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By using time-of-flight neutron spectroscopy with polarization analysis, we have separated coherent and incoherent contributions to the scattering of deuterated tetrahydrofuran in a wide scattering vector (Q)-range from meso- to inter-molecular length scales. The results are compared with those recently reported for water to address the influence of the nature of inter-molecular interactions (van der Waals vs hydrogen bond) on the dynamics. The phenomenology found is qualitatively similar in both systems. Both collective and self-scattering functions are satisfactorily described in terms of a convolution model that considers vibrations, diffusion, and a Q-independent mode. We observe a crossover in the structural relaxation from being dominated by the Q-independent mode at the mesoscale to being dominated by diffusion at inter-molecular length scales. The characteristic time of the Q-independent mode is the same for collective and self-motions and, contrary to water, faster and with a lower activation energy (≈1.4 Kcal/mol) than the structural relaxation time at inter-molecular length scales. This follows the macroscopic viscosity behavior. The collective diffusive time is well described by the de Gennes narrowing relation proposed for simple monoatomic liquids in a wide Q-range entering the intermediate length scales, in contraposition to the case of water.

We have synthesized single-crystal samples of $\beta$-VOSO$_4$ and fully characterized their magnetic properties. Our magnetic susceptibility, high field magnetization and powder inelastic neutron scattering results are in excellent agreement with theoretical expressions for a one-dimensional spin-1/2 Heisenberg chain with an exchange parameter of $3.83(2)$ meV. Ab initio calculations identify the superexchange pathway, revealing that the spin-chain does not run along the expected crystallographic chain $a$ direction but instead between V$^{4+}$O$_{6}$ octahedra that are linked via SO$_{4}$ tetrahedra along the $b$ axis. We do not detect any phase transition to a long-range magnetic order within our experimental conditions, indicating $\beta$-VOSO$_4$ is very close to an ideal one-dimensional magnetic system.

The A-site ordered chromate spinels LiGa1-xInxCr4O8 host a network of size-alternating spin-3/2 Cr3+ tetrahedra known as a 'breathing' pyrochlore lattice. For the x=0.05 composition, the complex magneto-structural ordering observed in the parent x=0 material is replaced by a single transition at Tf=11 K, ascribed to the collinear nematic order caused by strong spin-lattice coupling. We present here an inelastic neutron scattering study of the spin dynamics in this composition. Above Tf , the dynamical scattering function S(Q,E) is ungapped and quasi-elastic, similar to undoped LiGaCr4O8. Below Tf , the spectral weight splits between a broad inelastic feature at 5.8 meV and toward the elastic line. The former feature can be ascribed to spin precessions within antiferromagnetic loops, lifted to finite energy by the effective biquadratic spin-lattice term in the spin Hamiltonian.

The B-site sublattice in the double perovskite oxides A2BB'O6 (B: magnetic cation; B': nonmagnetic cation) causes spin frustration, but the relationship between the structure and spin frustration remains unclear although a number of compounds have been studied. The present study systematically investigated A2MnIIB'O6 (S = 5/2) and found that the frustration factor, defined by f = |θW|/TN (θW: Weiss temperature; TN: Neel temperature), scales linearly with the tolerance factor t, i.e., octahedral rotation. Unexpectedly, La2MnTaO5N (space group: P21/n) synthesized under high pressure is more frustrated (f = 6) than oxides with similar t values, despite the large octahedral rotation due to the small t value of 0.914. Structural analysis suggests that the enhanced frustration can be attributed to the site preference of nitride anions at the equatorial positions, which reduces the variance of neighboring Mn-Mn distances. Our findings provide a new guide to control and improve spin frustration in double perovskites with multiple anions.

The realization of magnetic frustration in a metallic van der Waals (vdW) coupled material has been sought as a promising platform to explore novel phenomena both in bulk matter and in exfoliated devices. However, a suitable material platform has been lacking so far. Here, we demonstrate that CeSiI hosts itinerant electrons coexisting with exotic magnetism. In CeSiI, the magnetic cerium atoms form a triangular bilayer structure sandwiched by van der Waals stacked iodine layers. From resistivity and magnetometry measurements, we confirm the coexistence of itinerant electrons with magnetism with dominant antiferromagnetic exchange between the strongly Ising-like Ce moments below 7 K. Neutron diffraction directly confirms magnetic order with an incommensurate propagation vector k ~ (0.28, 0, 0.19) at 1.6 K, which points to the importance of further neighbor magnetic interactions in this system. The presence of a two-step magnetic-field-induced phase transition along c axis further suggests magnetic frustration in the ground state. Our findings provide a novel material platform hosting a coexistence of itinerant electron and frustrated magnetism in a vdW system, where exotic phenomena arising from rich interplay between spin, charge and lattice in low dimension can be explored., 15 pages, 9 figures

We report neutron scattering and thermodynamic measurements of the quasi-one-dimensional Ising magnet $\mathrm{Sr}{\mathrm{Co}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{8}$ under transverse fields along the tetragonal $a/b$ direction. Our experiments reveal a N\'eel-type magnetic order in zero field, which successively changes into a coplanar antiferromagnetic order in applied fields. The detailed evolution of the noncollinear magnetic order in magnetic fields can be understood by considering the competition between the intrinsic magnetic interactions, uniform transverse field, and effective staggered field. Moreover, a series of gapped discrete confined-spinon modes are observed in zero field. With increasing field, the spin gap progressively softens, reaching a minimum value at $\ensuremath{\sim}7\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ where the N\'eel-type magnetic ordering moment is completely suppressed. This corresponds to the three-dimensional quantum critical point (QCP). The implications for multiple QCPs in this class of materials are discussed.

${\mathrm{Ba}}_{2}\mathrm{Y}\mathrm{Re}{\mathrm{O}}_{6}$ is a double perovskite material where the ${\mathrm{Re}}^{5+}$ (${d}^{2}$) ions occupy a frustrated face-centered cubic lattice. Despite strong antiferromagnetic interactions between the Re ions, as indicated by a large negative Weiss constant $\ensuremath{\theta}=\ensuremath{-}616$ K, spin freezing in ${\mathrm{Ba}}_{2}\mathrm{Y}\mathrm{Re}{\mathrm{O}}_{6}$ only occurs at 45 K. Since no long-range order of magnetic dipoles has previously been found in either muon spin rotation or neutron diffraction experiments below this temperature, it has been assumed that the low-temperature state is a spin glass. In stark contrast with these findings, we here show that ${\mathrm{Ba}}_{2}\mathrm{Y}\mathrm{Re}{\mathrm{O}}_{6}$ does in fact order with a strongly reduced dipole moment ${\ensuremath{\mu}}_{0}=0.29--0.42\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{B}$ via a polarized neutron diffraction experiment on a powder sample. Using the symmetries of the two likeliest magnetic structures and the properties of the $5{d}^{2}$ configuration in the presence of strong spin-orbit coupling and strong crystal fields, we use a recently derived single-ion wave function for ${\mathrm{Re}}^{5+}$ to predict a large quadrupolar moment based on the experimental ${\ensuremath{\mu}}_{o}$.

We present an experimental study of the magnetic structure and dynamics of two frustrated hyperkagome compounds, Gd3Ga5O12 and Gd3Al5O12. It has previously been shown that Gd3Ga5O12 exhibits long-range correlations of multipolar directors, that are formed from antiferromagnetic spins on loops of ten ions. Using neutron diffraction and Reverse Monte Carlo simulations we prove the existence of similar magnetic correlations in Gd3Al5O12, showing the ubiquity of these complex structures in frustrated hyperkagome materials. Using inelastic neutron scattering we shed further light on the director state and the associated low lying magnetic excitations. In addition we have measured quasielastic dynamics that show evidence of spin diffusion. Finally, we present AC susceptibility measurements on both Gd3Ga5O12 and Gd3Al5O12, revealing a large difference in the low frequency dynamics between the two otherwise similar compounds., Comment: 13 pages, 12 figures, Accepted by Physical Review B

We report the crystal structures and magnetic properties of two pseudopolymorphs of the $S=1/2 {\mathrm{Ti}}^{3+}$ coordination framework, $\mathrm{K}\mathrm{Ti}{({\mathrm{C}}_{2}{\mathrm{O}}_{4})}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}x{\mathrm{H}}_{2}\mathrm{O}$. Single-crystal x-ray and powder neutron diffraction measurements on $\ensuremath{\alpha}\ensuremath{-}\mathrm{K}\mathrm{Ti}{({\mathrm{C}}_{2}{\mathrm{O}}_{4})}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}x{\mathrm{H}}_{2}\mathrm{O}$ confirm its structure in the tetragonal $I4/mcm$ space group with a square planar arrangement of ${\mathrm{Ti}}^{3+}$ ions. Magnetometry and specific heat measurements reveal weak antiferromagnetic interactions, with ${J}_{1}\ensuremath{\approx}7$ K and ${J}_{2}/{J}_{1}=0.11$ indicating a slight frustration of nearest- and next-nearest-neighbor interactions. Below 1.8 K, $\ensuremath{\alpha}\ensuremath{-}\mathrm{K}\mathrm{Ti}{({\mathrm{C}}_{2}{\mathrm{O}}_{4})}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}x{\mathrm{H}}_{2}\mathrm{O}$ undergoes a transition to G-type antiferromagnetic order with magnetic moments aligned along the $c$ axis of the tetragonal structure. The estimated ordered moment of ${\mathrm{Ti}}^{3+}$ in $\ensuremath{\alpha}\ensuremath{-}\mathrm{K}\mathrm{Ti}{({\mathrm{C}}_{2}{\mathrm{O}}_{4})}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}x{\mathrm{H}}_{2}\mathrm{O}$ is suppressed from its spin-only value to $0.62(3)\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{B}$, thus verifying the two-dimensional nature of the magnetic interactions within the system. $\ensuremath{\beta}\ensuremath{-}\mathrm{K}\mathrm{Ti}{({\mathrm{C}}_{2}{\mathrm{O}}_{4})}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}2{\mathrm{H}}_{2}\mathrm{O}$, on the other hand, realizes a three-dimensional diamondlike magnetic network of ${\mathrm{Ti}}^{3+}$ moments within a hexagonal $P{6}_{2}22$ structure. An antiferromagnetic exchange coupling of $J\ensuremath{\approx}54$ K---an order of magnitude larger than in $\ensuremath{\alpha}\ensuremath{-}\mathrm{K}\mathrm{Ti}{({\mathrm{C}}_{2}{\mathrm{O}}_{4})}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}x{\mathrm{H}}_{2}\mathrm{O}$---is extracted from magnetometry and specific heat data. $\ensuremath{\beta}\ensuremath{-}\mathrm{K}\mathrm{Ti}{({\mathrm{C}}_{2}{\mathrm{O}}_{4})}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}2{\mathrm{H}}_{2}\mathrm{O}$ undergoes N\'eel ordering at ${T}_{N}=28$ K, with the magnetic moments aligned within the $ab$ plane and a slightly reduced ordered moment of $0.79\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{B}$ per ${\mathrm{Ti}}^{3+}$. Through density-functional theory calculations, we address the origin of the large difference in the exchange parameters between the $\ensuremath{\alpha}$ and $\ensuremath{\beta}$ pseudopolymorphs. Given their observed magnetic behaviors, we propose $\ensuremath{\alpha}\ensuremath{-}\mathrm{K}\mathrm{Ti}{({\mathrm{C}}_{2}{\mathrm{O}}_{4})}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}x{\mathrm{H}}_{2}\mathrm{O}$ and $\ensuremath{\beta}\ensuremath{-}\mathrm{K}\mathrm{Ti}{({\mathrm{C}}_{2}{\mathrm{O}}_{4})}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}2{\mathrm{H}}_{2}\mathrm{O}$ as close to ideal model $S=1/2$ Heisenberg square and diamond lattice antiferromagnets, respectively.

We performed inelastic neutron scattering measurements on a polycrystalline sample of a classical kagome antiferromagnet ${\mathrm{NaBa}}_{2}{\mathrm{Mn}}_{3}{\mathrm{F}}_{11}$ to investigate the possibility of a dispersionless zero-energy excitation associated with rotation of spins along the chains. The observed spectra indeed exhibit such an excitation with strong intensity at low energy, as well as dispersive excitations with weak intensity at high energy. Combining the measurements with calculations from linear spin-wave theory reveals that ${\mathrm{NaBa}}_{2}{\mathrm{Mn}}_{3}{\mathrm{F}}_{11}$ is a good realization of the classical kagome antiferromagnet which exhibits a dispersionless mode lifted by the magnetic dipole-dipole interaction.

Spin liquids are highly correlated yet disordered states formed by the entanglement of magnetic dipoles$^1$. Theories typically define such states using gauge fields and deconfined quasiparticle excitations that emerge from a simple rule governing the local ground state of a frustrated magnet. For example, the '2-in-2-out' ice rule for dipole moments on a tetrahedron can lead to a quantum spin ice in rare-earth pyrochlores - a state described by a lattice gauge theory of quantum electrodynamics$^{2-4}$. However, f-electron ions often carry multipole degrees of freedom of higher rank than dipoles, leading to intriguing behaviours and 'hidden' orders$^{5-6}$. Here we show that the correlated ground state of a Ce$^{3+}$-based pyrochlore, Ce$_2$Sn$_2$O$_7$, is a quantum liquid of magnetic octupoles. Our neutron scattering results are consistent with the formation of a fluid-like state of matter, but the intensity distribution is weighted to larger scattering vectors, which indicates that the correlated degrees of freedom have a more complex magnetization density than that typical of magnetic dipoles in a spin liquid. The temperature evolution of the bulk properties in the correlated regime below 1 Kelvin is well reproduced using a model of dipole-octupole doublets on a pyrochlore lattice$^{7-8}$. The nature and strength of the octupole-octupole couplings, together with the existence of a continuum of excitations attributed to spinons, provides further evidence for a quantum ice of octupoles governed by a '2-plus-2-minus' rule. Our work identifies Ce$_2$Sn$_2$O$_7$ as a unique example of a material where frustrated multipoles form a 'hidden' topological order, thus generalizing observations on quantum spin liquids to multipolar phases that can support novel types of emergent fields and excitations.

We report results from neutron diffraction with polarization analysis of the effect of In dopant level on the hydrogen concentration in hydrated samples of the proton conducting perovskite type oxide BaZr1−xInxO3−x/2with x = 0, 0.175, 0.20, 0.225, 0.25, and 0.275. Analysis of the neutron data establishes a trend of increasing hydrogen concentration, which is comparable to that obtained from thermogravimetric measurements on the same samples, as well as an increased unit-cell size and local structural disorder as a function of increasing In dopant level. These results encourage further use of neutron diffraction with polarization analysis as a non-destructive technique for the determination of the total hydrogen concentration, and structural properties, of proton conducting oxides as well as other hydrogen-containing materials.

Acceptor-doped barium zirconates are currently receiving considerable interest because of their high proton conductivity at intermediate temperatures, making them applicable as electrolytes in various electrochemical devices, but the mechanism of proton conduction is unclear. Here, we investigate the role of the acceptor-dopant level in the localized proton motions, i.e. proton transfers between oxygens and O-H reorientations, in hydrated samples of the proton conducting, acceptor-doped, perovskites BaZr1-xInxO3-x/2 with x = 0.10 and 0.20, using quasielastic neutron scattering (QENS). Analysis of the QENS spectra reveals that several proton transfer and O-H reorientational motions contribute to the QENS signal, as a consequence of the locally disordered nature of the structure due to the In doping of these materials, and establishes a generic and complex picture of localized proton dynamics in acceptor-doped barium zirconate based proton conductors. A comparison of the QENS results with vibrational spectroscopy data of the same materials, as reported in the literature, suggests a predominance of O-H reorientational motions in the observed dynamics. The highest doping level corresponds to a more distorted structure and faster dynamics, which thus indicates that some degree of structural disorder is favourable for high local proton mobility.

The instrumental divergence parameters and resolution for the D7 neutron diffuse scattering spectrometer at the Institut Laue-Langevin, France, are presented. The resolution parameters are calibrated against measurements of powders, single crystals, and the incoherent scattering from vanadium. We find that the powder diffraction resolution is well described by the Cagliotti function, the single crystal resolution function can be parameterized using the Cooper-Nathans formalism, and that in time-of-flight mode the energy resolution is consistent with monochromatic focussing.

We present an extensive experimental and theoretical study on the low-temperature magnetic properties of the monoclinic anhydrous alum compound BaMo(PO4)(2). The magnetic susceptibility reveals strong antiferromagnetic interactions theta(CW) = -167 K and long-range magnetic order at T-N = 22 K, in agreement with a recent report. Powder neutron diffraction furthermore shows that the order is collinear, with the moments near the ac plane. Neutron spectroscopy reveals a large excitation gap Delta = 15 meV in the low-temperature ordered phase, suggesting a much larger easy-axis spin anisotropy than anticipated. However, the large anisotropy justifies the relatively high ordered moment, Neel temperature, and collinear order observed experimentally and is furthermore reproduced in a first-principles calculations by using a new computational scheme. We therefore propose BaMo(PO4)(2) to host S = 1 antiferromagnetic chains with large easy-axis anisotropy, which has been theoretically predicted to realize novel excitation continua.

It is well established that in the low-temperature limit, the two-dimensional quantum Heisenberg antiferromagnet on a square lattice (2DQHAFSL) exhibits an anomaly in its spectrum at short-wavelengths on the zone-boundary. In the vicinity of the $(\pi,0)$ point the pole in the one-magnon response exhibits a downward dispersion, is heavily damped and attenuated, giving way to an isotropic continuum of excitations extending to high energies. The origin of the anomaly and the presence of the continuum are of current theoretical interest, with suggestions focused around the idea that the latter evidences the existence of spinons in a two-dimensional system. Here we present the results of neutron inelastic scattering experiments and Quantum Monte Carlo calculations on the metallo-organic compound Cu(DCOO)$_2\cdot 4$D$_2$O (CFTD), an excellent physical realisation of the 2DQHAFSL, designed to investigate how the anomaly at $(\pi,0)$ evolves up to finite temperatures $T/J\sim2/3$. Our data reveal that on warming the anomaly survives the loss of long-range, three-dimensional order, and is thus a robust feature of the two-dimensional system. With further increase of temperature the zone-boundary response gradually softens and broadens, washing out the $(\pi,0)$ anomaly. This is confirmed by a comparison of our data with the results of finite-temperature Quantum Monte Carlo simulations where the two are found to be in good accord. At lower energies, in the vicinity of the antiferromagnetic zone centre, there was no significant softening of the magnetic excitations over the range of temperatures investigated., Comment: Dedicated to the life and work of Professor Roger Cowley. 22 pages, 8 figures

Terbium gallium garnet (TGG), ${\mathrm{Tb}}_{3}{\mathrm{Ga}}_{5}{\mathrm{O}}_{12}$, is well known for its applications in laser optics but also exhibits complex low-temperature magnetism that is not yet fully understood. Its low-temperature magnetic order is determined by means of time-of-flight neutron powder diffraction. It is found to be a multiaxial antiferromagnet with magnetic ${\mathrm{Tb}}^{3+}$ ions forming six sublattices of magnetic moments aligned parallel and antiparallel to the $\ensuremath{\langle}100\ensuremath{\rangle}$ crystallographic directions of the cubic unit cell. The structure displays strong easy-axis anisotropy with respect to a twofold axis of symmetry in the local orthorhombic environment of the ${\mathrm{Tb}}^{3+}$ sites. The crystal-field splitting within the single-ion ground-state manifold is investigated by inelastic neutron scattering on powder samples. A strong temperature dependence of the quasidoublet ground state is observed and revised parameters of the crystal-field Hamiltonian are given. The results of bulk magnetic susceptibility and magnetization measurements are in good agreement with values based on the crystal-field model down to 20 K, where the onset of magnetic correlations is observed.

We report neutron-scattering and ac magnetic susceptibility measurements of the two-dimensional spin-1/2 frustrated magnet $\mathrm{BaCdVO}{({\mathrm{PO}}_{4})}_{2}$. At temperatures well below ${T}_{\mathsf{N}}\ensuremath{\approx}1\phantom{\rule{4.pt}{0ex}}\text{K}$, we show that only 34% of the spin moment orders in an up-up-down-down stripe structure. Dominant magnetic diffuse scattering and comparison to published $\mathrm{muon}\ensuremath{-}\mathrm{spin}\ensuremath{-}\mathrm{rotation}$ measurements indicates that the remaining 66% is fluctuating. This demonstrates the presence of strong frustration, associated with competing ferromagnetic and antiferromagnetic interactions, and points to a subtle ordering mechanism driven by magnon interactions. On applying magnetic field, we find that at $T=0.1$ K the magnetic order vanishes at 3.8 T, whereas magnetic saturation is reached only above 4.5 T. We argue that the putative high-field phase is a realization of the long-sought bond-spin-nematic state.

Pyrochlore systems are ideally suited to the exploration of geometrical frustration in three dimensions, and their rich phenomenology encompasses topological order and fractional excitations. Classical spin ices provide the first context in which it is possible to control emergent magnetic monopoles, and anisotropic exchange leads to even richer behaviour associated with large quantum fluctuations. Whether the magnetic ground state of Yb2Ti2O7 is a quantum spin liquid or a ferromagnetic phase induced by a Higgs transition appears to be sample dependent. Here we have determined the role of structural defects on the magnetic ground state via the diffuse scattering of neutrons. We find that oxygen vacancies stabilise the spin liquid phase and the stuffing of Ti sites by Yb suppresses it. Samples in which the oxygen vacancies have been eliminated by annealing in oxygen exhibit a transition to a ferromagnetic phase, and this is the true magnetic ground state., Exploring the role of structural defect is essential to understand the exotic quantum spin phenoma in rare earth pyrochlores. Here the authors show oxygen vacancies can stabilise the spin liquid phase and reveal the ferromagnetic ground state when oxygen vacancies are eliminated in Yb2Ti2O7.

Barlowite, ${\mathrm{Cu}}_{4}{(\mathrm{OH})}_{6}\mathrm{FBr}$, has attracted much attention as the parent compound of a new series of quantum spin-liquid candidates, ${\mathrm{Zn}}_{x}{\mathrm{Cu}}_{4\ensuremath{-}x}{(\mathrm{OH})}_{6}\mathrm{FBr}$. While it is known to undergo a magnetic phase transition to a long-range ordered state at ${T}_{N}=15$ K, there is still no consensus over either its nuclear or magnetic structures. Here, we use comprehensive, high-flux powder neutron diffraction studies on deuterated samples of barlowite to demonstrate that the only space group consistent with the observed nuclear and magnetic diffraction at low temperatures is the orthorhombic $Pnma$ space group. We furthermore conclude that the magnetic intensity at $Tl{T}_{N}$ is correctly described by the $P{n}^{\ensuremath{'}}{m}^{\ensuremath{'}}a$ magnetic space group, which crucially allows the ferromagnetic component observed in previous single-crystal and powder magnetization measurements. As such, the magnetic structure of barlowite resembles that of the related material clinoatacamite, ${\mathrm{Cu}}_{4}{(\mathrm{OH})}_{6}{\mathrm{Cl}}_{2}$, the parent compound of the well-known quantum spin-liquid candidate hebertsmithite, ${\mathrm{ZnCu}}_{3}{(\mathrm{OH})}_{6}{\mathrm{Cl}}_{2}$.

In geometrically frustrated magnets the crystal lattice prevents competing magnetic interactions from being simultaneously satisfied. Consequently these materials often possess fascinating magnetic properties such as unusual short-range magnetic correlations. However, studying short-range magnetic structure has historically been a notoriously difficult experimental task. Here, Frandsen $e\phantom{\rule{0}{0ex}}t$ $a\phantom{\rule{0}{0ex}}l$. apply a new experimental technique---magnetic pair distribution function (mPDF) analysis---to uncover the local magnetic structure and temperature dependence of recently-discovered frustrated magnets NaMCo${}^{2}$F${}^{7}$ (M=Ca,Sr). The results further establish these materials as an important new class of geometrically frustrated magnets with observable frustration effects at unusually high temperatures (~200 K), while also opening the door for a plethora of future studies of frustrated magnets using the mPDF method.

The charge ordered structure of ions and vacancies characterizing rare-earth pyrochlore oxides serves as a model for the study of geometrically frustrated magnetism. The organization of magnetic ions into networks of corner-sharing tetrahedra gives rise to highly correlated magnetic phases with strong fluctuations, including spin liquids and spin ices. It is an open question how these ground states governed by local rules are affected by disorder. In the pyrochlore Tb$_2$Hf$_2$O$_7$, we demonstrate that the vicinity of the disordering transition towards a defective fluorite structure translates into a tunable density of anion Frenkel disorder while cations remain ordered. Quenched random crystal fields and disordered exchange interactions can therefore be introduced into otherwise perfect pyrochlore lattices of magnetic ions. We show that disorder can play a crucial role in preventing long-range magnetic order at low temperatures, and instead induces a strongly-fluctuating Coulomb spin liquid with defect-induced frozen magnetic degrees of freedom., 18 pages, 4 figures, + Supplementary Information (4 figures)

We present the results of a combined $^{7}\mathrm{Li}$-NMR and diffraction study on ${\mathrm{LiGa}}_{0.95}{\mathrm{In}}_{0.05}{\mathrm{Cr}}_{4}{\mathrm{O}}_{8}$, a member of the ${\mathrm{LiGa}}_{1\ensuremath{-}x}{\mathrm{In}}_{x}{\mathrm{Cr}}_{4}{\mathrm{O}}_{8}$ ``breathing'' pyrochlore family. Via specific heat and NMR measurements, we find that the complex sequence of first-order transitions observed for ${\mathrm{LiGaCr}}_{4}{\mathrm{O}}_{8}$ is replaced by a single second-order transition at ${T}_{f}=11\text{ }\text{ }\mathrm{K}$. Neutron and x-ray diffraction rule out both structural symmetry lowering and magnetic long-range order as the origin of this transition. Instead, reverse Monte Carlo fitting of the magnetic diffuse scattering indicates that the low-temperature phase may be described as a collinear spin nematic state, characterized by a quadrupolar order parameter. This state also shows signs of short-range order between collinear spin arrangements on tetrahedra, revealed by mapping the reverse Monte Carlo spin configurations onto a three-state color model.

The layered compound ${\mathrm{KCu}}_{3}{\mathrm{As}}_{2}{\mathrm{O}}_{7}{(\mathrm{O}\mathrm{D})}_{3}$, comprising distorted kagome planes of $S=1/2\phantom{\rule{4pt}{0ex}}{\mathrm{Cu}}^{2+}$ ions, is a recent addition to the family of type-II multiferroics. Previous zero-field neutron diffraction work has found two helically ordered regimes in ${\mathrm{KCu}}_{3}{\mathrm{As}}_{2}{\mathrm{O}}_{7}{(\mathrm{O}\mathrm{D})}_{3}$, each showing a distinct coupling between the magnetic and ferroelectric order parameters. Here, we extend this work to magnetic fields up to 20 T using neutron powder diffraction, capacitance, polarization, and high-field magnetization measurements, hence determining the $H\ensuremath{-}T$ phase diagram. We find metamagnetic transitions in both low-temperature phases around ${\ensuremath{\mu}}_{0}{H}_{c}\ensuremath{\sim}3.7$ T, which neutron powder diffraction reveals to correspond to rotations of the helix plane away from the easy plane, as well as a small change in the propagation vector. Furthermore, we show that the sign of the ferroelectric polarization is reversible in a magnetic field, although no change is observed (or expected on the basis of the magnetic structure) due to the transition at 3.7 T. We finally justify the temperature dependence of the polarization in both zero-field ordered phases by a symmetry analysis of the free energy expansion, and attempt to account for the metamagnetic transition by adding anisotropic exchange interactions to our existing model for ${\mathrm{KCu}}_{3}{\mathrm{As}}_{2}{\mathrm{O}}_{7}{(\mathrm{O}\mathrm{D})}_{3}$.

Magnetic frustration and low dimensionality can prevent long range magnetic order and lead to exotic correlated ground states. SrDy$_2$O$_4$ consists of magnetic Dy$^{3+}$ ions forming magnetically frustrated zig-zag chains along the c-axis and shows no long range order to temperatures as low as $T=60$ mK. We carried out neutron scattering and AC magnetic susceptibility measurements using powder and single crystals of SrDy$_2$O$_4$. Diffuse neutron scattering indicates strong one-dimensional (1D) magnetic correlations along the chain direction that can be qualitatively accounted for by the axial next-nearest neighbour Ising (ANNNI) model with nearest-neighbor and next-nearest-neighbor exchange $J_1=0.3$ meV and $J_2=0.2$ meV, respectively. Three-dimensional (3D) correlations become important below $T^*\approx0.7$ K. At $T=60$ mK, the short range correlations are characterized by a putative propagation vector $\textbf{k}_{1/2}=(0,\frac{1}{2},\frac{1}{2})$. We argue that the absence of long range order arises from the presence of slowly decaying 1D domain walls that are trapped due to 3D correlations. This stabilizes a low-temperature phase without long range magnetic order, but with well-ordered chain segments separated by slowly-moving domain walls.

The concept for a proposed upgrade to the D7 diffuse scattering spectrometer at the Institut Laue-Langevin, Grenoble, is presented. The upgrade consists of rebuilding the primary spectrometer, substantially relaxing the incident beam divergence to match the divergence that can be accepted by the secondary spectrometer. Monte-Carlo simulations and acceptance diagram calculations have been used to verify the performance of the proposed instrument relative to the current D7. The proposed upgrade is not the final instrument plan, as modifications and compromises will need to be applied for space compatibility with neighbouring instruments. The concept offers an intensity upgrade for a nominal sample of a factor > 20 . The relative wavelength spread of the incident beam will be reduced by a factor ≈ 3 . The increased divergence will result in a degraded Q -resolution for small scattering angles from the sample, but a reduced resolution at large scattering angles.

International audience; Hydrated samples of the two proton conducting perovskites BaZr 0.9 M 0.1 O 2.95 (M = Y and Sc) were investigated using neutron spin−echo spectroscopy together with thermal gravimetric measurements, polarized neutron diffraction, and infrared spectroscopy, with the aim to determine how the atomic-scale proton dynamics depend on temperature, and type of dopant atom, M. The results show the presence of pronounced localized proton motions for temperatures above ca. 300 K, characterized by relaxation times on the order of picoseconds to nanoseconds and governed by a wide distribution of activation energies due to a heterogeneous distribution of proton sites present, with no strong dependence on the type of dopant atom.

A magnetic insulator, LiInCr${}_{4}$O${}_{8}$ realizes a so-called ``breathing'' pyrochlore lattice of $S=3/2$ spins on corner-sharing tetrahedra that alternate in size. In this manuscript, a collaboration of researchers from Institut Laue-Langevin in Grenoble and Institute for Solid State Physics in Tokyo present a detailed neutron scattering study that elucidates the sequence of structural and magnetic phase transitions that occur at low temperatures in this material.

The spinel oxides LiGaCr4O8 and LiInCr4O8 contain size-alternating pyrochlore lattices of spin-3/2 Cr3+ tetrahedra with different magnitudes of alternation. We show here that the solid solutions LiGa1-xInxCr4O8 between these two 'breathing' pyrochlore compounds display (i) rapid suppression of magnetic and structural transitions upon doping the end members, (ii) spin-glass-like freezing above 2 K in the range 0.1 $\lesssim$ x $\lesssim$ 0.6, and (iii) apparent spin-gap behavior for x $\gtrsim$ 0.7. Furthermore, no transitions are observed above 2 K at x ~ 0.9, where magnetic susceptibility remains finite at 2 K and magnetic heat capacity shows a quadratic temperature dependence at 1-5 K. Our work shows that breathing pyrochlore compounds provide a unique opportunity for studying both geometrical frustration and bond alternation., 5 pages, 4 figures

Here we investigated diffuse magnetic scattering in the highly frustrated double perovskite Ba2YRuO6 using polarized neutrons. Consistent with previous reports, the material shows two apparent transitions at 47 and 36 K to an eventual type I face-centered-cubic magnetic ground state. The (100) magnetic reflection shows different behavior from the five other observed reflections upon heating from 1.8 K, with the former broadening well beyond the resolution limit near 36 K. Closer examination of the latter group reveals a small, but clear, increase in peak widths between 36 and 47 K, indicating that this regime is dominated by short-range spin correlations. Diffuse magnetic scattering persists above 47 K near the position of (100) to at least 200 K, consistent with strong frustration. Reverse Monte Carlo (RMC) modeling of the diffuse scattering from 45 to 200 K finds that the spin-spin correlations between nearest and next-nearest neighbors are antiferromagnetic and ferromagnetic, respectively, at temperatures near the upper ordering temperature, but both become antiferromagnetic and of similar magnitude above 100 K. The significance of this unusual crossover is discussed in light of the super-superexchange interactions between nearest and next-nearest neighbors in this material and the demands of type I order. The dimensionality ofmore » the correlations is addressed by reconstructing the scattering in the (hk0) plane using the RMC spin configurations. This indicates that one-dimensional spin correlations dominate at temperatures close to the first transition. In addition, a comparison between mean-field calculations and (hk0) scattering implies that further neighbor couplings play a significant role in the selection of the ground state. Finally, the results and interpretation are compared with those recently published for monoclinic Sr2YRuO6, and similarities and differences are emphasized.« less

In many materials it remains a challenge to reveal the nature of magnetic correlations, including antiferromagnetism and spin disorder. Revealing the spin structure in magnetic nanoparticles is further complicated by the large incoherent neutron scattering cross section from water adsorbed at the particle surfaces and by the broadening of diffraction peaks due to the finite crystallite size. Moreover, the spin structure in magnetic nanoparticles may deviate significantly from that of the corresponding bulk material because of the low-symmetry surroundings of surface atoms and the large relative surface contribution to the magnetic anisotropy. Here we explore the potential use of polarized neutron diffraction to reveal the magnetic structure in NiO bulk and nanoparticle powders by applying the XYZ-polarization analysis method. Our investigations address in particular the spin orientation in bulk NiO and platelet-shaped NiO nanoparticles with thickness from greater than 200 nm down to 2.0 nm. The advantage of the applied method is that it is able to clearly separate the structural, the magnetic, and the spin-incoherent scattering signals for all particle sizes. For platelet-shaped particles with thickness from greater than 200 nm down to 2.2 nm we find that the spin orientation deviates about 16 degrees from the primary (111) plane of the platelet-shaped particles. In the smallest particles (2.0 nm thick) we find the spins are oriented with a 30 degrees. average angle to the primary (111) plane of the particles. The results show that polarization analyzed neutron powder diffraction is a viable method to investigate magnetic order in powders of antiferromagnetic nanoparticles. (Less)

We present a design for implementing uniaxial polarisation analysis on the LET cold neutron time-of-flight spectrometer, installed on the second target station at ISIS. The polarised neutron beam is to be produced by a transmission-based supermirror polariser with the polarising mirrors arranged in a "double-V" formation. This will be followed by a Mezei-type precession coil spin flipper, selected for its small spatial requirements, as well as a permanent magnet guide field to transport the beam polarisation to the sample position. The sample area will contain a set of holding field coils, whose purpose is to produce a highly homogenous magnetic field for the wide-angle 3He analyser cell. To facilitate fast cell changes and reduce the risk of cell failure, we intend to separate the cell and cryostat from the vacuum of the sample tank by installing both in a vessel at atmospheric pressure. When the instrument upgrade is complete, the performance of LET is expected to be commensurate with existing and planned polarised cold neutron spectrometers at other sources. Finally, we discuss the implications of performing uniaxial polarisation analysis only, and identify quasi-elastic neutron scattering (QENS) on ionic conducting materials as an interesting area to apply the technique.

We present new magnetic heat capacity and neutron scattering results for two magnetically frustrated molybdate pyrochlores: $S=1$ oxide Lu$_2$Mo$_2$O$_7$ and $S={\frac{1}{2}}$ oxynitride Lu$_2$Mo$_2$O$_5$N$_2$. Lu$_2$Mo$_2$O$_7$ undergoes a transition to an unconventional spin glass ground state at $T_f {\sim} 16$ K. However, the preparation of the corresponding oxynitride tunes the nature of the ground state from spin glass to quantum spin liquid. The comparison of the static and dynamic spin correlations within the oxide and oxynitride phases presented here reveals the crucial role played by quantum fluctuations in the selection of a ground state. Furthermore, we estimate an upper limit for a gap in the spin excitation spectrum of the quantum spin liquid state of the oxynitride of ${\Delta} {\sim} 0.05$ meV or ${\frac{\Delta}{|\theta|}}\sim0.004$, in units of its antiferromagnetic Weiss constant ${\theta} {\sim}-121$ K., Comment: 8 pages, 9 figures

Several ${\mathrm{Cu}}^{2+}$ hydroxide minerals have been recently identified as candidate realizations of the $S=1/2$ kagome Heisenberg model. In this context, we have studied the distorted system ${\mathrm{KCu}}_{3}{\mathrm{As}}_{2}{\mathrm{O}}_{7}$(OD)${}_{3}$ using neutron scattering and bulk measurements. Although the distortion favors magnetic order over a spin liquid ground state, refinement of the magnetic diffraction pattern below ${T}_{N1}=7.05(5)$ K yields a complex helical structure with $\mathbf{k}=(0.77,0,0.11)$. This structure, as well as the spin excitation spectrum, are well described by a classical Heisenberg model with ferromagnetic nearest neighbor couplings. Multiferroicity is observed below ${T}_{N1}$, with an unusual crossover between improper and pseudoproper behavior occurring at ${T}_{N2}=5.5$ K. The polarization at $T=2$ K is $P=1.5\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}$Cm${}^{\ensuremath{-}2}$. The properties of ${\mathrm{KCu}}_{3}{\mathrm{As}}_{2}{\mathrm{O}}_{7}$(OD)${}_{3}$ highlight the variety of physics which arise from the interplay of spin and orbital degrees of freedom in ${\mathrm{Cu}}^{2+}$ kagome systems.

Keywords: Magnetic-Properties ; Spin Chain ; Competing Interactions ; Dimerization ; Srcuo2 ; Field Reference EPFL-ARTICLE-158016doi:10.1021/cm7023263View record in Web of Science Record created on 2010-11-24, modified on 2017-05-12

The compound KTi(SO4)(2)center dot H2O was recently reported as a quasi-one-dimensional spin-1/2 compound with competing antiferromagnetic nearest-neighbor exchange J(1) and next-nearest-neighbor exchange J(2) along the chain with a frustration ratio alpha = J(2)/J(1) approximate to 0.29 [G. J. Nilsen, H. M. Ronnow, A. M. Lauchli, F. P. A. Fabbiani, J. Sanchez-Benitez, K. V. Kamenev, and A. Harrison, Chem. Mater. 20, 8 (2008)]. Here, we report a microscopically based magnetic model for this compound derived from density functional theory (DFT) based electronic structure calculations along with respective tight-binding models. Our (LSDA+U-d) calculations confirm the quasi-one-dimensional nature of the system with antiferromagnetic J(1) and J(2), but suggest a significantly larger frustration ratio alpha(DFT) approximate to 0.94-1.4, depending on the choice of U-d and structural parameters. Based on transfer matrix renormalization group (TMRG) calculations we find alpha(TMRG) = 1.5. Due to an intrinsic symmetry of the J(1)-J(2) model, our larger frustration ratio alpha is also consistent with the previous thermodynamic data. To identify the frustration ratio alpha unambiguously, we propose performing high-field magnetization and low-temperature susceptibility measurements.

A unique type of frustrated lattice is found in two $A$-site ordered spinel oxides, ${\mathrm{LiGaCr}}_{4}{\mathrm{O}}_{8}$ and ${\mathrm{LiInCr}}_{4}{\mathrm{O}}_{8}$. Because of the large size mismatch between ${\mathrm{Li}}^{+}$ and ${\mathrm{Ga}}^{3+}/{\mathrm{In}}^{3+}$ ions at the $A$ site, the pyrochlore lattice, made up of ${\mathrm{Cr}}^{3+}$ ions carrying spin $3/2$, becomes an alternating array of small and large tetrahedra, i.e., a ``breathing'' pyrochlore lattice. We introduce a parameter, the breathing factor ${B}_{f}$, which quantifies the degree of frustration in the pyrochlore lattice: ${B}_{f}$ is defined as ${J}^{\ensuremath{'}}/J$, where ${J}^{\ensuremath{'}}$ and $J$ are nearest-neighbor magnetic interactions in the large and small tetrahedra, respectively. ${\mathrm{LiGaCr}}_{4}{\mathrm{O}}_{8}$ with ${B}_{f}\ensuremath{\sim}0.6$ shows magnetic susceptibility similar to that of conventional Cr spinel oxides such as ${\mathrm{ZnCr}}_{2}{\mathrm{O}}_{4}$. In contrast, ${\mathrm{LiInCr}}_{4}{\mathrm{O}}_{8}$ with a small ${B}_{f}\ensuremath{\sim}0.1$ exhibits a spin-gap behavior in its magnetic susceptibility, suggesting a proximity to an exotic singlet ground state. Magnetic long-range order occurs at 13.8 and 15.9 K for ${\mathrm{LiGaCr}}_{4}{\mathrm{O}}_{8}$ and ${\mathrm{LiInCr}}_{4}{\mathrm{O}}_{8}$, respectively, in both cases likely owing to the coupling to structural distortions.

A new polymorph of the title compound is hydrothermally synthesized from a solution of CuO and V2O5 in 1% aq, HNO3 (autoclave, 443 K, 10 d).

A unique type of frustrated lattice is found in two A-site ordered spinel oxides, LiGaCr(4)O(8) and LiInCr(4)O(8). Because of the large size mismatch between Li(+) and Ga(3+)/In(3+) ions at the A site, the pyrochlore lattice, made up of Cr(3+) ions carrying spin 3/2, becomes an alternating array of small and large tetrahedra, i.e., a "breathing" pyrochlore lattice. We introduce a parameter, the breathing factor B(f), which quantifies the degree of frustration in the pyrochlore lattice: B(f) is defined as J'/J, where J' and J are nearest-neighbor magnetic interactions in the large and small tetrahedra, respectively. LiGaCr(4)O(8) with B(f)~0.6 shows magnetic susceptibility similar to that of conventional Cr spinel oxides such as ZnCr(2)O(4). In contrast, LiInCr(4)O(8) with a small B(f)~0.1 exhibits a spin-gap behavior in its magnetic susceptibility, suggesting a proximity to an exotic singlet ground state. Magnetic long-range order occurs at 13.8 and 15.9 K for LiGaCr(4)O(8) and LiInCr(4)O(8), respectively, in both cases likely owing to the coupling to structural distortions.

The orbital is one of the four fundamental degrees of freedom in a solid, besides spin, charge and lattice. In transition metal compounds, it is usually the d orbitals which play deciding roles in determining the crystallographic and physical properties. Here we report the discovery of a unique structural transition in single crystals of the spin-1/2 quasi-kagomé antiferromagnet volborthite, Cu(3)V(2)O(7)(OH)(2)·2H(2)O, whereby the unpaired electron 'switches' from one d orbital to another upon cooling. This is not a conventional orbital order-disorder transition, but rather an orbital switching that has not previously been observed. The structural transition is found to profoundly affect the magnetic properties of volborthite, because magnetic interactions between Cu spins in the kagomé lattice are considerably modified by the orbital switching. This finding provides us with an interesting example to illustrate the intimate interplay between the orbital degree of freedom and competing magnetic interactions in a frustrated magnet.

We study polycrystalline samples of KCu3As2O7(OH)3, a new candidate spin-1/2 kagome antiferromagnet, by magnetic susceptibility and heat capacity measurements above 2 K. The unique arrangement of the 3z2 - r2 and x2 - y2 orbitals on the Cu2+ kagome net is noted and compared to the orbital patterns found in other kagome minerals. It is suggested that this orbital arrangement gives rise to one antiferromagnetic and two ferromagnetic interactions on isosceles triangles forming a highly distorted kagome lattice. KCu3As2O7(OH)3 is found to show an antiferromagnetic long-range order at T_N = 7.2 K. Remarkably, a spin entropy is more gradually released upon cooling below T_N compared with a conventional magnetic long-range order, which may originate from the geometrical frustration still present in this highly distorted kagome lattice., Comment: 5 pages, 4 figures

A new polymorph of volborthite [tricopper(II) divanadium(V) heptaoxide dihydroxide dihydrate], Cu3V2O7(OH)2 2H2O, has been discovered in a single crystal prepared by hydrothermal synthesis. X-ray analysis reveals that the monoclinic structure has the space group C2/c at room temperature, which is different from that of the previously reported C2/m structure. Both structures have Cu3O6(OH)2 layers composed of edge-sharing CuO4(OH)2 octahedra, with V2O7 pillars and water molecules between the layers. The Cu atoms occupy two and three independent crystallographic sites in the C2/m and C2/c structures, respectively, likely giving rise to different magnetic interactions between CuII spins in the kagome lattices embedded in the Cu3O6(OH)2 layers., Comment: 7pages, 2figures

We present spatial and dynamic information on the s=1/2 distorted kagome antiferromagnet volborthite, Cu3V2O7(OD)2.2D2O, obtained by polarized and inelastic neutron scattering. The instantaneous structure factor, S(Q), is dominated by nearest neighbor pair correlations, with short range order at wave vectors Q1=0.65(3) {\AA}^-1 and Q2=1.15(5) {\AA}^-1 emerging below 5 K. The excitation spectrum, S(Q,{\omega}), reveals two steep branches dispersing from Q1 and Q2, and a flat mode at {\omega}=5.0(2) meV. The results allow us to identify the cross-over at T*=1 K in 51V NMR and specific heat measurements as the build-up of correlations at Q_1. We compare our data to theoretical models proposed for volborthite, and demonstrate that the excitation spectrum can be explained by spin-wave-like excitations with anisotropic exchange parameters, as also suggested by recent local density calculations., Comment: Rewritten article resubmitted to Phys. Rev. Lett. 0211