We report the results of an inelastic neutron scattering experiment on nearly percolating Heisenberg antiferromagnets (RbMncMg1− cF3), in which the Mn concentrations (c = 0.31, 0.34 and 0.39) are very close to the percolation threshold (cp = 0.312). A broad-peak superimposed on Ising-cluster excitations was observed throughout the Brillouin zone. The intensity of a broad peak increased on approaching the percolation threshold. The origin of this broad peak is attributed to the excitation of fractons in a percolating network.

The dispersion curves for magnetic excitations along the principal directions of the Brillouin zone of the singlet ground state compound PrNi2Si2 have been measured at 30 K by inelastic neutron-scattering experiments in the paramagnetic phase. From these data we have extracted information about the main exchange interaction coupling parameters J(ij) that are responsible for the appearance of an amplitude modulated magnetic order in this compound below T-N=20 K. The results are consistent with the long-range character of Ruderman-Kittel-Kasuya-Yosida interactions because in order to describe the dispersion curves observed at 30 K it is necessary to consider the influence of the interaction to the eighth nearest-neighbor Pr3+ ion along the c direction. In addition, we have studied the thermal dependence of the excitations at several representative points in the Brillouin zone that show an important softening as the temperature is lowered to T-N. [S0163-1829(97)03042-7].

We have developed a computer program to calculate the thermal diffuse scattering (TDS) intensity distribution for single-crystal specimens in a diffractometer with no energy analysis. We assumed that the phonon frequencies are approximated by those of elastic waves and that the elastic constants, density and lattice parameters of the system under study are known. The results of the calculations were compared to experimental data obtain for single crystals of Si, diamond and NiAl at the wide-angle neutron diffractometer (WAND) at the HFIR at Oak Ridge National Laboratory. Excellent agreement was found between the calculations and the experimental observations.

Results of inelastic neutron scattering in the amplitude-modulated magnetic phase of PrNi2Si2 are presented. The main, lowest longitudinal branch of the dispersion curve exhibits a shape much more dispersive than that present above TN = 20 K. Close to the incommensurate vector Q = (0, 0, 0.875), a critical behaviour occurs, characterized by the appearance of a new longitudinal magnetic excitation at low energy, showing a nearly complete softening for q = Q. Both low-energy excitations could be interpreted as phasons and amplitudons. Results are analyzed by a generalized susceptibility formalism.

The extinction-free neutron scattering for a 3.4 mu m epitaxial film of the dilute antiferromagnet Fe0.52Zn0.48F2 has been studied near The (100) antiferromagnetic Bragg point. For the H=0 Bragg scattering we observe the random-exchange Ising model behavior I similar to\t\(2 beta) with beta=0.35. For 0<H less than or equal to 4.5 T the random-field Ising peak intensity vs T has the opposite curvature from the H=0 case near T-c(H). We argue that this has to do with the formation of two weakly interacting, interpenetrating, antiferromagnetically ordered domains with interfaces primarily falling on vacancy sites.

The twisting of the spin-density wave (SDW) in Cr in the transverse SDW phase has been examined by neutron scattering experiments in a magnetic field. Twisting beyond experimental errors was observed, but the twisting angle is very small compared to the rotation angle of the SDW in the magnetic field. This result is consistent with the fact that the transverse spin fluctuation in the Cr SDW is strongly suppressed at low excitation energies. The anisotropy energy of the SDW in Cr is also discussed.

The thermal vibrations of lithium oxide were studied in the temperature range from 293 to 1120K using a high-resolution powder diffractometer. The isotropic thermal parameter of the oxygen ion has a linear relationship with the temperature. The parameter of the lithium ion increases rapidly above 450 K. To clarify the nature of the thermal vibration of lithium ion, inelastic neutron scattering measurements for a transverse optical mode were also carried out in the temperature range from 293 to 1000 K. No temperature dependence of the phonon energy was observed for the TO mode at the zone center, while for the Raman-active mode the phonon energy has a strong temperature dependence above 450K.

In order to obtain information about the origin of dips observed previously in the x-ray thermal diffuse scattering (TDS) for pyrolytic graphite, phonon frequencies and lifetimes were investigated by neutron inelastic scattering techniques. Although no anomalous dispersion was detected, significant broadening of phonon peaks was found in the region of wave-vector space corresponding to the dips in the TDS. The neutron and x-ray data are discussed on the basis of phonons coupled with an excitation having a two-dimensional dispersion relation. Surface phonons were examined as a possible candidate for such an excitation.

The magnetic excitations in a single crystal of PrNi2Si2 have been studied by inelastic neutron scattering. Dispersion curves have been followed through the centered tetragonal Brillouin zone. In the paramagnetic phase, at 30 K, the lowest energy magnetic branch exhibits strong dispersion ranging from 1.9 to 3.5 meV, the minimum energy occurring at the wave vector of the ordered phase; from the observed dispersion the inter-ionic isotropic bilinear exchange parameters are deduced. At 4.2 K, in the modulated phase, the same branch presents slightly larger, but not significantly different, dispersion.

In order to examine the influence of exchange frustration on spin-wave excitations, inelastic neutron scattering experiments have been performed on a random-exchange system FexMn1−xTiO3 with x = 0.00, 0.10, 0.25, 0.33. For all three mixed systems, the magnetic excitation spectra consist of two components: a well-defined spin-wave component and a broad damped component. The latter can well fit with the spectral-weight function for a damped harmonic oscillator.

Using neutron-scattering techniques, we have investigated the magnetic correlations in dilute antiferromagnets close to the percolation threshold, in which the magnetic connectivity takes a fractal form. Recent experimental results concerning the self-similarity of the magnetic order, and magnetic excitations in two-dimensional Ising and three-dimensional Heisenberg antiferromagnets are presented.

The extinction-free neutron scattering for a 3.4 μm epitaxial film of the dilute antiferromagnet Fe0.5Zn0.5F2 has been studied near the (100) magnetic Bragg point. For the H = 0 Bragg scattering we observe the random-exchange Ising model behavior I ≈ |t|2β with β = 0.35. For 0 < H < = 4.5 T the random-field Ising peak intensity vs. T has the opposite curvature from the H = 0 case near Tc(H).

The magnetic excitations of an incommensurate spin-density-wave system Cr98.5Fe1.5 were studied at the commensurate position (Q = (0 0 1)) using neutron inelastic scattering. Above TN, the magnetic excitation centered at (0 0 1) dominates the scattering,just as reported for pure Cr, and the line profiles of constant-E scans at several energy transfers are described well by single Gaussians centered at (0 0 1). The commensurate inelastic scattering intensity shows a maximum at TN just like critical scattering. The origin of the commensurate inelastic scattering is discussed.

Results from neutron inelastic-scattering experiments on Fe, Cr, and three bcc Fe-Cr alloys were analyzed with a Born-von Karmin model to obtain phonon density-of-states (DOS) curves. We compared the phonon DOS of the bce Fe-Cr alloys to the composite phonon DOS from appropriate fractions of the phonon DOS of the pure metals Fe and Cr. In the high-temperature limit, we obtained the vibrational entropy of mixing of Fe and Cr to be 0.141, 0.201, and 0.214 k(B)/atom for alloys of Fe70Cr30, Fe53Cr47, and Fe30Cr70, respectively, with the disordered solid solution having the larger vibrational entropy. Some expected effects of vibrational entropy on the chemical unmixing transformation in Fe-Cr are discussed.

The magnetic excitation spectrum for the longitudinally modulated incommensurate spin structure of Er metal has been studied by inelastic neutron scattering. Since the Er spins order parallel to the $c$ axis of the hcp crystal structure, it is possible to separate the longitudinal and transverse spin fluctuations. We find no sharp spin waves for the transverse fluctuations owing to the spatial variations of the exchange and anisotropy energies. The longitudinal fluctuations, however, produce neutron scattering, at small wave vectors, which possesses rather sharp peaks that have a linear dispersion relation.

Using neutron inelastic scattering techniques we investigated the magnetic excitations in a diluted Heisenberg antiferromagnet (RbMn(c)Mg1-cF3 with c = 0.39) close to the percolation threshold of c(p) = 0.312. No indication of propagating spin waves was observed throughout the Brillouin zone, but a broad peak superimposed on Ising-cluster excitations was observed. The origin of this broad peak is attributed to the excitation of fractons in a percolating network.

We have performed an inelastic neutron scattering experiment in order to investigate the pressure dependence of the magnon dispersion of terbium in its ferromagnetic phase. our measurements were performed along the crystal c axis at 90 K (well below the Curie temperature T(c) = 220 K) at ambient pressure, and at 4.3 and 15.2 kbar of applied hydrostatic pressure. The difference between the magnon dispersion curves at ambient pressure and at 4.3 kbar is small, while at 15.2 kbar the dispersion curve shifts appreciably towards higher energies. The measured magnon dispersion curves have been analyzed using a Hamiltonian that includes Heisenberg exchange as well as single-ion anisotropy terms. For each dispersion curve we have calculated five exchange constants and two anisotropy terms. The energy gap at q = 0 due to the anisotropies is enhanced with pressure and exchange interactions acting between c planes appear to decrease with the application of high pressure.