Two magnon light scattering in K2MnF4

The light scattering spectra of the two dimensional Heisenberg antiferromagnet K₂MnF₄ are reported. At low temperatures a two magnon band is observed and four bands due to phonons as well.     

Resonant two magnon Raman scattering in α-Fe2O3

Strong resonant enhancement of two-magnon Raman scattering was observed in antiferromagnetic α-Fe₂O₃ for laser frequencies near the intrinsic absorption edge. One magnon scattering, although allowed by symmetry, was not observed. The laser frequency dependence of the resonance enhancement could be well presented by the square of the magnitude of the first derivative of the electric susceptibility. Based on these facts, we conclude that the scattering arises from an exchange mechanism in the excited state.     

First order phonon and magnon Raman scattering in KCoF3

At T=2°K, a magnon and a phonon Raman line is observed respectively at 38.5 and 56.7 cm^?1. The last one arises from an acoustic phonon at R point in the paramagnetic Brillouin zone, coupled to the magnon in the magnetically ordered phase.     

Determination of zone-boundary magnon energy and damping in RbMnF3 by means of light scattering experiments

New experimental data of two-magnon Raman scattering on the cubic antiferromagnet RbMnF₃ are used to obtain information on the single spin dynamics in all the ordered phase. The comparisons with the existing second order theory and with the neutron scattering data shows that the light scattering gives correct values for energy and lifetime of the zone boundary magnetic excitations.     

Inelastic neutron scattering from transverse acoustic modes in K2SnCl6

In cubic K₂SnCl₆ the dispersion curve für q q; [110] of [1$\text{1}$0] polarized acoustic phonons has been measured at three temperatures near the phase transition temperature T_c1. The acoustic branch shows no temperature dependence in the low wave vector region and stiffens slightly near the X-point of the fcc-Brillouin zone as the temperature approaches the phase transition T_c1= 262 K from above. The results support a previously developed model on acoustic anomalies in this compound.     

Inelastic neutron scattering study of acoustical magnons in MnFe2O4

The room-temperature spin wave spectrum in MnFe₂O₄ has been investigated by means of inelastic neutron scattering using the time-of-flight technique. The acoustical branch could be obtained in the [100]- and the [110]-directions up to nearly the Brillouin zone boundary. Values for the effective exchange parameters J_AB and J_BB have been derived. In both symmetry directions some low lying phonon branches could also be observed.     

Brillouin light scattering in likso4

Brillouin shift for LIkSO₄ was measured as a function of temperature inthe range 300–363°K. Elastic stiffness coefficients C₁₁, C₃₃ and C₆₆ were also evaluated in this range. A discontinuity of their values was observed at 333°K, suggesting thereby a structural phase transition.     

Inelastic neutron scattering study in cubic NaNbO3

Inelastic neutron scattering measurements performed on NaNbO₃ close to the cubic to tetragonal phase transition show that the 2D fluctuations earlier revealed by X-ray diffuse scattering correspond to an overdamped type dynamics. The sharp decrease in the spectral width observed at T-T_c =10° is interpreted in terms of the increase in correlation lengths and the cross-over between 2D and 3D behaviour of the fluctuations.     

Localized magnon gap modes in the 2-d Ising antiferromagnets K2CoF4 and Rb2CoF4

The defect (Mn²⁺,Ni²⁺,Fe²⁺) induced magnon gap modes in the layered antiferromagnets K₂CoF₄ and Rb₂CoF₄ were investigated with the methods of FIR absorption-and IR emission spectroscopy. The anisotropic exchange-parameters describing the strongly localized Mn²⁺ spin excitations far below the host lattice magnon band and the Ni²⁺ excitations in the vacinity of this band are presented. In the diluted system K₂Co_1-cMn_cF₄ localized Mn²⁺ cluster modes up to about C≈0.1 were observed. The excitation energy of these modes can only be explained by assuming an anisotropic Mn²⁺-Mn²⁺ exchange which is in contrast to the pure isomorphous system K₂MnF₄. In the spin mismatch system K₂CoF₄: Fe the magnetic moments of the isolated Fe²⁺ impurities are pulled from the plane perpendicular to the c-axis and aligned parallel to the easy axis of the magnetic crystal.     

Field dependence of uniform magnon energies in lamellar CoCl2 and CoBr2 by AFMR experiments

AFMR experiments in the frequency range 77<v<600 GHz and in the magnetic field range 0<H<75 kG on antiferromagnetic CoCl₂ and CoBr₂ single crystals at 4.2 K are reported. The waves propagate along the c axis of the crystals and the steady magnetic field is in the plane of the layers, which is also the easy plane of the spins.The influence of a rotation of the field in the easy magnetization plane is investigated, showing the existence of antiferromagnetic domains.The results of the field dependence of uniform magnon energies are reported in detail for the two compounds CoCl₂ and CoBr₂. In CoBr₂, the two magnon modes at zero field are non degenerate, their frequencies are v₁(0) = 60 GHzandv₂(0) = 675 GHz, and the field required for saturation is H_s⊥ = 74.2 kG.     

Temperature renormalization of the magnon energies of FeBr2

The temperature renormalization of the magnon energies is investigated in FeBr₂, a S = 1 Heisenberg ferromagnet with rather large single-ion and exchange anisotropies. Two theories are applied: a self-consistently-renormalized (SCR) spin-wave theory (which is not affected by “kinematic” inconsistencies) and a calculation of the self-energy in the T-matrix formalism. At low temperature both theories are found to give a small renormalization of the energy gap in agreement with neutron scattering experiments. On the other hand the two approximations give significantly different renormalization at higher temperature. Measurements in this temperature range should be able to put in evidence non linear effects in the anisotropy.     

Brillouin scattering in Hg2Cl2

Hg₂Cl₂ undergoes a second order phase transition around 185 K. All the elastic constants have been measured above (tetragonal phase) and below (orthorhombic phase) the transition point. The results support the hypothesis that the transition is induced by the condensation of a transversal acoustical mode at an X point of the first Brillouin zone in the tetragonal phase. The coefficients of the coupling terms in the free energy are evaluated and the relative values of the spontaneous deformations are evaluated.     

Raman scattering from magnons in CoCl2 and FeCl2

One-magnon Raman scattering has been observed in the metamagnets CoCl₂ and FeCl₂. The k = 0 magnon energies are 16 ± 1 cm^-1 at 21 K and 16.4 ± 0.4 cm^-1 at 12 K, respectively and these values are in good agreement with previous AFMR and neutron scattering results. A search for two-magnon scattering in both compounds was unsuccessful, largely because of masking from nearby first-order phonons and a weak temperature dependent broad band at 140 cm^-1 in CoCl₂, which is assigned to two-phonon scattering from acoustic phonons.     

Estimation of interlayer interactions in CdI2 by light scattering techniques

The velocity of transverse acoustic wave propagating along the direction perpendicular to the layer and the frequency of the rigid layer mode in CdI₂ have been measured by Brillouin and Raman scattering, respectively, at several temperatures. The interlayer forces have been deduced independently from these two different measurements. A comparison between them has led to the conclusion that the nearest neighbour interlayer interaction dominates the interlayer interaction.     

Direct evidence for the magnetic ordering of Nd ions in NdMn2Si2 and NdMn2Ge2 by high resolution inelastic neutron scattering

We have investigated the low energy nuclear spin excitations in NdMn₂Si₂ and NdMn₂Ge₂ by high resolution inelastic neutron scattering. Previous neutron diffraction investigations gave ambiguous results about Nd magnetic ordering at low temperatures. The present element-specific technique gave direct evidence for the magnetic ordering of Nd ions. We found considerable difference in the process of the Nd magnetic ordering at low temperature in NdMn₂Si₂ and NdMn₂Ge₂. Our results are consistent with those of magnetization and recent neutron diffraction measurements.     

Collision induced light scattering in gaseous CH4

Collision induced light scattering in gaseous CH₄ has been investigated up to 110 Amagats. The long-range induced polarizability appears to give the dominant contribution to the spectrum which extends up to 360 cm^-1.     

Diffusional and vibrational dynamics of ZnCl2 aqueous solutions by inelastic neutron scattering

We have studied saturated solutions of ZnCl₂ (and NiCl₂) in D₂O by time-of-flight quasi-elastic and inelastic neutron scattering spectroscopy. Spectra were taken at room temperature as a function of transferred momentum. We find that ionic diffusion in these solutions is best described by a crystal vacancy diffusion model. We also find evidence of collective excitations in the region of ～ 20 meV.     

Magnetic field dependence of spin-flip cross section for polarized neutron scattering of CeCu2Si2

The magnetic field dependence of the spin-flip cross section for quasielastic polarized neutron scattering is investigated for the Ce Kondo system at zero temperature. An exact calculation is done for the differential cross section with a small energy transfer and also for the total cross section. In particular the latter case is discussed in detail in order to investigate the recent experiment done for the polycrystalline CeCu₂Si₂. With the use of the obtained results, two possibilities are discussed about the wavefunction for the crystal-field ground doublet of the f-electron in CeCu₂Si₂.     

An inelastic neutron scattering study of C2H2 adsorbed on type 13X zeolites

The inelastic neutron scattering spectra of C₂H₂ and C₂D₂ adsorbed on a Ag⁺ exchanged 13X zeolite (0–800 cm^?1) and of C₂H₂ on the Na⁺ form (0–300 cm^?1) have been obtained. For the Na-13X system no distinct vibrational modes were observed, however for the Ag-13X systems the low frequency intramolecular modes of the adsorbed gas and some of the vibrations of the adsorbed gas relative to the surface have been assigned. From the deuteration shifts it appears that C₂H₂ and C₂D₂, adsorbed on Ag-13X, are non-linear.     

Quasi-elastic neutron scattering study of AlCl3 aqueous solution

Quasi-elastic neutron scattering from protons in AlCl₃ aqueous solution has been measured by the use of a triple-axis spectrometer. Assuming the existence of two different kinds of water molecules, either in free state or in bound state, the whole shape of energy spectra of scattered neutrons was analyzed. About eight water molecules were found to be bound to an Al ion, being almost independent of the solute concentration. The diffusion constant, the rest time and the jump length of the remaining free water molecules were estimated based on a jump diffusion model.