high-resolution neutron spectroscopy.

We have investigated the ferromagnetic phase transition of elemental Co by high-resolution neutron backscattering spectroscopy. We monitored the splitting of the nuclear levels by the hyperfine field at the Co nucleus. The energy of this hyperfine splitting is identified as the order parameter of the ferromagnetic phase transition.

Setup for polarized neutron imaging using in situ He cells at the Oak Ridge National Laboratory High Flux Isotope Reactor CG-1D beamline.

In the present study, we report a new setup for polarized neutron imaging at the ORNL High Flux Isotope Reactor CG-1D beamline using an in situ He polarizer and analyzer. This development is very important for extending the capabilities of the imaging instrument at ORNL providing a polarized beam with a large field-of-view, which can be further used in combination with optical devices like Wolter optics, focusing guides, or other lenses for the development of microscope arrangement. Such a setup can be of advantage for the existing and future imaging beamlines at the pulsed neutron sources.

Magnetic glass state and magnetoresistance in SrLaFeCoO double perovskite.

Unusual features in magnetization resembling the kinetic arrest of a magnetic glass state are observed in the La-doped double perovskite, SrLaFeCoO. Neutron powder diffraction experiments confirm the presence of antisite disorder as well as a lack of long-range magnetic order down to 4 K in this double perovskite which displays spin glass-like features in dc and ac susceptibilities. Magnetic relaxation observed through cooling and heating under unequal fields (CHUF) point towards unusual domain dynamics which is supported by a broad memory effect.

Spin-driven symmetry breaking in the frustrated fcc pyrite MnS2.

We report the characterisation of natural samples of the cubic pyrite mineral MnS2 using very high resolution synchrotron x-ray diffraction techniques. At low temperatures we find a new low temperature polymorph, which results from coupling between magnetic and lattice degrees of freedom. Below the magnetic ordering temperature T(N) = 48 K, we detect a pseudo-tetragonal distortion with a tiny c/a ratio of 1.

Pressure control of magnetic clusters in strongly inhomogeneous ferromagnetic chalcopyrites.

Room-temperature ferromagnetism in Mn-doped chalcopyrites is a desire aspect when applying those materials to spin electronics. However, dominance of high Curie-temperatures due to cluster formation or inhomogeneities limited their consideration. Here we report how an external perturbation such as applied hydrostatic pressure in CdGeP₂:Mn induces a two serial magnetic transitions from ferromagnet to non-magnet state at room temperature.

Giant pressure-induced volume collapse in the pyrite mineral MnS2.

Dramatic volume collapses under pressure are fundamental to geochemistry and of increasing importance to fields as diverse as hydrogen storage and high-temperature superconductivity. In transition metal materials, collapses are usually driven by so-called spin-state transitions, the interplay between the single-ion crystal field and the size of the magnetic moment. Here we show that the classical S = 5/2 mineral hauerite (MnS2) undergoes an unprecedented (ΔV ~ 22%) collapse driven by a conceptually different magnetic mechanism.

Effect of size reduction on the structural and magnetic order in LaMnO(3+δ) (δ ≈ 0:03) nanocrystals: a neutron diffraction study.

We report a structural transition from the orthorhombic to the rhombohedral phase upon size reduction in nanocrystalline LaMnO(3+δ) (δ ≈ 0:03) as revealed through neutron diffraction studies. The transition occurs when the average particle (crystallite) size is taken below ~50 nm without change of δ, which is fixed at around 0.03 as measured by a number of characterization tools.

Direct evidence for nuclear spin waves in Nd2CuO4 by high-resolution neutron-spin-echo spectroscopy.

The possibility of coupling through the hyperfine interaction of nuclear spins with the electronic spin system has given rise to hope for potential novel applications in spintronics and quantum computations. We investigated the dispersion of nuclear spin waves in such a coupled system, Nd2CuO4, by using neutron-spin-echo spectroscopy at millikelvin temperatures. Our results show the existence of dispersion of nuclear spin waves in Nd2CuO4 at T D 40 mK.

Low energy nuclear spin excitations in Ho metal investigated by high resolution neutron spectroscopy.

We have investigated the low energy excitations in metallic Ho by high resolution neutron spectroscopy. We found at T = 3 K clear inelastic peaks in the energy loss and energy gain sides, along with the central elastic peak. The energy of this low energy excitation, which is 26.

Magnetoelastic effects in multiferroic YMnO3.

We have investigated magnetoelastic effects in multiferroic YMnO(3) below the antiferromagnetic phase transition, T(N) ≈ 70 K, using neutron powder diffraction. The a lattice parameter of the hexagonal unit cell of YMnO(3) decreases normally above T(N), but decreases anomalously below T(N), whereas the c lattice parameter increases with decreasing temperature and then increases anomalously below T(N). The unit cell volume also undergoes an anomalous contraction below T(N).

Magnetic ordering in double perovskites R2CoMnO6 (R = Y, Tb) investigated by high resolution neutron spectroscopy.

We have investigated low energy nuclear spin excitations in double perovskite compounds R(2)CoMnO(6) (R=Y, Tb) by inelastic neutron scattering with a high resolution back-scattering spectrometer. We observed inelastic signals at about 2.1 μeV for Y(2)CoMnO(6) and also for Tb(2)CoMnO(6) at T = 2 K in both energy-loss and energy-gain sides.

Magnetoelastic effects in Jahn-Teller distorted CrF₂ and CuF₂ studied by neutron powder diffraction.

We have studied the temperature dependence of the crystal and magnetic structures of the Jahn-Teller distorted transition metal difluorides CrF₂ and CuF₂ by neutron powder diffraction in the temperature range 2-280 K. The lattice parameters and the unit cell volume show magnetoelastic effects below the Néel temperature. The lattice strain due to the magnetostriction effect couples with the square of the order parameter of the antiferromagnetic phase transition.

Structural and thermal properties of LaMnO3 from neutron diffraction and first principles studies.

Neutron diffraction experiments have been performed on powder samples of LaMnO(3) below and above the Jahn-Teller transition temperature of 750 K. Experimental investigations are assisted by density functional theory calculations. Theoretical studies are carried out for the orbitally ordered state of LaMnO(3) which allows one to compare the behavior of the orbitally ordered and disordered structures as a function of temperature.

Magnetoelastic effect in MF2 (M = Mn, Fe, Ni) investigated by neutron powder diffraction.

We have investigated the magnetoelastic effects in MF(2) (M = Mn, Fe, Ni) associated with the antiferromagnetic phase transition temperature T(N) by neutron powder diffraction. The temperature variation of the lattice parameters and the unit cell volume has been determined accurately with small temperature steps. From the temperature variation of the lattice parameters a, c and V the lattice strains Δa, Δc and ΔV associated with the antiferromagnetic phase transition have been extracted.

Magnetic lattice dynamics of the oxygen-free FeAs pnictides: how sensitive are phonons to magnetic ordering?

To shed light on the role of magnetism on the superconducting mechanism of the oxygen-free FeAs pnictides, we investigate the effect of magnetic ordering on phonon dynamics in the low-temperature orthorhombic parent compounds, which present a spin density wave. The study covers both the 122 (AFe(2)As(2); A = Ca, Sr, Ba) and 1111 (AFeAsF; A = Ca, Sr) phases. We extend our recent work on the Ca (122 and 1111) and Ba (122) cases by treating, computationally and experimentally, the 122 and 1111 Sr compounds.

The magnetoelastic effect in CoF2 investigated by means of neutron powder diffraction.

We have investigated the magnetoelastic effects in CoF(2) associated with the antiferromagnetic phase transition temperature T(N)≈39 K by means of neutron powder diffraction. The temperature variation of the lattice parameters and the unit cell volume has been determined accurately with small temperature steps. From the temperature variation of the lattice parameter c we extracted the lattice strain Δc associated with the antiferromagnetic phase transition.

Anomalous hyperfine interaction in CoF(2) investigated by high resolution neutron spectroscopy.

We investigated the low energy excitations in CoF(2) in the µeV range with a back-scattering neutron spectrometer. The energy scans on a CoF(2) powder sample revealed inelastic peaks at E = 0.728 ± 0.

Neutron diffraction investigation of the magnetic structure and magnetoelastic effects in NdMnO(3).

We investigated the evolution of the magnetic structure and magnetoelastic effects of NdMnO(3) by neutron powder diffraction. We confirmed the A-type antiferromagnetic (AF) structure of NdMnO(3) below T(N)≈82 K with magnetic moments parallel to the b axis of the orthorhombic crystal structure (space group Pbnm). We found that the magnetic moments of Nd order below about T(N)(Nd)≈20 K in a ferromagnetic structure with moments parallel to the c axis.

Similarities between structural distortions under pressure and chemical doping in superconducting BaFe2As2.

The discovery of a new family of high-T(C) materials, the iron arsenides (FeAs), has led to a resurgence of interest in superconductivity. Several important traits of these materials are now apparent: for example, layers of iron tetrahedrally coordinated by arsenic are crucial structural ingredients. It is also now well established that the parent non-superconducting phases are itinerant magnets, and that superconductivity can be induced by either chemical substitution or application of pressure, in sharp contrast to the cuprate family of materials.

Direct evidence for the Nd magnetic ordering in NdMnO(3) from the hyperfine field splitting of Nd nuclear levels.

We investigated the low energy excitations in NdMnO(3) in the µeV range by a backscattering neutron spectrometer. The energy spectra on polycrystalline NdMnO(3) samples revealed inelastic peaks at E = 2.15 ± 0.