Search

Non-collinear magnetic order arises for various reasons in several magnetic systems and exhibits interesting spin dynamics. Despite its ubiquitous presence, little is known of how magnons, otherwise stable quasiparticles, decay in these systems, particularly in metallic magnets. Using inelastic neutron scattering, we examine the magnetic excitation spectra in a metallic non-collinear antiferromagnet CrB$_{2}$, in which Cr atoms form a triangular lattice and display incommensurate magnetic order. Our data show intrinsic magnon damping and continuum-like excitations that cannot be explained by linear spin wave theory. The intrinsic magnon linewidth $\Gamma(q,E_{q})$ shows very unusual momentum dependence, which our analysis shows to originate from the combination of two-magnon decay and the Stoner continuum. By comparing the theoretical predictions with the experiments, we identify where in the momentum and energy space one of the two factors becomes more dominant. Our work constitutes a rare comprehensive study of the spin dynamics in metallic non-collinear antiferromagnets. It reveals, for the first time, definite experimental evidence of the higher-order effects in metallic antiferromagnets., Comment: 6 pages, 4 figures, accepted for publication in PRL

Understanding the nature of the electronic nematic phase in iron pnictide superconductors is important for elucidating its impact on high-temperature superconductivity. Here we use transport and inelastic neutron scattering to study spin excitations and in-plane resistivity anisotropy in uniaxial pressure detwinned ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ and ${\mathrm{SrFe}}_{2}{\mathrm{As}}_{2}$, the parent compounds of iron pnictide superconductors. While ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ exhibits weakly first-order tetragonal-to-orthorhombic structural and antiferromagnetic (AF) phase transitions below ${T}_{s}g{T}_{N}\ensuremath{\approx}138\phantom{\rule{0.16em}{0ex}}\mathrm{K}, {\mathrm{SrFe}}_{2}{\mathrm{As}}_{2}$ has strongly coupled first-order structural and AF transitions below ${T}_{s}={T}_{N}\ensuremath{\approx}210\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. We find that the direct signatures of the nematic phase persist to lower temperatures above the phase transition in the case of ${\mathrm{SrFe}}_{2}{\mathrm{As}}_{2}$ compared to ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$. Our findings support the conclusion that the strongly first-order nature of the magnetic transition in ${\mathrm{SrFe}}_{2}{\mathrm{As}}_{2}$ weakens the nematic phase and resistivity anisotropy in the system.

We use inelastic neutron scattering to study energy and wave vector dependence of spin fluctuations in SrCo_{2}As_{2}, derived from SrFe_{2-x}Co_{x}As_{2} iron pnictide superconductors. Our data reveal the coexistence of antiferromagnetic (AF) and ferromagnetic (FM) spin fluctuations at wave vectors Q_{AF}=(1,0) and Q_{FM}=(0,0)/(2,0), respectively. By comparing neutron scattering results with those of dynamic mean field theory calculation and angle-resolved photoemission spectroscopy experiments, we conclude that both AF and FM spin fluctuations in SrCo_{2}As_{2} are closely associated with a flatband of the e_{g} orbitals near the Fermi level, different from the t_{2g} orbitals in superconducting SrFe_{2-x}Co_{x}As_{2}. Therefore, Co substitution in SrFe_{2-x}Co_{x}As_{2} induces a t_{2g} to e_{g} orbital switching, and is responsible for FM spin fluctuations detrimental to the singlet pairing superconductivity.

We report a neutron scattering study of the metallic triangular lattice antiferromagnet PdCrO$_2$. Powder neutron diffraction measurements confirm that the crystalline space group symmetry remains $R\bar{3}m$ below $T_N$. This implies that magnetic interactions consistent with the crystal symmetry do not stabilise the non-coplanar magnetic structure which was one of two structures previously proposed on the basis of single crystal neutron diffraction measurements. Inelastic neutron scattering measurements find two gaps at low energies which can be explained as arising from a dipolar-type exchange interaction. This symmetric anisotropic interaction also stabilises a magnetic structure very similar to the coplanar magnetic structure which was also suggested by the single crystal diffraction study. The higher energy magnon dispersion can be modelled by linear spin wave theory with exchange interactions up to sixth nearest-neighbors, but discrepancies remain which hint at additional effects unexplained by the linear theory., Comment: 10 pages, 5 figures, submitted to Phys. Rev. B

We have used time-of-flight inelastic neutron scattering to measure the spin wave spectrum of the canonical half-doped manganite Pr$_{0.5}$Ca$_{0.5}$MnO$_{3}$, in its magnetic and orbitally ordered phase. Comparison of the data, which cover multiple Brillouin zones and the entire energy range of the excitations, with several different models shows that only the CE-type ordered state provides an adequate description of the magnetic ground state, provided interactions beyond nearest neighbor are included. We are able to rule out a ground state in which there exist pairs of dimerized spins which interact only with their nearest neighbors. The Zener polaron ground state, which comprises strongly bound magnetic dimers, can be ruled out on the basis of gross features of the observed spin wave spectrum. A model with weaker dimerization reproduces the observed dispersion but can be ruled out on the basis of subtle discrepancies between the calculated and observed structure factors at certain positions in reciprocal space. Adding further neighbor interactions results in almost no dimerization, i.e. interpolating back to the CE model. These results are consistent with theoretical analysis of the degenerate double exchange model for half-doping.

We use time-of-flight (TOF) inelastic neutron scattering (INS) spectroscopy to investigate the doping dependence of magnetic excitations across the phase diagram of NaFe1-xCoxAs with x = 0, 0.0175, 0.0215, 0.05, and 0.11. The effect of electron-doping by partially substituting Fe by Co is to form resonances that couple with superconductivity, broaden and suppress low energy (E 80 meV) spin excitations compared with spin waves in undoped NaFeAs. However, high energy (E > 80 meV) spin excitations are weakly Co-doping dependent. Integration of the local spin dynamic susceptibility "(!) of NaFe1-xCoxAs reveals a total fluctuating moment of 3.6 μ2 B/Fe and a small but systematic reduction with electron doping. The presence of a large spin gap in the Cooverdoped nonsuperconducting NaFe0.89Co0.11As suggests that Fermi surface nesting is responsible for low-energy spin excitations. These results parallel Ni-doping evolution of spin excitations in BaFe2-xNixAs2, confirming the notion that low-energy spin excitations coupling with itinerant electrons are important for superconductivity, while weakly doping dependent high-energy spin excitations result from localized moments.

Author(s): Wang, M; Valdivia, P; Yi, M; Chen, JX; Zhang, WL; Ewings, RA; Perring, TG; Zhao, Y; Harriger, LW; Lynn, JW; Bourret-Courchesne, E; Dai, P; Lee, DH; Yao, DX; Birgeneau, RJ

An inelastic neutron scattering study of the spin waves corresponding to the stripe antiferromagnetic order in insulating ${\mathrm{Rb}}_{0.8}{\mathrm{Fe}}_{1.5}{\mathrm{S}}_{2}$ throughout the Brillouin zone is reported. The spin wave spectra are well described by a Heisenberg Hamiltonian with anisotropic in-plane exchange interactions. Integrating the ordered moment and the spin fluctuations results in a total moment squared of $27.6\ifmmode\pm\else\textpm\fi{}4.2{\ensuremath{\mu}}_{B}^{2}/\text{Fe}$, consistent with $S\ensuremath{\approx}2$. Unlike $X{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ ($X=\text{Ca}$, Sr, and Ba), where the itinerant electrons have a significant contribution, our data suggest that this stripe antiferromagnetically ordered phase in ${\mathrm{Rb}}_{0.8}{\mathrm{Fe}}_{1.5}{\mathrm{S}}_{2}$ is a Mott-like insulator with fully localized $3d$ electrons and a high-spin ground state configuration. Nevertheless, the anisotropic exchange couplings appear to be universal in the stripe phase of Fe pnictides and chalcogenides.

We report neutron inelastic-scattering measurements and analysis of the spectrum of magnons propagating within the Fe2O4 bilayers of LuFe2O4. The observed spectrum is consistent with six magnetic modes and a single prominent gap, which is compatible with a single bilayer magnetic unit cell containing six spins. We model the magnon dispersion by linear spin-wave theory and find very good agreement with the domain-averaged spectrum of a spin-charge bilayer superstructure comprising one Fe3+-rich monolayer and one Fe2+-rich monolayer. These findings indicate the existence of polar bilayers in LuFe2O4. We also discuss a model of charge-segregated nonpolar bilayer order which we argue is not consistent with our data but which we cannot exclude definitively. Weak scattering observed below the magnon gap suggests that a fraction of the bilayers contain other combinations of charged monolayers not included in the model. Refined values for the dominant exchange interactions are reported.

The magnetic excitations of the square-lattice spin-1/2 antiferromagnet and high- T(c) parent compound La2CuO4 are determined using high-resolution inelastic neutron scattering. Sharp spin waves with absolute intensities in agreement with theory including quantum corrections are found throughout the Brillouin zone. The observed dispersion relation shows evidence for substantial interactions beyond the nearest-neighbor Heisenberg term which can be understood in terms of a cyclic or ring exchange due to the strong hybridization path around the Cu4O4 square plaquettes.

Inelastic neutron scattering reveals strong spin fluctuations with energies as high as 0.4 eV in the nearly antiferromagnetic metal ${\mathrm{Cr}}_{0.95}{\mathrm{V}}_{0.05}$. The magnetic response is well described by a modified Millis-Monien-Pines function. From the low-energy response, we deduce a large exchange enhancement, more than an order of magnitude larger than the corresponding enhancement of the low-temperature electronic heat capacity $\ensuremath{\gamma}T$. A scaling relationship between $\ensuremath{\gamma}$ and the inverse of the wave vector--averaged spin relaxation rate ${\ensuremath{\Gamma}}_{\mathrm{ave}}$ is demonstrated for a number of magnetically correlated metals.

The spin dynamics in $S=3/2,$ one-dimensional (1D) Heisenberg antiferromagnets, ${\mathrm{CsVCl}}_{3}$ and ${\mathrm{CsVBr}}_{3},$ was investigated using inelastic neutron scattering. The magnetic excitations were measured at temperatures above the three-dimensional ordering temperature in order to discuss the 1D properties in the present systems. We report on the spin dynamics at low temperatures and its temperature (T) dependence. From the observed dispersion relation, using the renormalization constant calculated from the quantum Monte Carlo method, we obtained the exchange constant (J) in good agreement with those taken from bulk-susceptibility measurements. The observed structure factor was well described by that calculated from the parameters describing the dispersion relation. The energy width (\ensuremath{\Gamma}) was independent of the 1D momentum transfer (q) at large energy transfers, however, its q dependence exhibits the minimum at the magnetic zone center. The parameters describing the energy scale in the spin dynamics at low T were found to be scaled by J. We also investigated the T dependence of the spin dynamics: $\ensuremath{\Gamma}(T)$ can be scaled by J, $\ensuremath{\Gamma}(T)$ at $TgJ$ is proportional to T as predicted by classical theory but at $TgJ,$ \ensuremath{\Gamma} decreases with decreasing T and becomes finite. On the other hand, we found that $\ensuremath{\kappa}(T)$ is proportional to T at any T. The scaling by J in the dynamics at low T as well as $\ensuremath{\Gamma}(T)$ suggests that the observed spin dynamics is of 1D origin. We conclude that the spin dynamics at high temperatures $TgJ,$ is well described by classical theory, however, at low temperatures $TlJ,$ the finite width indicates some quantum fluctuations. Therefore, we observed the crossover from the quantum state at low T to the classical state at high T in $S=3/2$ systems.

The layered compound La2-2xSr1+2xMn2O7 (x=0.3) consists of bilayers of metallic MnO2 sheets separated by insulating material. The compound exhibits markedly anisotropic magnetoresistance at temperatures well below the three-dimensional magnetic ordering temperature TC=90 K in addition to colossal magnetoresistance around TC. We present neutron diffraction data which show that the magnetic structure of this material switches from antiferromagnetic stacking of the (ferromagnetically ordered) sheets in zero field to ferromagnetic stacking in a field of 1.5 Tesla. The data are the first to be collected on any manganite as a function of applied field, exactly as the magnetoresistance data themselves are collected. They provide a natural explanation of the low-field magnetoresistance in the ordered phase in terms of spin-polarised tunnelling between the magnetic layers and suggest that the material is a bulk stack of spin-valve devices., 11 pages and 3 figures To be published in Phys. Rev. B Rapid Communications

The magnetic critical scattering of the near-ideal three-dimensional Heisenberg antiferromagnet (AF) RbMnF3 has been remeasured using neutron scattering. The critical dynamics has been studied in detail in the temperature range 0.77T(N) < T < 1.11T(N), where T-N is the Neel temperature. In agreement with previous measurements, at T-N and for wave vectors away from the AF zone center, the scattering has a quasielastic component in addition to the inelastic response predicted by renormalization-group and mode-coupling theories. Both components scale with the dynamic exponent z = 1.43 +/- 0.04, in agreement with dynamic scaling. On cooling below T-N the inelastic peaks transform into the transverse spin waves and a crossover has been observed in the dispersion from a power-law relation omega(q) = Aq(z) at T-N to a linear behavior omega(q) = cq in the hydrodynamic region below T-N. The quasielastic component evolves below T-N into the longitudinal susceptibility identified in an earlier polarized neutron experiment. The intensity and energy width of the longitudinal scattering decrease on cooling below T-N. Down to the lowest temperatures where the longitudinal susceptibility could be measured the leading term in the scaling behavior of the energy width was gamma(q) approximate to q(1.58+/-0.03) (hydrodynamic theory predicts a q(2) law). Possible explanations for the observed behavior of the longitudinal susceptibility are discussed.

The bilayer manganite ${\mathrm{La}}_{1.2}{\mathrm{Sr}}_{1.8}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ exhibits a transition between nearly insulating paramagnetic and metallic ferromagnetic states at ${T}_{C}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}126\mathrm{K}$. We use magnetic neutron scattering to show that the paramagnetic state contains long-lived antiferromagnetic clusters coexisting with ferromagnetic critical fluctuations. The principal implication is that at least in two dimensions, Anderson localization effects must also be taken into account in attempts to explain giant magnetoresistance in manganites.

Pulsed neutron spectroscopy is used to absolute measurements of the dynamic magnetic susceptibility of insulating YBa{sub 2}Cu{sub 3}O{sub 6.15}. Acoustic and optical modes, derived from in- and out-of-phase oscillation of spins in adjacent CuO{sub 2} planes, dominate the spectra and are observed up to 250 meV. The optical modes appear first at 74{plus_minus}5 meV. Linear-spin-wave theory gives an excellent description of the data and yields intralayer and interlayer exchange constants of {ital J}{sub {parallel}}=125{plus_minus}5 meV and {ital J}{sub {perpendicular}}=11{plus_minus}2 meV, respectively, and a spin-wave intensity renormalization {ital Z}{sub {chi}}=0.4{plus_minus}0.1. {copyright} {ital 1996 The American Physical Society.}

We use inelastic neutron scattering to measure the spin wave dispersion throughout the Brillouin zone of the double-exchange ferromagnet La0.7Pb0.3MnO3. Magnons with energies as high as 95 meV are directly observed and an unexpectedly simple Heisenberg Hamiltonian, with solely a nearest-neighbor coupling of 8.79 +/- 0.21 meV, accounts for the entire dispersion relation. The calculated Curie temperature for this local moment Hamiltonian overestimates the measured Curie point (355 K) by only 15%. Raising temperature yields unusual broadening of the high frequency spin waves, even within the ferromagnetic phase.

We show by inelastic neutron scattering that the frustrated magnet ${\mathrm{SrCr}}_{9p}{\mathrm{Ga}}_{12\ensuremath{-}9p}{\mathrm{O}}_{19}$ $[p\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.92(5)]$ has dispersionless magnetic excitations at energies 18.6(1) and 37.2(5) meV. The wave vector and temperature dependence of the excitations as well as the ratio 1:2 of excitation energies is perfectly accounted for by isolated antiferromagnetically exchange coupled pairs of spins $s\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}3/2$. Consideration of the layered structure of magnetic chromium ions and the ligand environment indicates that these pairs are adjacent spins in neighboring triangular lattice planes which separate two-dimensional sublattices of interacting kagom\'e-triangle-kagom\'e layers.

Systematic inelastic neutron scattering studies have been performed on La${}_{1.85}$Sr${}_{0.15}$Cu${}_{1\ensuremath{-}y}$Ni${}_{y}$O${}_{4}$ in order to elucidate the microscopic mechanism of magnetic Ni impurity doping effects on spin dynamics and superconductivity. In contrast to Zn doping, which precipitates static or quasistatic spin correlations around dopant, Ni doping remarkably reduces ${E}_{\mathrm{comme}}$, the minimum energy of the upward branch of the hourglass-shaped magnetic dispersion where a commensurate magnetic peak is observed. Ni doping at $y\ensuremath{\sim}0.04$ further reduces ${E}_{\mathrm{comme}}$ to zero energy and broadens the distribution of ${E}_{\mathrm{comme}}$, corresponding to the appearance of the normal state at base temperature ($T\ensuremath{\sim}4$ K) as well as the broadening of the superconducting transition.

We use inelastic neutron scattering to study temperature dependence of the paramagnetic spin excitations in iron pnictide BaFe${}_{2}$As${}_{2}$ throughout the Brillouin zone. In contrast to a conventional local moment Heisenberg system, where paramagnetic spin excitations are expected to have a Lorentzian function centered at zero energy transfer, the high-energy ($\ensuremath{\hbar}\ensuremath{\omega}g100$ meV) paramagnetic spin excitations in BaFe${}_{2}$As${}_{2}$ exhibit spin-wave-like features up to at least 290 K ($T=2.1{T}_{N}$). Furthermore, we find that the sizes of the fluctuating magnetic moments $\ensuremath{\langle}{m}^{2}\ensuremath{\rangle}\ensuremath{\approx}3.6{\ensuremath{\mu}}_{B}^{2}$ per Fe are essentially temperature independent from the antiferromagnetic ordered state at $0.05{T}_{N}$ to $2.1{T}_{N}$, which differs considerably from the temperature dependent fluctuating moment observed in the iron chalcogenide Fe${}_{1.1}$Te [Zaliznyak et al., Phys. Rev. Lett. 107, 216403 (2011)]. These results suggest unconventional magnetism and strong electron correlation effects in BaFe${}_{2}$As${}_{2}$.

We report neutron inelastic scattering measurements on the normal and superconducting states of single-crystalline Cs0.8Fe1.9Se2. Consistent with previous measurements on RbxFe2−ySe2, we observe two distinct spin excitation signals: (i) spin-wave excitations characteristic of the block antiferromagnetic order found in insulating AxFe2−ySe2 compounds, and (ii) a resonance-like magnetic peak localized in energy at 11 meV and at an in-plane wave-vector of (0.25,0.5). The resonance peak increases below Tc=27 K, and has a similar absolute intensity to the resonance peaks observed in other Fe-based superconductors. The existence of a magnetic resonance in the spectrum of RbxFe2−ySe2 and now of CsxFe2−ySe2 suggests that this is a common feature of superconductivity in this family. The low-energy spin-wave excitations in Cs0.8Fe1.9Se2 show no measurable response to superconductivity, consistent with the notion of spatially separate magnetic and superconducting phases.

We report inelastic neutron-scattering measurements of the magnetic excitations in SrFe${}_{2}$As${}_{2}$, the parent of a family of iron-based superconductors. The data extend throughout the Brillouin zone and up to energies of $~$$260$ meV. The spectrum calculated from a ${J}_{1}$-${J}_{2}$ model does not accurately describe our data, and we show that some of the qualitative features that the model fails to describe are readily explained by calculations from a five-band itinerant mean-field model. In particular, the high-energy part of the spectra recorded above ${T}_{\mathrm{N}}$ do not differ significantly from those at low temperature, which is explained by the itinerant model and which has implications for theories of electronic nematic and orbital ordering.

We use neutron scattering to show that spin waves in the iron chalcogenide ${\mathrm{Fe}}_{1.05}\mathrm{Te}$ display novel dispersion clearly different from both the first principles density functional calculations and recent observations in the related iron pnictide ${\mathrm{CaFe}}_{2}{\mathrm{As}}_{2}$. By fitting to a Heisenberg Hamiltonian, we find that although the nearest-neighbor exchange couplings in the two systems are quite different, their next-nearest-neighbor (NNN) couplings are similar. This suggests that superconductivity in the pnictides and chalcogenides share a common magnetic origin that is intimately associated with the NNN magnetic coupling between the irons.

Inelastic neutron scattering is used to investigate the collective magnetic excitations of the high-temperature superconductor-parent antiferromagnet ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4}$. We find that while the lower energy excitations are well described by spin-wave theory, including one- and two-magnon scattering processes, the high-energy spin waves are strongly damped near the $(1/2,0)$ position in reciprocal space and merge into a momentum dependent continuum. This anomalous damping indicates the decay of spin waves into other excitations, possibly unbound spinon pairs.

Magnetic correlations in the paramagnetic phase of CaFe2As2(TN=172 K) have been examined by means of inelastic neutron scattering from 180 K ( 1.05TN) up to 300 K (1.8TN). Despite the first-order nature of the magnetic ordering, strong but short-ranged antiferromagnetic (AFM) correlations are clearly observed. These correlations, which consist of quasielastic scattering centered at the wave vector QAFM of the low-temperature AFM structure, are observed up to the highest measured temperature of 300 K and at high energy transfer ( >60 meV). The L dependence of the scattering implies rather weak interlayer coupling in the tetragonal c direction corresponding to nearly two-dimensional fluctuations in the (ab) plane. The spin correlation lengths within the Fe layer are found to be anisotropic, consistent with underlying fluctuations of the AFM stripe structure. Similar to the cobalt-doped superconducting BaFe2As2 compounds, these experimental features can be adequately reproduced by a scattering model that describes short-ranged and anisotropic spin correlations with overdamped dynamics.

Inelastic neutron scattering is used to probe the collective spin excitations of the near optimally doped superconductor $\text{Ba}{({\text{Fe}}_{1\ensuremath{-}x}{\text{Co}}_{x})}_{2}{\text{As}}_{2}$ $(x=0.065)$. Previous measurements on the antiferromagnetically ordered parents of this material show a strongly anisotropic spin-wave velocity. Here we measure the magnetic excitations up to 80 meV and show that a similar anisotropy persists for superconducting compositions. The dispersive mode measured here connects directly with the spin resonance previously observed in this compound. When placed on an absolute scale, our measurements show that the local- or wave-vector-integrated susceptibility is larger in magnitude than that of the ordered parents over the energy range probed.

Neutron scattering measurements of the magnetic excitations in single crystals of antiferromagnetic CaFe2As2 reveal steeply dispersive and well-defined spin waves up to an energy of approximately 100 meV. Magnetic excitations above 100 meV and up to the maximum energy of 200 meV are however broader in energy and momentum than the experimental resolution. While the low energy modes can be fit to a Heisenberg model, the total spectrum cannot be described as arising from excitations of a local moment system. Ab initio calculations of the dynamic magnetic susceptibility suggest that the high energy behavior is dominated by the damping of spin waves by particle-hole excitations.

We use inelastic neutron scattering to measure the magnetic excitations in underdoped La2-xSrxCuO4 (x=0.085, T_{c}=22 K) for large energy (5

Time-of-flight spectrocopy using neutrons produced by a spallation source is used to measure the one-magnon scattering throughout the Brillouin zone for La{sub 2}CuO{sub 4}. The zone-boundary magnons have an energy {h bar}{omega}{sub ZB}=0.312{plus minus}0.005 eV and are good eigenstates of the quantum Heisenberg Hamiltonian in that they possess lifetimes {gt}10/{omega}. A multiplicative renormalization of the overall frequency scale of classical spin-wave theory accounts for the quantum effects in the one-magnon spectrum.

We report neutron-scattering measurements of cooperative spin excitations in antiferromagnetically ordered ${\text{BaFe}}_{2}{\text{As}}_{2}$, the parent phase of an iron pnictide superconductor. The data extend up to $\ensuremath{\sim}100\text{ }\text{meV}$ and show that the spin excitation spectrum is sharp and highly dispersive. By fitting the spectrum to a linear spin-wave model we estimate the magnon bandwidth to be in the region of 0.17 eV. The large characteristic spin-fluctuation energy suggests that magnetism could play a role in the formation of the superconducting state.

Spin correlations in the pyrochlore antiferromagnet ${\mathrm{Y}}_{2}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ with Curie-Weiss temperature ${\ensuremath{\Theta}}_{CW}*\ensuremath{-}1100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and critical temperature ${T}_{N}=77\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ were examined through neutron scattering. For ${T}_{N}lTl{\ensuremath{\Theta}}_{CW}∕3$ the data show spin relaxation with a rate $\ensuremath{\hbar}\ensuremath{\Gamma}=1.17(9){k}_{B}T$. For $Tl{T}_{N}$ spectral weight moves to higher energies with substantial changes up to $4{k}_{B}{T}_{N}$. For $T⪡{T}_{N}$ there is a $\ensuremath{\Delta}=11(1)\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ energy gap and a pronounced spectral maximum at $19.7\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. Throughout the temperature range examined, the wave vector dependence of inelastic scattering exhibits a broad peak for $Qd\ensuremath{\approx}3.8$ ($d$ is the $\mathrm{Ru}\text{\ensuremath{-}}\mathrm{Ru}$ spacing) consistent with dipolar spin correlations.

We report a detailed inelastic neutron scattering study of the collective magnetic excitations of overdoped superconducting ${\mathrm{La}}_{1.78}{\mathrm{Sr}}_{0.22}{\mathrm{CuO}}_{4}$ for the energy range 0--160 meV. Our measurements show that overdoping suppresses the strong response present for optimally doped ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ which is peaked near 50 meV. The remaining response is peaked at incommensurate wave vectors for all energies investigated. We observe a strong high-frequency magnetic response for $E\ensuremath{\gtrsim}80\text{ }\text{ }\mathrm{meV}$ suggesting that significant antiferromagnetic exchange couplings persist well into the overdoped part of the cuprate phase diagram.

We use inelastic neutron scattering to study the temperature dependence of magnetic excitations (for energies up to 100 meV) in the cuprate La1.875Ba0.125CuO4. This compound exhibits stripe order below a temperature of �50 K; previous measurements have shown that the magnetic excitations of the stripe-ordered phase have an hour-glass-like dispersion, with a saddle point at � 50 meV. Here we compare measurements in the disordered phase taken at 65 and 300 K. At energies on the scale of kBT , there is substantial momentum-broadening of the signal, and the low-energy incommensurate features can no longer be resolved at 300 K. In contrast, there is remarkably little change in the scattered signal for energies greater than kBT . In fact, the momentum-integrated dynamic susceptibility is almost constant with temperature. We suggest that the continuity of higher-energy magnetic excitations is strong evidence for dynamic stripes in the high-temperature, disordered phase. We reconsider the nature of the magnetic dispersion, and we discuss the correspondences between the thermal evolution of magnetic stripe correlations with other electronic properties of the cuprates.

Using time-of-flight and triple-axis inelastic neutron spectroscopy, we determine spin-wave excitations throughout the Brillouin zone for ferromagnetic manganites La1-xCaxMnO3 (x=0.25,0.3) in their low-temperature metallic states. While spin-wave excitations in the long-wavelength limit (spin stiffness D) have similar values for both compounds, the excitations near the Brillouin-zone boundary of La0.7Ca0.3MnO3 are considerably softened in all symmetry directions compared to that of La0.75Ca0.25MnO3. A Heisenberg model with the nearest neighbor and the fourth neighbor exchange interactions can describe the overall dispersion curves fairly well. We compare the data with various theoretical models describing the spin excitations of ferromagnetic manganites.

The magnetic dynamic structure factor of the one-dimensional $S=1∕2$ chain system $\mathrm{Ba}{\mathrm{Cu}}_{2}{({\mathrm{Si}}_{0.5}{\mathrm{Ge}}_{0.5})}_{2}{\mathrm{O}}_{7}$ is studied in a wide range of energy transfers and temperatures. Contrary to previous erroneous reports [T. Masuda et al., Phys. Rev. Lett. 93, 077206 (2004)], the scaling properties observed in the range $0.5--25\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ are found to be fully consistent with expectations for a Luttinger spin liquid. At higher energies, a breakdown of scaling laws is observed and attributed to lattice effects. The results are complementary to those found in literature for other $S=1∕2$ chain compounds, such as $\mathrm{K}\mathrm{Cu}{\mathrm{F}}_{3}$ and Cu benzoate.

High resolution neutron scattering measurements on a single crystal of SrCu{sub 2-x}Mg{sub x}(BO{sub 3}){sub 2} with x{approx}0.05 reveal the presence of new spin excitations within the gap of this quasi-two-dimensional, singlet ground state system. The application of a magnetic field induces Zeeman-split states associated with S=1/2 unpaired spins which are antiferromagnetically correlated with the bulk singlet. Substantial broadening of both the one- and two-triplet excitations in the doped single crystal is observed, as compared with pure SrCu{sub 2}(BO{sub 3}){sub 2}. Theoretical calculations using a variational algorithm and a single quenched magnetic vacancy on an infinite lattice are shown to qualitatively account for these effects.

Using the MAPS spectrometer at the ISIS spallation source, we have measured the magnetic excitations of single-crystal samples of stripe-ordered ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Ni}{\mathrm{O}}_{4}$ with $x=0.333$ and 0.275. The full two-dimensional spin-wave dispersions were obtained using incident energies of 60 and 160 meV. To analyze the excitations, we have evaluated a spin-only Hamiltonian describing diagonal, site-centered stripes in the linear spin-wave approximation. Besides the superexchange energy $J$ within antiferromagnetic domains, we have considered effective exchange couplings ${J}_{1}$ and ${J}_{2}$ across a charge stripe coupling second-neighbor Ni sites along Ni-O bond directions and along the plaquette diagonal, respectively. From least-squares fits of the model to the measurements on the $x=1∕3$ sample at $T=10\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, we find that the dispersions are well described by a model using just $J$ and ${J}_{1}$, but not $J$ and ${J}_{2}$. Consistent with an analysis of previous measurements, we find that $J$ is about 90% of the superexchange energy of undoped ${\mathrm{La}}_{2}\mathrm{Ni}{\mathrm{O}}_{4}$ and ${J}_{1}∕J\ensuremath{\approx}0.5$. The excitations observed for $x=0.275$ are surprisingly similar to those for $x=1∕3$, despite the differing magnetic-ordering wave vectors; the main difference is a broadening of the excitations for $x=0.275$. For both samples, we find that one spin-wave branch has a gap of $\ensuremath{\sim}20\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, confirming a previous observation for $x=1∕3$. We discuss the possible origin of this gap.

We report that Bi doping drives Pr 2-x BixRu2O7 from an antiferromagnetic insulator (x = 0) to a metallic paramagnet (x approximately 1) with a broad low T maximum in C/T. Neutron scattering reveals local low energy spin excitations (variant Planck's omega approximately 1 meV) with a spectrum that is unaffected by heating to k(B)T >> variant Planck's omega. We show that a continuous distribution of splittings of the non-Kramers Pr3+ ground-state doublet such as might result from various types of lattice strain can account for all the data.

High-resolution neutron scattering experiments on optimally doped La2-xSrxCuO4 (x=0.16) reveal that the magnetic excitations are dispersive. The dispersion is the same as in YBa2Cu3O6.85, and is quantitatively related to that observed with charge sensitive probes. The associated velocity in La2-xSrxCuO4 is only weakly dependent on doping with a value close to the spin-wave velocity of the insulating (x=0) parent compound. In contrast with the insulator, the excitations broaden rapidly with increasing energy, forming a continuum at higher energy and bear a remarkable resemblance to multiparticle excitations observed in 1D S=1/2 antiferromagnets. The magnetic correlations are 2D, and so rule out the simplest scenarios where the copper oxide planes are subdivided into weakly interacting 1D magnets.

We have investigated the spin dynamics in the strongly correlated chain copper oxide ${\mathrm{S}\mathrm{r}\mathrm{C}\mathrm{u}\mathrm{O}}_{2}$ for energies up to $\ensuremath{\gtrsim}0.6\text{ }\mathrm{e}\mathrm{V}$ using inelastic neutron scattering. We observe a gapless continuum of magnetic excitations, which is well described by the ``M\"uller ansatz'' for the two-spinon continuum in the $S=1/2$ antiferromagnetic Heisenberg spin chain. The lower boundary of the continuum extends up to $\ensuremath{\approx}360\text{ }\mathrm{m}\mathrm{e}\mathrm{V}$, which corresponds to an exchange constant $J=226(12)\text{ }\mathrm{m}\mathrm{e}\mathrm{V}$.

We have measured the bulk properties as well as the elastic and inelastic neutron scattering of ${\mathrm{YMnO}}_{3}$ in order to understand the static and dynamic properties of the Mn moments. Our measurements clearly show that above ${T}_{N}$ there are short range correlations between spins at the nearest neighbor and next nearest neighbor Mn sites, which also fluctuate in time. This, together with other bulk properties, demonstrates the presence of a spin liquid phase above ${T}_{N}$ arising from the geometrically frustrated Mn moments. Below ${T}_{N},$ a well-defined spin wave develops and we can understand the experimentally measured spin wave spectrum in terms of a Heisenberg Hamiltonian with a small easy plane anisotropy. However, even in the ordered phase we have found evidence of short range spin correlations.

The magnetic excitation spectrum in the two-dimensional (2D) S = 1/2 Heisenberg antiferromagnet copper deuteroformate tetradeuterate has been measured for temperatures up to T similar to J/2, where J = 6.31 +/- 0.02 meV is the 2D exchange coupling. For T << J, a dispersion of the zone boundary energy is observed, which is attributed to a wave vector dependent quantum renormalization. At higher temperatures, spin-wavelike excitations persist, but are found to broaden and soften. By combining our data with numerical calculations, and with existing theoretical work, a consistent description of the behavior of the model system is found over the whole temperature interval investigated.