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In quantum magnets, magnetic moments fluctuate heavily and are strongly entangled with each other, a fundamental distinction from classical magnetism. Here, with inelastic neutron scattering measurements, we probe the spin correlations of the honeycomb lattice quantum magnet YbCl3. A linear spin wave theory with a single Heisenberg interaction on the honeycomb lattice, including both transverse and longitudinal channels of the neutron response, reproduces all of the key features in the spectrum. In particular, we identify a Van Hove singularity, a clearly observable sharp feature within a continuum response. The demonstration of such a Van Hove singularity in a two-magnon continuum is important as a confirmation of broadly held notions of continua in quantum magnetism and additionally because analogous features in two-spinon continua could be used to distinguish quantum spin liquids from merely disordered systems. These results establish YbCl3 as a benchmark material for quantum magnetism on the honeycomb lattice., Honeycomb lattices with interacting spins can host rich magnetic behaviour; however, typically features are complicated by additional interactions. Here, the authors perform neutron scattering on YbCl3, which exhibits near perfect two-dimensional magnetism, providing a benchmark for other materials.

New surprises continue to be revealed about ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4}$, the parent compound of the original cuprate superconductor. Here we present neutron scattering evidence that the structural symmetry is lower than commonly assumed. The static distortion results in anisotropic Cu-O bonds within the ${\mathrm{CuO}}_{2}$ planes; such anisotropy is relevant to pinning charge stripes in hole-doped samples. Associated with the extra structural modulation is a soft phonon mode. If this phonon were to soften completely, the resulting change in ${\mathrm{CuO}}_{6}$ octahedral tilts would lead to weak ferromagnetism. Hence, we suggest that this mode may be the ``chiral'' phonon inferred from recent studies of the thermal Hall effect. We also note the absence of interaction between the antiferromagnetic spin waves and low-energy optical phonons, in contrast to what is observed in hole-doped samples.

Single crystals of the underscreened Kondo ferromagnet Yb14MnSb11 were investigated using polarised neutron diffraction and magnetisation measurements. The magnitude and direction of the ma...

We investigate the physical properties of the Shastry-Sutherland lattice material BaNd$_2$ZnO$_5$. Neutron diffraction, magnetic susceptibility, and specific heat measurements reveal antiferromagnetic order below 1.65 K. The magnetic order is found to be a 2-$\boldsymbol{Q}$ magnetic structure with the magnetic moments lying in the Shastry-Sutherland lattice planes comprising the tetragonal crystal structure of BaNd$_2$ZnO$_5$. The ordered moment for this structure is 1.9(1) $\mu_B$ per Nd ion. Inelastic neutron scattering measurements reveal that the crystal field ground state doublet is well separated from the first excited state at 8 meV. The crystal field Hamiltonian is determined through simultaneous refinement of models with both the LS coupling and intermediate coupling approximations to the inelastic neutron scattering and magnetic susceptibility data. The ground state doublet indicates that the magnetic moments lie primarily in the basal plane with magnitude consistent with the size of the determined ordered moment., Comment: 13 pages, 8 figures

In conventional quasi-one-dimensional antiferromagnets with quantum spins, magnetic excitations are carried by either magnons or spinons in different energy regimes: they do not coexist independently, nor could they interact with each other. In this Letter, by combining inelastic neutron scattering, quantum Monte Carlo simulations and Random Phase Approximation calculations, we report the discovery and discuss the physics of the coexistence of magnons and spinons and their interactions in Botallackite-Cu2(OH)3Br. This is a unique quantum antiferromagnet consisting of alternating ferromagnetic and antiferromagnetic Spin-1/2 chains with weak inter-chain couplings. Our study presents a new paradigm where one can study the interaction between two different types of magnetic quasiparticles, magnons and spinons., Comment: Accepted by PRL

${\mathrm{La}}_{1.67}{\mathrm{Sr}}_{0.33}{\mathrm{NiO}}_{4}$ develops charge and spin stripe orders at temperatures of roughly 200 K, with modulation wave vectors that are temperature independent. Various probes of spin and charge response have provided independent evidence for some sort of change below $\ensuremath{\sim}50$ K. In combination with a new set of neutron scattering measurements, we propose a unified interpretation of all of these observations in terms of a freezing of Ni-centered charges stripes, together with a glassy ordering of the spin stripes that shows up in neutron scattering as a slight rotation of the average spin direction.

The structural and magnetic properties of a glaserite-type Na2BaFe(VO4)2 compound, featuring a triangular magnetic lattice of Fe2+ (S = 2), are reported. Temperature dependent X-ray single crystal studies indicate that at room temperature the system adopts a trigonal P3m1 structure and undergoes a structural phase transition to a C2/c monoclinic phase slightly below room temperature (Ts = 288 K). This structural transition involves a tilting of Fe-O-V bond angles and strongly influences the magnetic correlation within the Fe triangular lattice. The magnetic susceptibility measurements reveal a ferromagnetic transition near 7 K. Single crystal neutron diffraction confirms the structural distortion and the ferromagnetic spin ordering in Na2BaFe(VO4)2. The magnetic structure of the ordered state is modeled in the magnetic space group C2'/c' that implies a ferromagnetic order of the a and c moment components and antiferromagnetic arrangement for the b components. Overall, the Fe magnetic moments form ferromagnetic layers that are stacked along the c-axis, where the spins point along one of the (111) facets of the FeO6 octahedron.

We report inelastic neutron scattering experiments in Ca2Y2Cu5O10 and map out the full one magnon dispersion which extends up to a record value of 53 meV for frustrated ferromagnetic (FM) edge-sharing CuO2 chain (FFESC) cuprates. A homogeneous spin-1/2 chain model with a FM nearest-neighbor (NN), an antiferromagnetic (AFM) next-nearest-neighbor (NNN) inchain, and two diagonal AFM interchain couplings (ICs) analyzed within linear spin-wave theory (LSWT) reproduces well the observed strong dispersion along the chains and a weak one perpendicularly. The ratio R=|J_{a2}/J_{a1}| of the FM NN and the AFM NNN couplings is found as ~0.23, close to the critical point Rc=1/4 which separates ferromagnetically and antiferromagnetically correlated spiral magnetic ground states in single chains, whereas Rc>0.25 for coupled chains is considerably upshifted even for relatively weak IC. Although the measured dispersion can be described by homogeneous LSWT, the scattering intensity appears to be considerably reduced at ~11.5 and ~28 meV. The gap-like feature at 11.5 meV is attributed to magnon-phonon coupling whereas based on DMRG simulations of the dynamical structure factor the gap at 28 meV is considered to stem partly from quantum effects due to the AFM IC. Another contribution is ascribed to the intrinsic superstructure from the distorting incommensurate pattern of CaY cationic chains adjacent to the CuO2 ones. It gives rise to non-equivalent CuO4 units and Cu-O-Cu bond angles Phi and a resulting distribution of all exchange integrals. The J's fitted by homogeneous LSWT are regarded as average values. The record value of the FM NN integral J1=24 meV among FFESC cuprates can be explained by a non-universal Phi (not 90 deg.) and Cu-O bond length dependent anisotropic mean direct FM Cu-O exchange K_{pd}~120 meV. Enhanced K_{pd} values are also needed to compensate a significant AFM J_{dd} > ~6 meV.

$\mathrm{S}{\mathrm{r}}_{2}\mathrm{Ru}{\mathrm{O}}_{4}$, an unconventional superconductor, is known to possess an incommensurate spin-density wave instability driven by Fermi surface nesting. Here we report a static spin-density wave ordering with a commensurate propagation vector ${\mathbf{q}}_{\mathrm{c}}=(0.25\phantom{\rule{0.16em}{0ex}}0.25\phantom{\rule{0.16em}{0ex}}0)$ in Fe-doped $\mathrm{S}{\mathrm{r}}_{2}\mathrm{Ru}{\mathrm{O}}_{4}$, despite the magnetic fluctuations persisting at the incommensurate wave vectors ${\mathbf{q}}_{\mathrm{ic}}=(0.3\phantom{\rule{0.16em}{0ex}}0.3\phantom{\rule{0.16em}{0ex}}L)$ as in the parent compound. The latter feature is corroborated by the first-principles calculations, which show that Fe substitution barely changes the nesting vector of the Fermi surface. These results suggest that in addition to the known incommensurate magnetic instability, $\mathrm{S}{\mathrm{r}}_{2}\mathrm{Ru}{\mathrm{O}}_{4}$ is also in proximity to a commensurate magnetic tendency that can be stabilized via Fe doping.

While it is mostly well-known for its excellent thermoelectric properties at high temperatures, Yb${}_{14}$MnSb${}_{11}$ has also come to be characterized as a ferromagnetic semiconductor. Such systems support ferromagnetic correlations similar to metallic compounds, although with semiconducting electronic configurations. As such, they are promising materials for use in spintronic devices. The authors here use neutron scattering techniques to examine the distribution of the magnetization density and the energy scale of the magnetic exchange interactions in the Yb${}_{14}$MnSb${}_{11}$ compound. They find that there is a large ferromagnetic moment localized to the Mn site in the crystal structure with a `Kondo cloud' of negative moments distributed over the other atomic sites. The results provide long sought answers to questions regarding the valence, magnetization distribution, and energy scales of the magnetic interactions in this compound. The ferromagnetic spin-wave spectrum supports this premise and is well characterized by inclusion of nearest and next-nearest neighbor exchange interactions.

$\mathrm{GeC}{\mathrm{u}}_{2}{\mathrm{O}}_{4}$ spinel exhibits a tetragonal structure due to the strong Jahn-Teller distortion associated with $\mathrm{C}{\mathrm{u}}^{2+}$ ions. We show that its magnetic structure can be described as slabs composed of a pair of layers with orthogonally oriented spin-$\frac{1}{2}$ Cu chains in the basal $ab$ plane. The spins between the two layers within a slab are collinearly aligned while the spin directions of neighboring slabs are perpendicular to each other. Interestingly, we find that spins along each chain form an unusual up-up-down-down (UUDD) pattern, suggesting a non-negligible nearest-neighbor biquadratic exchange interaction in the effective classical spin Hamiltonian. We hypothesize that spin-orbit coupling and orbital mixing of $\mathrm{C}{\mathrm{u}}^{2+}$ ions in this system are non-negligible, which calls for future calculations using perturbation theory with extended Hilbert (spin and orbital) space and calculations based on density functional theory including spin-orbit coupling and looking at the global stability of the UUDD state.

We report quantum transport and Dirac fermions in YbMnBi2 single crystals. YbMnBi2 is a layered material with anisotropic conductivity and magnetic order below 290 K. Magnetotransport properties, nonzero Berry phase, and small cyclotron mass indicate the presence of Dirac fermions. Lastly, angular-dependent magnetoresistance indicates a possible quasi-two-dimensional Fermi surface, whereas the deviation from the nontrivial Berry phase expected for Dirac states suggests the contribution of parabolic bands at the Fermi level or spin-orbit coupling.

The low energy spin excitation spectrum of the breathing pyrochlore ${\mathrm{Ba}}_{3}{\mathrm{Yb}}_{2}{\mathrm{Zn}}_{5}{\mathrm{O}}_{11}$ has been investigated with inelastic neutron scattering. Several nearly resolution limited modes with no observable dispersion are observed at 250 mK while, at elevated temperatures, transitions between excited levels become visible. To gain deeper insight, a theoretical model of isolated ${\mathrm{Yb}}^{3+}$ tetrahedra parametrized by four anisotropic exchange constants is constructed. The model reproduces the inelastic neutron scattering data, specific heat, and magnetic susceptibility with high fidelity. The fitted exchange parameters reveal a Heisenberg antiferromagnet with a very large Dzyaloshinskii-Moriya interaction. Using this model, we predict the appearance of an unusual octupolar paramagnet at low temperatures and speculate on the development of intertetrahedron correlations.

The descloizite-type compound, SrMn(VO4)(OH), was synthesized as large single crystals (1-2mm) using a high-temperature high-pressure hydrothermal technique. X-ray single crystal structure analysis reveals that the material crystallizes in the acentric orthorhombic space group of P212121 (no. 19), Z = 4. The structure exhibits a one-dimensional feature, with [MnO4] chains propagating along the a-axis which are interconnected by VO4 tetrahedra. Raman and infrared spectra were obtained to identify the fundamental vanadate and hydroxide vibrational modes. Magnetization data reveal a broad maximum at approximately 80 K, arising from one-dimensional magnetic correlations with intrachain exchange constant of J/kB = 9.97(3) K between nearest Mn neighbors and a canted antiferromagnetic behavior below TN = 30 K. Single crystal neutron diffraction at 4 K yielded a magnetic structure solution in the lower symmetry of the magnetic space group P21 with two unique chains displaying antiferromagnetically ordered Mn moments oriented nearly perpendicular to the chain axis. Lastly, the presence of the Dzyaloshinskii Moriya antisymmetric exchange interaction leads to a slight canting of the spins and gives rise to a weak ferromagnetic component along the chain direction.

Polycrystalline samples of the quarter-doped manganites $R_{0.75}$Ca$_{0.25}$MnO$_3$ ($R$ = Y, Tb, Dy, Ho, and Er) were studied by X-ray diffraction and AC/DC susceptibility measurements. All five samples are orthorhombic and exhibit similar magnetic properties: enhanced ferromagnetism below $T_1$ ($\sim80$ K) and a spin glass (SG) state below $T_{SG}$ ($\sim30$ K). With increasing $R^{3+}$ ionic size, both $T_1$ and $T_{SG}$ generally increase. The single crystal neutron diffraction results on Tb$_{0.75}$Ca$_{0.25}$MnO$_3$ revealed that the SG state is mainly composed of a short-range ordered version of a novel canted (i.e. noncollinear) antiferromagnetic spin state. Furthermore, calculations based on the double exchange model for quarter-doped manganites reveal that this new magnetic phase provides a transition state between the ferromagnetic state and the theoretically predicted spin-orthogonal stripe phase., Comment: 10 pages, 10 figures. Submitted to Physical Review B

We have studied the lattice dynamics of a Ni 54 Mn 23 Al 23 (at.%) Heusler single-crystalline alloy by means of neutron scattering and ultrasonic techniques. Results show the existence of a number of precursor phenomena. We have found an anomaly (dip) in the low TA 2 phonon branch at the wave number ξ 0 ≈ 0.33 (in reciprocal lattice units) that becomes more pronounced (phonon softening) with decreasing temperature. We have also observed softening of the associated shear elastic constant ( C ′ ) with decreasing temperature. Ultrasonic measurements under applied magnetic field, both isothermally and varying the temperature show that the values of elastic constants depend on magnetic order thus evidencing magnetoelastic coupling.

In this study, we describe why Ising spin chains with competing interactions in SrHo2O4 segregate into ordered and disordered ensembles at low temperatures (T). Using elastic neutron scattering, magnetization, and specific heat measurements, the two distinct spin chains are inferred to have Neel (↑↓↑↓) and double-Neel (↑↑↓↓) ground states, respectively. Below TN = 0.68(2)K, the Neel chains develop three-dimensional long range order (LRO), which arrests further thermal equilibration of the double-Neel chains so they remain in a disordered incommensurate state for T below TS = 0.52(2)K. SrHo2O4 distills an important feature of incommensurate low dimensional magnetism: kinetically trapped topological defects in a quasi–d–dimensional spin system can preclude order in d + 1 dimensions.

The ferrimagnetic spinel $\mathrm{CoV_2O_4}$ has been a topic of intense recent interest, both as a frustrated insulator with unquenched orbital degeneracy and as a near-itinerant magnet which can be driven metallic with moderate applied pressure. Here, we report on our recent neutron diffraction and inelastic scattering measurements on powders with minimal cation site disorder. Our main new result is the identification of a weak ($\frac{\Delta a}{a} \sim 10^{-4}$), first order structural phase transition at $T^*$ = 90 K, the same temperature where spin canting was seen in recent single crystal measurements. This transition is characterized by a short-range distortion of oxygen octahedral positions, and inelastic data further establish a weak $\Delta\sim 1.25 meV$ spin gap at low temperature. Together, these findings provide strong support for the local orbital picture and the existence of an orbital glass state at temperatures below $T^*$., Comment: 8 pages, 5 figures, To be published in Physical Review B

The $A{R}_{2}{\text{O}}_{4}$ family ($R=\text{rare}$ earth) has recently been attracting interest as a new series of frustrated magnets, with the magnetic $R$ atoms forming zigzag chains running along the $c$ axis. We have investigated polycrystalline ${\mathrm{BaNd}}_{2}{\mathrm{O}}_{4}$ with a combination of magnetization, heat-capacity, and neutron powder diffraction measurements. Magnetic Bragg peaks are observed below ${T}_{N}=1.7\phantom{\rule{4pt}{0ex}}\mathrm{K}$, and they can be indexed with a propagation vector of $\stackrel{P\vec}{k}=(0,1/2,1/2)$. The signal from magnetic diffraction is well described by long-range ordering of only one of the two types of Nd zigzag chains, with collinear up-up-down-down intrachain spin configurations (double N\'eel state). Furthermore, low-temperature magnetization and heat-capacity measurements reveal two magnetic-field-induced spin transitions at 2.75 and 4 T for $T=0.46\phantom{\rule{4pt}{0ex}}\mathrm{K}$. The high-field phase is paramagnetic, while the intermediate-field state may arise from a spin transition of the long-range ordered Nd chains. One possible candidate for the field-induced ordered state corresponds to an up-up-down intrachain spin configuration, as predicted for a classical ${J}_{1}\ensuremath{-}{J}_{2}$ Ising chain with a double N\'eel ground state in zero field.

The spinel vanadates have become a model family for exploring orbital order on the frustrated pyrochlore lattice, and recent debate has focused on the symmetry of local crystal fields at the cation sites. Here, we present neutron scattering measurements of the magnetic excitation spectrum in $\mathrm{FeV_2O_4}$, a recent example of a ferrimagnetic spinel vanadate which is available in single crystal form. We report the existence of two emergent magnon modes at low temperatures, which draw strong parallels with the closely related material, $\mathrm{MnV_2O_4}$. We were able to reproduce the essential elements of both the magnetic ordering pattern and the dispersion of the inelastic modes with semi- classical spin wave calculations, using a minimal model that implies a sizeable single-ion anisotropy on the vanadium sublattice. Taking into account the direction of ordered spins, we associate this anisotropy with the large trigonal distortion of $\mathrm{VO_6}$ octahedra, previously observed via neutron powder diffraction measurements. We further report on the spin gap, which is an order-of-magnitude larger than that observed in $\mathrm{MnV_2O_4}$. By looking at the overall temperature dependence, we were able to show that the gap magnitude is largely associated with the ferro-orbital order known to exist on the iron sublattice, but the contribution to the gap from the vanadium sublattice is in fact comparable to what is reported in the Mn compound. This reinforces the conclusion that the spin canting transition is associated with the ordering of vanadium orbitals in this system, and closer analysis indicates closely related physics underlying orbital transitions in $\mathrm{FeV_2O_4}$ and $\mathrm{MnV_2O_4}$., Comment: 8 pages, 6 figures, includes supplementary figure; accepted for publication in Physical Review B

Neutron diffraction, X-ray diffraction, magnetic susceptibility, and specific heat measurements have been used to investigate the magnetic and structural phase transitions of the spinel system Fe1+xCr2 xO4 (0.0 x 1.0). The temperature versus Fe concentration (x) phase diagram contains two magnetically ordered phases and four structural phases below 420 K. The complicated transitions are closely related to the change in the spin and orbital degrees of freedom induced by substitution of Fe ions for Cr ions. The systematic change in the crystal structure is explained by the combined effects of Jahn-Teller distortion, spin-lattice interaction, Fe2+-Fe3+ hopping, and disorder among Fe2+, Fe3+, and Cr3+ ions.

The hole-doped kagome lattice of Cu${}^{2+}$ ions in LaCuO${}_{2.66}$ was investigated by nuclear quadrupole resonance, electron spin resonance, electrical resistivity, bulk magnetization, and specific-heat measurements. For temperatures above $\ensuremath{\sim}180$ K, the spin and charge properties show an activated behavior suggestive of a narrow-gap semiconductor. At lower temperatures, the results indicate an insulating ground state which may or may not be charge ordered. While the frustrated spins in remaining patches of the original kagome lattice might not be directly detected here, the observation of coexisting nonmagnetic sites, free spins, and frozen moments reveals an intrinsically inhomogeneous magnetism. Numerical simulations of a $\frac{1}{3}$-diluted kagome lattice rationalize this magnetic state in terms of a heterogeneous distribution of cluster sizes and morphologies near the site-percolation threshold.

We report the structural and magnetic properties of a new class of cobaltates with the chemical formula (BaSr)${}_{4\ensuremath{-}x}$La${}_{2x}$Co${}_{4}$O${}_{15}$ ($x=0$, 0.5, and 1). These compounds crystallize in a hexagonal structure in which cobalt ions are distributed among two distinct crystallographic sites with different oxygen coordination. Three Co-O tetrahedra and one octahedron are linked by shared oxygen atoms to form Co${}_{4}$O${}_{15}$ clusters, which are packed together into a honeycomblike network. Partial substitution of Sr and/or Ba atoms by La allows one to adjust the degree of Co valence mixing, but all compositions remain subject to a random distribution of charge. Magnetic susceptibility together with neutron scattering measurements reveal that all studied specimens are characterized by competing ferro- and antiferro-magnetic exchange interactions that give rise to a three-dimensional Heisenberg spin-glass state. Neutron spectroscopy shows a clear trend of slowing down of spin-dynamics upon increasing La concentration, suggesting a reduction in charge randomness in the doped samples.

We present a neutron-diffraction study of FeV${}_{2}$O${}_{4}$, which is rare in exhibiting spin and orbital degrees of freedom on both cation sublattices of the spinel structure. Our data confirm the existence of three structural phase transitions previously identified with x-ray powder diffraction and reveal that the lower two transitions are associated with sequential collinear and canted ferrimagnetic transitions involving both cation sites. Through consideration of local crystal and spin symmetry, we further conclude that Fe${}^{2+}$ cations are ferro-orbitally ordered below 135 K and V${}^{3+}$ orbitals order at 60 K, in accordance with predictions for vanadium spinels with large trigonal distortions and strong spin-orbit coupling. Intriguingly, the direction of ordered vanadium spins at low temperature obey ``ice rules'' more commonly associated with the frustrated rare-earth pyrochlore systems.

We present a neutron and x-ray scattering investigation of Ca${}_{3}$LiOsO${}_{6}$, 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 Ca${}_{3}$LiOsO${}_{6}$, 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.

The metal-insulator transition (MIT) is one of the most dramatic manifestations of electron correlations in materials. Various mechanisms producing MITs have been extensively considered, including the Mott (electron localization via Coulomb repulsion), Anderson (localization via disorder) and Peierls (localization via distortion of a periodic 1D lattice). One additional route to a MIT proposed by Slater, in which long-range magnetic order in a three dimensional system drives the MIT, has received relatively little attention. Using neutron and X-ray scattering we show that the MIT in NaOsO3 is coincident with the onset of long-range commensurate three dimensional magnetic order. Whilst candidate materials have been suggested, our experimental methodology allows the first definitive demonstration of the long predicted Slater MIT. We discuss our results in the light of recent reports of a Mott spin-orbit insulating state in other 5d oxides.

We study excitations in weakly interacting pairs of quantum spin ladders coupled through geometrically frustrated bonds. The ground state is a disordered spin liquid, that at high fields is replaced by an ordered chiral helimagnetic phase. The spectra observed by high-field inelastic neutron scattering experiments on the prototype compound Sul-Cu2Cl4 are qualitatively different from those in the previously studied frustration-free spin liquids. Beyond the critical field Hc=3.7 T, the soft mode that drives the quantum phase transition spawns two separate excitations: a gapless Goldstone mode and a massive magnon. Additional massive quasiparticles are clearly visible below Hc, but are destroyed in the ordered phase. In their place one observes a sharply bound excitation continuum., Comment: 6 pages, 4 figures

Neutron inelastic scattering and diffraction techniques have been used to study the MnV2O4 spinel system. Our measurements show the existence of two transitions to long-range ordered ferrimagnetic states, the first collinear and the second noncollinear. The lower temperature transition, characterized by development of antiferromagnetic components in the basal plane, is accompanied by a tetragonal distortion and the appearance of a gap in the magnetic excitation spectrum. The low-temperature noncollinear magnetic structure has been definitively resolved. Taken together, the crystal and magnetic structures indicate a staggered ordering of the V d orbitals. The anisotropy gap is a consequence of unquenched V orbital angular momentum.

Inelastic neutron scattering is used to investigate magnetic excitations in the quasi-one-dimensional quantum spin-liquid system ${\mathrm{Cu}}_{2}{\mathrm{Cl}}_{4}\ifmmode\cdot\else\textperiodcentered\fi{}{\mathrm{D}}_{8}{\mathrm{C}}_{4}{\mathrm{SO}}_{2}$. Contrary to previously conjectured models that relied on bond-alternating nearest-neighbor interactions in the spin chains, the dominant interactions are actually next-nearest-neighbor in-chain antiferromagnetic couplings. The appropriate Heisenberg Hamiltonian is equivalent to that of a $S=1/2$ 4-leg spin-tube with almost perfect one dimensionality and no bond alternation. A partial geometric frustration of rung interactions induces a small incommensurability of short-range spin correlations.

Neutron scattering is used to study magnetic field induced ordering in the quasi-1D quantum spin-tube compound Sul--Cu$_2$Cl$_4$ that in zero field has a non-magnetic spin-liquid ground state. The experiments reveal an incommensurate chiral high-field phase stabilized by a geometric frustration of the magnetic interactions. The measured critical exponents $\beta\approx0.235$ and $\nu\approx0.34$ at $H_c\approx3.7$ T point to an unusual sub-critical scaling regime and may reflect the chiral nature of the quantum critical point., Comment: 4 pages

Inelastic and elastic neutron scattering is used to study spin correlations in the quasi-one-dimensional quantum antiferromagnet IPA-CuCl3 in strong applied magnetic fields. A condensation of magnons and commensurate transverse long-range ordering is observe at a critical field Hc=9.5 T. The field dependencies of the energies and polarizations of all magnon branches are investigated both below and above the transition point. Their dispersion is measured across the entire one-dimensional Brillouin zone in magnetic fields up to 14 T. The critical wave vector of magnon spectrum truncation Masuda et al., Phys. Rev. Lett. 96, 047210 2006 is found to shift from hc0,35 at HHC to hc=0.25 for HHC. A drastic reduction of magnon bandwidths in the ordered phase Garlea et al., Phys. Rev. Lett. 98, 167202 2007 is observed and studied in detail. New features of the spectrum, presumably related to this bandwidth collapse, are observed just above the transition field.

The weakly coupled quasi-one-dimensional spin ladder compound (CH$_3$)$_2$CHNH$_3$CuCl$_3$ is studied by neutron scattering in magnetic fields exceeding the critical field of Bose-Einstein condensation of magnons. Commensurate long-range order and the associated Goldstone mode are detected and found to be similar to those in a reference 3D quantum magnet. However, for the upper two massive magnon branches the observed behavior is totally different, culminating in a drastic collapse of excitation bandwidth beyond the transition point., Comment: 4 pages, 4 figures

Spin-wave dispersions in the antiferromagnetic state of single-crystal LiFePO4 were determined by inelastic neutron scattering measurements. The dispersion curves measured from the 0,1,0 reflection along both a * and b * reciprocal-space directions reflect the anisotropic coupling of the layered Fe 2+ S=2 spin system. The spin-wave dispersion curves were theoretically modeled using linear spin-wave theory by including in the spin Hamiltonian in-plane nearest- and next-nearest-neighbor interactions J1 and J2, inter-plane nearest-neighbor interactions J and a single-ion anisotropy D .Aw eak0,1,0 magnetic peak was observed in elastic neutron scattering studies of the same crystal indicating that the ground state of the staggered iron moments is not along the 0,1,0 direction, as previously reported from polycrystalline samples studies, but slightly rotated away from this axis.

We report the results of a neutron diffraction study, carried out on both single crystalline and polycrystalline samples of ${\mathrm{Tb}}_{5}{\mathrm{Si}}_{2.2}{\mathrm{Ge}}_{1.8}$ and polycrystalline ${\mathrm{Tb}}_{5}{\mathrm{Si}}_{2.5}{\mathrm{Ge}}_{1.5}$. On cooling, at approximately 120 K, the ${\mathrm{Tb}}_{5}{\mathrm{Si}}_{2.2}{\mathrm{Ge}}_{1.8}$ system undergoes a magnetoelastic transition from a high-temperature monoclinic-paramagnetic to a low-temperature orthorhombic-ferromagnetic structure. Between 120 K and 75 K, the magnetic structure has a net ferromagnetic component along the $a$ axis direction. The moments are slightly canted with respect to the $a$ axis, while the components along the $b$ and $c$ axes are ordered antiferromagnetically. A second magnetic transition occurs at approximately 75 K. Below this temperature, the magnetic structure consists of ferromagnetically aligned ${\ensuremath{\mu}}_{x}$ and ${\ensuremath{\mu}}_{z}$ projections of the magnetic moments and an antiferromagnetic arrangement of the ${\ensuremath{\mu}}_{y}$ moment components. Magnetic structures of ${\mathrm{Tb}}_{5}{\mathrm{Si}}_{2.5}{\mathrm{Ge}}_{1.5}$ are nearly identical to those of ${\mathrm{Tb}}_{5}{\mathrm{Si}}_{2.2}{\mathrm{Ge}}_{1.8}$.

Magnetization and neutron diffraction studies on a polycrystalline Tb2 Ti3 Ge4 sample (orthorhombic Sm5 Ge4 -type structure, space group Pnma, No. 62) have been carried out. This compound is found to order antiferromagnetically at ?18 K (TN). The magnetization (M) versus field (H) isotherms obtained at 2, 3, 5, and 10 K indicate a field-induced antiferromagnetic to ferromagnetic transition in fields of the order of 0.5 T. The saturation magnetization value at 2.5 K (M extrapolated to 1/H?0) is only ?5.6 ?B / Tb3+, suggesting the possible presence of crystal field effects with or without a persisting antiferromagnetic component. Neutron powder diffraction data at 10 K confirm the existence of a magnetic long range order. Modeling of the magnetic scattering reveals a complex and incommensurate antiferromagnetic spin structure below TN. � 2009 American Institute of Physics.