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Inelastic neutron scattering was used to study the wavevector- and frequency-dependent magnetic fluctuations in single crystals of superconducting YBa$_2$Cu$_3$O$_{6+x}$. The spectra contain several important features, including a gap in the superconducting state, a pseudogap in the normal state and the much-discussed resonance peak. The appearance of the pseudogap determined from transport and nuclear resonance coincides with formation of the resonance in the magnetic excitations. The exchange energy associated with the resonance has the temperature and doping dependences as well as the magnitude to describe approximately the electronic specific heat near the superconducting transition temperature ($T_c$)., Comment: 12 pages, 4 figures

We have used inelastic neutron scattering to study the magnetic excitations in CuO, a quasi 1-D antiferromagnet, along the chain direction, [zeta, 0, ]. The low-energy excitations can be described adequately by linear spin wave theory, but at high energies the magnetic scattering exhibits characteristics of the spinon model.

We review recent measurements of the high-frequency dynamic magnetic susceptibility in the high-$T_c$ superconducting systems La$_{2-x}$Sr$_{x}$CuO$_4$ and YBa$_2$Cu$_3$O$_{6+x}$. Experiments were performed using the chopper spectrometers HET and MARI at the ISIS spallation source. We have placed our measurements on an absolute intensity scale, this allows systematic trends to be seen and comparisons with theory to be made. We find that the insulating S=1/2 antiferromagnetic parent compounds show a dramatic renormalization of the spin wave intensity. The effect of doping on the response is to cause broadenings in wave vector and large redistributions of spectral weight in the frequency spectrum., Comment: 9 pages, 7 figures (To be published in Physica B). Typos corrected

We report neutron inelastic scattering measurements on polycrystalline LaFePO and Sr2ScO3FeP, two members of the iron phosphide families of superconductors. No evidence is found for any magnetic fluctuations in the spectrum of either material in the energy and wavevector ranges probed. Special attention is paid to the wavevector at which spin-density-wave-like fluctuations are seen in other iron-based superconductors. We estimate that the magnetic signal, if present, is at least a factor of four (Sr2ScO3FeP) or seven (LaFePO) smaller than in the related iron arsenide and chalcogenide superconductors. These results suggest that magnetic fluctuations are not as influential on the electronic properties of the iron phosphide systems as they are in other iron-based superconductors.

The spin excitation spectrum in the 2D square lattice Heisenberg antiferromagnet CFTD has been investigated using time-of-flight neutron scattering. The magnon dispersion and intensities have been determined as a function of wavevector Q in the full Brillouin zone. Both quantities are found to deviate significantly from the predictions of linear spin wave theory for wavevectors in the vicinity of Q = (pi, 0), whereas good agreement is found near Q = (pi/2, pi/2).

We review recent neutron scattering experiments to determine the magnetic fluctuations and order in the geometrically frustrated magnet SrCr 8 − x Ga 4 + x O 19 . The instruments used are the direct- and indirect-geometry instruments HET and IRIS at the ISIS pulsed spallation source. Results are compared with earlier data obtained using the triple-axis spectrometer TASI at the Riso DR3 reactor.

The authors review recent reactor- and spallation-source-based neutron scattering experiments on the magnetic fluctuations and order in a variety of doped transition metal oxides. In particular, data are shown for the NiO chain compound, Y{sub 2{minus}x}Ca{sub x}BaNiO{sub 5}, the two-dimensional cuprate superconductors La{sub 2{minus}x}Sr{sub x}CuO{sub 4} and YBa{sub 2}Cu{sub 3} O{sub 6+x}, and the classical three-dimensional ``Mott-Hubbard`` system V{sub 2{minus}y}O{sub 3}.

The correlations and fluctuations in Cu(DCOO)(2). 4D(2)O, which is a good physical realization of the 2D S = 1/2 Heisenberg antiferromagnet on a square lattice, have been measured by neutron scattering measurements. The quantum fluctuations cause a non-uniform. renormalization of the spin-wave dispersion with a zone boundary dispersion of 7%, while the spin wave amplitude is reduced to 51 +/- 4% of its classical value. The temperature dependence of the correlation length, the spin-wave damping and the spin-wave softening agrees with theoretical predictions over a large temperature range.

We report a study of the magnetic response of pure chromium in the paramagnetic state (T=334 K ) near the Neel temperature TN=311 K using inelastic neutron scattering. The response can be described over a wide energy range by a phenomenological response function. We compare our results to recent measurements on the nearly antiferromagnetic Cr0.95V0.05.

We have used inelastic neutron scattering to measure the spin wave dispersion throughout the Brillouin zone of the bilayer manganite La 1.2 Sr 1.8 Mn 2 O 7 . Due to the bilayer structure, the spin-wave spectrum consist of two modes, acoustic and optical. In the ferromagnetic phase ( T ≪ T C =121 K), the dispersion relations are well described by a two-dimensional bilayer Heisenberg Hamiltonian with nearest neighbour interactions only. The effective magnetic exchange between adjacent Mn ions in the plane, J ‖ S =10.3±0.2 meV, is approximately 4 times larger than that between the planes of the bilayer J ⊥ S =2.5±0.25 meV. This large anisotropy between ferromagnetic exchanges within and across the bilayer might be associated with a strong planar orbital polarisation of the e g band electrons which would favour a larger double-exchange mechanism within the plane.

The spin excitations in La2CuO4, a two-dimensional spin- 1 2 Heisenberg antiferromagnet and parent compound of high-Tc superconductors, have been measured throughout the Brillouin zone using high-resolution inelastic neutron scattering. The observed dispersion relations show evidence for substantial second- or third-nearest-neighbour exchange couplings in addition to the strong first nearest-neighbour antiferromagnetic exchange.

The magnetic excitations of a nearly ideal realization of a one-dimensional S=1/2 Heisenberg antiferromagnet, ${\mathrm{KCuF}}_{3}$, have been studied by inelastic neutron scattering. The experimments were performed using the pulsed neutron source ISIS and show that the magnetic scattering occurs at energies well above the top of the linearized spin-wave band. This result shows that it is essential to take account of quantum effects in discussing the excitations. The results are in excellent accord with theoretical predictions for the dynamical correlation function.

The dispersion relation of the spin-wave excitations in bcc iron has been studied by neutron inelastic scattering at the spallation neutron source ISIS. Magnetic intensity was followed up to 550 meV along the [100] direction. The general form of the dispersion curve is in qualitative agreement with that calculated from a spin-polarized band model, and in particular the prediction of propagating modes extending above 300 meV has been confirmed.

The critical scattering of the isotropic 3D Heisenberg antiferromagnet (AF) RbMnF3 has been re-measured using neutron scattering. In agreement with earlier measurements, the scattering at TN and for wave vectors away from the AF zone centre shows a quasielastic component in addition to the inelastic response predicted by renormalization group (RNG) and mode-coupling (MC) calculations. This component scales as predicted, and below TN the inelastic peaks evolve into the transverse spin-waves, while the quasielastic peak transforms into the longitudinal scattering.

The spin excitations of the s = 1 2 nearly one-dimensional Heisenberg antiferromagnet, KCuF 3 , have been determined using neutron scattering techniques. The results at high energies show that the excitations are described by pairs of unbound spinons but, at low temperatures and energies, the scattering is described by spin wave theory.

We have performed neutron inelastic scattering experiments in order to investigate the high-energy magnetic excitations in chromium. The commensurate diffuse mode has been measured up to an enerfy of 550 meV, the mode remains highly localised in Q and persists past an expected energy gap at 440 meV.

Inelastic neutron scattering has been used to study the magnetic excitations of a nearly ideal realisation of a one-dimensional S =½ Heisenberg antiferromagnet, KCuF3. The experiments were performed using the pulsed neutron source ISIS and it is seen that time of flight methods are particularly well suited to the study of low dimensional systems. The excitation spectrum consists of a quantum continuum of excited states extending up to energies double that of the usual antiferromagnetic zone boundary spin-wave mode. The scattering can be described very well by a theoretical ansatz for the dynamical correlation function.

We present new results for the spin wave dispersion relation measured along the [ξξ0] direction in bcc iron (12% silicon) by time-of-light, neutron inelastic scattering. The excitations were followed to the zone boundary, where they are spread over a range of energies around 300 meV.

We have measured the spinwave dispersion throughout the Brillouin zone of the doubleexchange ferromagnet La0.7Pb0.3MnO3 by using inelastic neutron scattering. An unexpectedly simple Heisenberg Hamiltonian with solely a nearestneighbour coupling accounts for the entire dispersion relation. The value of the Curie temperature Tc calculated from the derived coupling constant agrees with the experimental value to within 15%. The results, including damping of the zone boundary spin waves near Tc, can be fully understood in the framework of the doubleexchange Hamiltonian. In the paramagnetic phase of the twodimensional analogue La1.2Sr1.8Mn2O7, we have observed longlived antiferromagnetic clusters coexisting with ferromagnetic fluctuations. This effect must be included in theories that attempt to explain giant magnetoresistance in manganites, at least in two dimensions.