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Abstract: We have investigated the hyperfine interaction in Co2SiO4 by inelastic neutron scattering with a high resolution back-scattering neutron spectrometer. The energy spectrum measured from a Co2SiO4 powder sample revealed inelastic peaks at at T=3.5K on both energy gain and energy loss sides. The inelastic peaks move gradually towards lower energy with increasing temperature and finally merge with the elastic peak at the electronic magnetic ordering temperature . The inelastic peaks have been interpreted to be due to the transition between hyperfine-split nuclear level of the 59Co isotopes with spin . The temperature dependence of the energy of the inelastic peak in Co2SiO4 showed that this energy can be considered to be the order parameter of the antiferromagnetic phase transition. The determined hyperfine splitting in Co2SiO4 deviates from the linear relationship between the ordered electronic magnetic moment and the hyperfine splitting in Co, Co–P amorphous alloys and CoO presumably due to the presence of unquenched orbital moment. These results are very similar to those of CoF2 recently reported by Chatterji and Schneider [7]. [Copyright &y& Elsevier]

The discovery of a new family of high-TC 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. The structure and properties of chemically substituted samples are known to be intimately linked; however, remarkably little is known about this relationship when high pressure is used to induce superconductivity in undoped compounds. Here we show that the key structural features in BaFe2As2, namely suppression of the tetragonal-to-orthorhombic phase transition and reduction in the As–Fe–As bond angle and Fe–Fe distance, show the same behaviour under pressure as found in chemically substituted samples. Using experimentally derived structural data, we show that the electronic structure evolves similarly in both cases. These results suggest that modification of the Fermi surface by structural distortions is more important than charge doping for inducing superconductivity in BaFe2As2. [ABSTRACT FROM AUTHOR]

Multiferroic materials are the materials that show several cooperative ordering phenomena simultaneously. Here we consider only those materials that show both antiferromagnetism and ferroelectricity and in which the ordering parameters are coupled to each other. I shall review our recent neutron scattering investigations on multiferroic hexagonal manganites YMnO3. We have determined the spin wave dispersions in YMnO3 in symmetry directions by inelastic neutron scattering and have fitted the results with a simple nearest-neighbour anisotropic Heisenberg model. We have investigated the spin dynamics of YMnO3 above TN ≈ 70 K and have shown the existence of persistent albeit heavily damped spin waves. [ABSTRACT FROM AUTHOR]

Multiferroic materials are the materials that show several cooperative ordering phenomena simultaneously. Here we consider only those materials that show both antiferromagnetism and ferroelectricity and in which the ordering parameters are coupled to each other. I shall review our recent neutron scattering investigations on multiferroic hexagonal manganites YMnO3. We have determined the spin wave dispersions in YMnO3 in symmetry directions by inelastic neutron scattering and have fitted the results with a simple nearest-neighbour anisotropic Heisenberg model. We have investigated the spin dynamics of YMnO3 above TN ≈ 70 K and have shown the existence of persistent albeit heavily damped spin waves. [ABSTRACT FROM AUTHOR]

Abstract: We have performed muon spin relaxation () experiments on undoped CaMnO3 and electron-doped for and 0.3. Undoped CaMnO3 is a G-type antiferromagnet with a Néel temperature of . The signal shows three well-defined frequencies, which decrease continuously with temperature and becomes zero at . The electron-doped Ca0.85Sm0.15MnO3 which shows colossal magnetoresistive (CMR) behaviour, undergoes a structural and magnetic transition at . The signal shows heavily damped oscillating signal. The frequency decreases continuously with increasing temperature. Ca0.9Sm0.1MnO3 and Ca0.7 Sm0.3MnO3 show only relaxation and do not show any frequency. We have determined temperature variation of the muon relaxation rates below and above the magnetic and charge order transitions under zero and longitudinal magnetic fields, which give important information about the nature of these transitions. [Copyright &y& Elsevier]

We have investigated the Jahn–Teller transition accompanied by orbital order–disorder transition in La1-xBaxMnO3 by high temperature powder neutron diffraction in the low-doping region (0≤x≤0.1). The Jahn–Teller transition temperature which is about TJT≈750 K for the stoichiometric LaMnO3 decreases drastically on doping with Ba. We have shown that the previously identified metrically cubic orthorhombic O-phase for x=0 is metrically orthorhombic and the orthorhombicity increases with doping. The Jahn–Teller transition is accompanied only by the partial reduction of the distortion of the MnO6 octahedra. From the distortion of the MnO6 octahedra we have determined the orbital mixing coefficients. Negative thermal expansion has been observed in stoichiometric LaMnO3 in a temperature range of about 75 K just below TJT≈750 K. The negative thermal expansion has also been observed in La1-xBaxMnO3 below TJT but the effect is reduced with doping. [Copyright &y& Elsevier]

We have investigated temperature dependence of the lattice parameters and the unit cell volume of ZnF2 by neutron diffraction and have discovered negative thermal expansion (NTE) at low temperature. To understand why this simple compound exhibits NTE we performed first principle calculations. These calculations reproduce qualitatively the experimental temperature dependence of volume. [ABSTRACT FROM AUTHOR]

We have investigated temperature dependence of the lattice parameters and the unit cell volume of ZnF2 by neutron diffraction and have discovered negative thermal expansion (NTE) at low temperature. To understand why this simple compound exhibits NTE we performed first principle calculations. These calculations reproduce qualitatively the experimental temperature dependence of volume. [ABSTRACT FROM AUTHOR]

We reported previously [T. Chatterji et al., Phys. Rev. B 78, 134105 (2008)] negative thermal expansion (NTE) in ReO3 in the limited temperature range from 2 to 220 K. Here we discovered NTE in ReO3 at higher-temperature region from 600 to 680 K. We determined the temperature variation in the lattice parameter and the unit cell volume of ReO3 by neutron diffraction. The temperature variation in the lattice parameter and the unit cell volume show two regions of NTE and two minima. We attribute the NTE of ReO3 to be the result of anharmonicity and anomalous softening of M3 phonon mode. [ABSTRACT FROM AUTHOR]

We have shown, using high resolution x-ray diffraction studies of nanoparticles, that size reduction can lead to an arrest of the high temperature phase in perovskite materials, which prevents growth of the low temperature phase that needs a specific crystal structure for its stabilization. This has been shown in the context of ∼15 nm diameter nanocrystals of La0.5Ca0.5MnO3, in which the orthorhombic distortion at room temperature freezes in and, as a consequence, the charge order cannot set in and the ferromagnetic state that sets in at higher temperature is stabilized at low temperature. [ABSTRACT FROM AUTHOR]

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. This effect is related to the unconventional properties of created MnP magnetic clusters within the host material. Such behavior is also discussed in connection with ab initio density functional calculations, where the structural properties of MnP indicate magnetic transitions as function of pressure as observed experimentally. Our results point out new ways to obtain controlled response of embedded magnetic clusters. [ABSTRACT FROM AUTHOR]

We have shown, using high resolution x-ray diffraction studies of nanoparticles, that size reduction can lead to an arrest of the high temperature phase in perovskite materials, which prevents growth of the low temperature phase that needs a specific crystal structure for its stabilization. This has been shown in the context of ∼15 nm diameter nanocrystals of La0.5Ca0.5MnO3, in which the orthorhombic distortion at room temperature freezes in and, as a consequence, the charge order cannot set in and the ferromagnetic state that sets in at higher temperature is stabilized at low temperature. [ABSTRACT FROM AUTHOR]

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. Using synchrotron X-ray diffraction we show that cold compression induces the formation of a disordered intermediate. However, using an evolutionary algorithm we predict a new structure with edge-sharing chains. This is confirmed as the thermodynamic ground state using in situ laser heating. We show that magnetism is globally absent in the new phase, as low-spin quantum S = 1/2 moments are quenched by dimerization. Our results show how the emergence of metal-metal bonding can stabilize giant spin-lattice coupling in Earth's minerals. [ABSTRACT FROM AUTHOR]

We construct an effective spin Hamiltonian for YMn2O5 through analysis of first-principles density-functional theory results. Using our first-principles-derived spin Hamiltonian we calculate the magnon dispersion of this compound and compare with the experimentally measured spectra probed using inelastic neutron scattering. Our first-principles-derived set of magnetic interaction parameters show the importance of the Mn1-Mn2 interaction along the b axis, which was ignored in the literature. We provide justification in favor of our first-principles-derived parameters. [ABSTRACT FROM AUTHOR]

We report here the results of a high-resolution neutron powder diffraction study on the multiferroic solid solution system (Bi0.8Ba0.2)(Fe0.8Ti0.2)O3 in the temperature range 4 to 700 K. Using irreducible representation theory to analyze the magnetic structure by Rietveld refinement, we show that the magnetic structure is collinear G-type antiferromagnetic. Further, we confirm the occurrence of an isostructural phase transition (IPT) accompanying the magnetic ordering around ∼625 K in (Bi0.8Ba0.2)(Fe0.8Ti0.2)O3. It is shown that as a result of the IPT, the positions of all the atoms change significantly in the magnetically ordered phase, leading to an excess polarization which scales linearly with the sublattice magnetization obtained by Rietveld refinement of the magnetic structure. Structural evidence for magnetoelastic coupling for the magnetic transitions below room temperature is also presented. [ABSTRACT FROM AUTHOR]

The discovery of a new family of high-TC 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. The structure and properties of chemically substituted samples are known to be intimately linked; however, remarkably little is known about this relationship when high pressure is used to induce superconductivity in undoped compounds. Here we show that the key structural features in BaFe2As2, namely suppression of the tetragonal-to-orthorhombic phase transition and reduction in the As–Fe–As bond angle and Fe–Fe distance, show the same behaviour under pressure as found in chemically substituted samples. Using experimentally derived structural data, we show that the electronic structure evolves similarly in both cases. These results suggest that modification of the Fermi surface by structural distortions is more important than charge doping for inducing superconductivity in BaFe2As2. [ABSTRACT FROM AUTHOR]

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. By measuring the temperature dependence of the energy we determined the critical exponent [Formula: see text] and the ferromagnetic Curie temperature of [Formula: see text] K. The present result of the critical exponent agrees better with the predicted value (0.367) of the three-dimensional Heisenberg model than that determined previously by nuclear magnetic resonance.

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 3 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. The first proof-of-concept experiment is performed to study the ferromagnetic phase transition in the Fe 3 Pt sample. We also demonstrate that the polychromatic neutron beam in combination with in situ 3 He cells can be used as the initial step for the rapid measurement and qualitative analysis of radiographs.

Unusual features in magnetization resembling the kinetic arrest of a magnetic glass state are observed in the La-doped double perovskite, SrLaFeCoO 6 . 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. Among the two anomalies that are observed at [Formula: see text] 75 K and at [Formula: see text] 250 K in the magnetic measurements, the former is associated with a spin-freezing temperature below which the magnetic glass state is experimentally verified. The magnetometric experiments detailed in the paper bring out the non-equilibrium metastable magnetic states in this disordered magnetic system. The magnetic glass state described above manifests in the electrical resistivity [Formula: see text] through the formation of a 'hard gap' because of the spin-exchange energy following the formation of magnetic glass. It is observed that the combination of disorder and magnetic glass state leads to a large, negative magnetoresistance (MR) of  ≈47[Formula: see text] at 5 K in 8 T.

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. The change in the crystallographic structure is accompanied by a change in the magnetic order, where the canted antiferromagnetic order with moments in the basal (ab) plane for the bulk changes to collinear ferromagnetic order with spins along the c-axis for the nanocrystals. The spontaneous ferromagnetic moment ~3 μ(B) and the transition temperature of 260 K in LaMnO(3+δ) nanocrystals are similar to those found in La0:67Ca0:33MnO3 which has a much higher Mn(4+) content. The likely origin is traced to change in magnetic exchange interactions due to change in Mn-O bond lengths which become almost identical in the MnO6 octahedron in the rhombohedral structure in the absence of Jahn-Teller distortion. The study provides an example of structural and magnetic phase transition driven purely by size reduction and with no change in the chemical constituents.