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The Fe1+y,Te1-xSex series of materials is one of the prototype families of Fe-based superconductors. To provide further insight into these materials, we present systematic inelastic neutron scattering measurements of the low-energy spin excitations for x = 0.27, 0.36, 0.40, and 0.49. These measurements show an evolution of incommensurate spin excitations towards the (1/2,1/2,0) wave vector with doping. Concentrations (x = 0.40 and 0.49) which exhibit the most robust superconducting properties have spin excitations closest to (1/2,1/2,0) and also exhibit a strong spin resonance in the spin excitation spectrum below Tc. The resonance signal appears to be closer to (1/2,1/2,0) than the underlying spin excitations. We discuss the possible relationship between superconductivity and spin excitations at the (1/2,1/2,0) wave vector and the role that interstitial Fe may play. [ABSTRACT FROM AUTHOR]

We have performed elastic and inelastic neutron scattering experiments on single crystal samples of the coordination polymer compound CuF2(H2O)2(pyz) (pyz = pyrazine) to study the magnetic structure and excitations. The elastic neutron diffraction measurements indicate a collinear antiferromagnetic structure with moments oriented along the [0.7 0 1] real-space direction and an ordered moment of 0.60 ± 0.03μB/Cu. This value is significantly smaller than the single-ion magnetic moment, reflecting the presence of strong quantum fluctuations. The spin wave dispersion from magnetic zone center to the zone boundary points (0.5 1.5 0) and (0.5 0 1.5) can be described by a two-dimensional Heisenberg model with a nearest-neighbor magnetic exchange constant J2D = 0.934 ± 0.0025 meV. The interlayer interaction Jperp in this compound is less than 1.5% of J2D The spin excitation energy at the (0.5 0.5 0.5) zone boundary point is reduced when compared to the (0.5 1 0.5) zone boundary point by ~10.3% ± 1.4%. This zone boundary dispersion is consistent with quantum Monte Carlo and series expansion calculations for the S = 1/2 Heisenberg square lattice antiferromagnet, which include corrections for quantum fluctuations to linear spin wave theory. [ABSTRACT FROM AUTHOR]

We present a neutron and x-ray scattering investigation of Ca³LiO5O6, a 5d material predicted to host magnetic ordering solely through an extended superexchange pathway involving two anions. This contrasts with investigations of extended superexchange interactions that have been largely limited to low-dimensional 3d systems involving both superexchange and extended superexchange. Despite the apparent one-dimensional nature and triangular units of magnetic osmium ions in Ca³LiO5O6, the onset of magnetic correlations has been observed at a high temperature of 117 K in bulk measurements. We experimentally determine the magnetically ordered structure and show it to be long range and three dimensional. Our results support the model of extended superexchange interaction. [ABSTRACT FROM AUTHOR]

We determine the low-field ordered magnetic phases of the S=1 dimerized antiferromagnet Ba3Mn2O8 using single-crystal neutron diffraction. We find that for magnetic fields between μ0H=8.80T and 10.56T applied along the [110] direction the system exhibits spin density wave order with incommensurate wave vectors of type (η,η,ε). For μ0H>10.56T, the magnetic order changes to a spiral phase with incommensurate wave vectors only along the [hh0] direction. For both field-induced ordered phases, the magnetic moments are lying in the plane perpendicular to the field direction. The nature of these two transitions is fundamentally different: the low-field transition is a second-order transition to a spin density wave ground state, while the one at higher field, toward the spiral phase, is of first order. [ABSTRACT FROM AUTHOR]

The layered 5d transition-metal oxide Sr2IrO4 has been shown to host a novel Jeff=½Mott spin-orbit insulating state with antiferromagnetic ordering, leading to comparisons with the layered cuprates. Here we study the effect of substituting Mn for Ir in single crystals of Sr2Ir0.9Mn0.1O4 through an investigation involving bulk measurements and resonant x-ray and neutron scattering. We observe a new long-range magnetic structure emerge upon doping through a reordering of the spins from the basal plane to the c axis with a reduced ordering temperature compared to Sr2IrO4. The strpng enhancement of the magnetic x-ray scattering intensity at the L3 edge relative to the L2 edge indicates that the Jeff=½ state is robust and capable of hosting a variety of ground states. [ABSTRACT FROM AUTHOR]

We have performed unpolarized and polarized neutron diffraction experiments on monodisperse 8- and 13-nm antiferromagnetic MnO nanoparticles. For the 8-nm sample, the antiferromagnetic transition temperature TN (114 K) is suppressed compared to that in the bulk material (119 K), while for the 13-nm sample TN (120 K) is comparable to that in the bulk. The neutron diffraction data of the nanoparticles is well described using the bulk MnO magnetic structure but with a substantially reduced average magnetic moment of 4.2±0.3 μB/Mn for the 8-nm sample and 3.9±0.2 μB/Mn for the 13-nm sample. An analysis of the polarized neutron data on both samples shows that in an individual MnO nanoparticle about 80% of Mn ions order. These results can be explained by a structure in which the monodisperse nanoparticles studied here have a core that behaves similar to the bulk with a surface layer which does not contribute significantly to the magnetic order. [ABSTRACT FROM AUTHOR]

Intrinsic, two-dimensional ferromagnetic semiconductors are an important class of materials for overcoming the limitations of dilute magnetic semiconductors for spintronics applications. CrSiTe3 is a particularly interesting member of this class, since it can likely be exfoliated down to single layers, where TC is predicted to increase dramatically. Establishing the nature of the magnetism in the bulk is a necessary precursor to understanding the magnetic behavior in thin-film samples and the possible applications of this material. In this work, we use elastic and inelastic neutron scattering to measure the magnetic properties of single-crystalline CrSiTe3. We find that there is a very small single ion anisotropy favoring magnetic ordering along the c axis and that the measured spin waves fit well to a model where the moments are only weakly coupled along that direction. Finally, we find that both static and dynamic correlations persist within the ab plane up to at least 300 K, strong evidence of this material's two-dimensional characteristics that are relevant for future studies on thin-film and monolayer samples. [ABSTRACT FROM AUTHOR]

The structural and the magnetic properties of CeCu6-xAgx (0≤x≤0.85) and Cecu6-xPdx (0≤x≤0.4) have been studied using neutron diffraction, resonant ultrasound spectroscopy (RUS), x-ray diffraction measurements, and first principles calculations. The structural and magnetic phase diagrams of CeCu6-xAgx and CeCu6-xPdx as a function of Ag/Pd composition are reported. The end member, CeCu6, undergoes a structural phase transition from an orthorhombic (Pnma) to a monoclinic (P21/c) phase at 240 K. In CeCu6-xAgx, the structural phase transition temperature (Ts) decreases linearly with Ag concentration and extrapolates to zero at xS ≈0.1. The structural transition in CeCu6-xPdx remains unperturbed with Pd substitution within the range of our study. The lattice constant b slightly decreases with Ag/Pd doping, whereas a and c increase with an overall increase in the unit cell volume. Both systems, CeCu6-xAgx and CeCu6-xPdx, exhibit a magnetic quantum critical point (QCP), at x≈0.2 and x≈0.05, respectively. Near the QCP, long range antiferromagnetic ordering takes place at an incommensurate wave vector (δ1 0 δ2), where δ1~0.62,δ2~0.25,x=0.125 for CeCu6-xPdx and δ1~0.64,δ2~0.3,x=0.3 for CeCu6-xAgx. The magnetic structure consists of an amplitude modulation of the Ce moments which are aligned along the c axis of the orthorhombic unit cell. [ABSTRACT FROM AUTHOR]

We report elastic and inelastic neutron-scattering measurements of the high-TC ferromagnet Mn1+δSb. Measurements were performed on a large, TC=434 K, single crystal with an interstitial Mn content of δ≈0.13. The neutron-diffraction results reveal that the interstitial Mn has a magnetic moment, and that it is aligned antiparallel to the main Mn moment. We perform density functional theory calculations including the interstitial Mn and find the interstitial to be magnetic, in agreement with the diffraction data. The inelastic neutron-scattering measurements reveal two features in the magnetic dynamics: (i) a spin-wave-like dispersion emanating from ferromagnetic Bragg positions (HK2n), and (ii) a broad, nondispersive signal centered at forbidden Bragg positions (HK2n+1). The inelastic spectrum cannot be modeled by simple linear spin-wave-theory calculations and appears to be significantly altered by the presence of the interstitial Mn ions. The results show that the influence of the interstitial Mn on the magnetic state in this system is more important than previously understood. [ABSTRACT FROM AUTHOR]

The magnetic susceptibility, crystal and magnetic structures, and electronic structure of the double perovskite Sr2ScOsO6 are reported. Using both neutron and x-ray powder diffraction we find that the crystal structure is monoclinic P21/n from 3.5 to 300 K. Magnetization measurements indicate an antiferromagnetic transition at TN = 92K, one of the highest transition temperatures of any double perovskite hosting only one magnetic ion. Type I antiferromagnetic order is determined by neutron powder diffraction, with an Os moment of only 1.6(1)μB, close to half the spin-only value for a crystal field split 5d electron state with a t32g ground state. Density functional calculations show that this reduction is largely the result of strong Os-O hybridization, with spin-orbit coupling responsible for only a ~0.1μB reduction in the moment. [ABSTRACT FROM AUTHOR]

Neutron scattering is used to probe magnetic interactions as superconductivity develops in optimally doped Fe1+δSexTe1-x. Applying the first moment sum rule to comprehensive neutron scattering data, we extract the change in magnetic exchange energy Δ[JR-R' (SR · SR')] in the superconducting state referenced to the normal state. Oscillatory changes are observed for Fe-Fe displacements ∣ΔR∣ < ξ, where ξ = 1.3(1) nm is the superconducting coherence length. Dominated by a large reduction in the second nearest neighbor exchange energy [-1.2(2) meV/Fe], the overall reduction in magnetic interaction energy is Δ(Hmag) = -0.31(9) meV/Fe. Comparison to the superconducting condensation energy ΔESC = -0.013(1) meV/Fe, which we extract from specific heat data, suggests the modified magnetism we probe drives superconductivity in Fe1+δSexTe1-x. [ABSTRACT FROM AUTHOR]

Using inelastic neutron scattering, we show that the onset of superconductivity in underdoped Ba(Fe1-xCox)2As2 coincides with a crossover from well-defined spin waves to overdamped and diffusive spin excitations. This crossover occurs despite the presence of long-range stripe antiferromagnetic order for samples in a compositional range from x = 0.04 to 0.055, and is a consequence of the shrinking spin-density wave gap and a corresponding increase in the particle-hole (Landau) damping. The latter effect is captured by a simple itinerant model relating Co doping to changes in the hot spots of the Fermi surface. We argue that the overdamped spin fluctuations provide a pairing mechanism for superconductivity in these materials. [ABSTRACT FROM AUTHOR]

The Jeff = ½ state is manifested in systems with large cubic crystal field splitting and spin-orbit coupling that are comparable to the on-site Coulomb interaction, U. 5d transition-metal oxides host parameters in this regime and strong evidence for this state in Sr2IrO4, and additional iridates, has been presented. All the candidates, however, deviate from the cubic crystal field required to provide an unmixed canonical Jeff = ½ state, impacting the development of a robust model of this novel insulating and magnetic state. We present experimental and theoretical results that not only show Ca4IrO6 hosts the state, but furthermore uniquely resides in the limit required for a canonical unmixed Jeff = ½ state. [ABSTRACT FROM AUTHOR]

We present a single-crystal neutron scattering study of BaMn2Bi2, a recently synthesized material with the same ThCr2Si2-type structure found in several Fe-based unconventional superconducting materials. We show long-range magnetic order, in the form of a G-type antiferromagnetic structure, exists up to 390 K with an indication of a structural transition at 100 K. Utilizing inelastic neutron scattering, we observe a spin gap of 16 meV, with spin waves extending up to 55 meV. We find these magnetic excitations are well fit to a J1-J2-Jc Heisenberg model and present values for the exchange interactions. The spin-wave spectrum appears to be unchanged by the 100 K structural phase transition. [ABSTRACT FROM AUTHOR]

We have carried out a pressure study of the unconventional superconductor FeTe0.6Se0.4 up to 1.5 GPa by neutron scattering, resistivity, and magnetic susceptibility measurements. The neutron spin resonance energy and the superconducting transition temperature have been extracted as a function of applied pressure in samples obtained from the same crystal. Both increase with pressure up to a maximum at ≈1.3 GPa, directly demonstrating a correlation between these two fundamental parameters of unconventional superconductivity. A comparison between the quantitative evolution of Tc and the resonance energy as a function of applied pressure is also discussed. These measurements serve to demonstrate the feasibility of using pressure dependent inelastic neutron scattering to explore the relationship between the resonance energy and Tc in unconventional superconductors. [ABSTRACT FROM AUTHOR]

We study the effects of Cu substitution in Fe1.1 Te, the nonsuperconducting parent compound of the iron-based superconductor, Fe1+yTe1-xSex, utilizing neutron scattering techniques. It is found that the structural and magnetic transitions, which occur at ∼60 K without Cu, are monotonically depressed with increasing Cu content. By 10% Cu for Fe, the structural transition is hardly detectable, and the system becomes a spin glass below 22 K, with a slightly incommensurate ordering wave vector of (0.5 -- &dgr;, 0,0.5) with S being the incommensurability of 0.02, and correlation length of 12 Â along the a axis and 9 Â along the c axis. With 4% Cu, both transition temperatures are at 41 K, though short-range incommensurate order at (0.42,0,0.5) is present at 60 K. With further cooling, the incommensurability decreases linearly with temperature down to 37 K, below which there is a first-order transition to a long-range almost-commensurate antiferromagnetic structure. A spin anisotropy gap of 4.5 meV is also observed in this compound. Our results show that the weakly magnetic Cu has a large effect on the magnetic correlations; it is suggested that this is caused by the frustration of the exchange interactions between the coupled Fe spins. [ABSTRACT FROM AUTHOR]

Inelastic neutron scattering measurements on Ba(Fe0.925Mn0.075)2As2 manifest spin fluctuations at two different wave vectors, Qstripe = (&half;,&half; 1) and QNéel = (1,0,1), corresponding to the expected stripe spin-density wave order and checkerboard antiferromagnetic order in the tetragonal I4/mmm cell, respectively. Below TN = 80 K, long-range stripe magnetic ordering occurs and sharp spin wave excitations appear at Qstripe while broad and diffusive spin fluctuations remain at QNéel at all temperatures. Low concentrations of Mn dopants nucleate local moment spin fluctuations at QNéel that compete with itinerant spin fluctuations at Qstripe and may disrupt the development of superconductivity. [ABSTRACT FROM AUTHOR]

Lattice dynamics and magnetic excitations are investigated to elucidate the origin of magneto-dielectric effects in the S = 1/2 two-dimensional quantum spin compound K2V308. We find evidence for lattice instabilities at 110 K and 60 K as optical phonon anomalies and a soft mode at 26 cm-1 in A, symmetry. Two-magnon excitations in B1 symmetry show an unconventional double-peak structure and temperature dependence. This suggests the existence of a split mode near the zone boundary caused by a mixing of spin and lattice modes. [ABSTRACT FROM AUTHOR]

We use inelastic neutron scattering to study the temperature dependence of the low-energy spin excitations in single crystals of superconducting FeTe0.6Se0.4 (Tc = 14 K). In the low-temperature superconducting state, the imaginary part of the dynamic susceptibility at the electron and hole Fermi-surfaces nesting wave vector Q = (0.5,0.5), x "(Q,o), has a small spin gap, a two-dimensional neutron spin resonance above the spin gap, and increases linearly with increasing ha> for energies above the resonance. While the intensity of the resonance decreases like an order parameter with increasing temperature and disappears at temperature slightly above Tc, the energy of the mode is weakly temperature dependent and vanishes concurrently above Tc. This suggests that in spite of its similarities with the resonance in electron-doped superconducting BaFe2-x(Co,Ni)xAs2, the mode in FeTe0.6Se0.4 is not directly associated with the superconducting electronic gap. [ABSTRACT FROM AUTHOR]

We study the spin-wave excitation spectrum of the antiferromagnetic bilayer ruthenate Ca3Ru2O7 through inelastic neutron scattering measurements. The material behaves as a quasi-two-dimensional ferromagnetic bilayer system with very weak interbilayer antiferromagnetic coupling, and its magnetic dispersion can be well described with a nearest-neighbor Heisenberg model. We find that the intrabilayer exchange interaction along the out-of-plane c-axis is much stronger than the in-plane intralayer interaction, in sharp contrast to the previously reported bilayer manganite system. And our study reveals a finite lifetime effect of the magnetic excitation, presumably attributable to the existence of a small number of itinerant charge carriers within the planes. [ABSTRACT FROM AUTHOR]

We report inelastic neutron-scattering measurements of crystal-field transitions in PrFeAsO, PrFeAsO0.87F0.13, and NdFeAsO0.85F0.15. Doping with fluorine produces additional crystal-field excitations, providing evidence that there are two distinct charge environments around the rare-earth ions, with probabilities that are consistent with a random distribution of dopants on the oxygen sites. The 4f electrons of the Pr3+ and Nd3+ ions have nonmagnetic and magnetic ground states, respectively, indicating that the enhancement of Tc compared to LaFeAsO1-xFx is not due to rare-earth magnetism. [ABSTRACT FROM AUTHOR]

We present neutron diffraction measurements on single-crystal samples of nonsuperconducting Ba(Fe1-xCrx)2As2 as a function of Cr doping for 0 ⩽ x ⩽ 0.47. The average spin-density-wave moment is independent of concentration for x ⩽ 0.2 and decreases rapidly for x ⩾ 0.3. For concentrations in excess of 30% chromium, we find a new competing magnetic phase consistent with G-type antiferromagnetism which rapidly becomes the dominant magnetic ground state. Strong magnetism is observed for all concentrations measured, naturally explaining the absence of superconductivity in the Cr-doped materials. [ABSTRACT FROM AUTHOR]

We investigate the magnetic properties of the series Sr2Ir1-xRuxO4 with neutron, resonant x-ray, and magnetization measurements. The results indicate an evolution and coexistence of magnetic structures via a spin-flop transition from ab-plane to c-axis collinear order as the 5d Ir4+ ions are replaced with an increasing concentration of 4d Ru4+ ions. The magnetic structures within the ordered regime of the phase diagram (x<0.3) are reported. Despite the changes in magnetic structure no alteration of the Jeff=1/2 ground state is observed. The behavior of Sr2Ir1-xRuxO4 is consistent with electronic phase separation and diverges from a standard scenario of hole doping. The role of lattice alterations with doping on the magnetic and insulating behavior is considered. The results presented here provide insight into the magnetic insulating states in strong spin-orbit coupled materials and the role perturbations play in altering the behavior. [ABSTRACT FROM AUTHOR]