Spin relaxation in the amorphous spin glass Al2Mn3Si3O12

Neutron scattering measurements on the amorphous spin glass Al₂Mn₃Si₃O₁₂ have been made using the time-of-flight method. The scattering lawS(Q, ) reveals a quasielastic line with temperature andQ-dependent linewidth and an elastic line with constant intensity between 15 and 294K. The linewidth of the quasielastic scattering diminishes with decreasing temperature following an Arrhenius law at least down to 15K. Deviations from this exponential form are strictly correlated with an increase of the elastic intensity below 15K. We favour the opinion that this effect is caused by the instrumental resolution rather than by the onset of spin glass freezing.     

Magnetic relaxation spectra of YbPd2Si2

We present the results of inelastic neutron scattering experiments on the intermediate-valent system YbPd₂Si₂ to investigate the magnetic relaxation behaviour. We have performed measurements on polycrystalline samples with neutrons of incident energyE ₀=3.1 meV at temperatures between 1.5 K and 250 K, and withE ₀=12.7 meV andE ₀=50.8 meV at temperatures between 5 K and 50 K using time-of-flight spectrometers. At temperaturesT>50 K we find a pure quasielastic magnetic response with a rather broad linewidth typical for intermediate-valent systems. AtT50 K an inelastic excitation line appears at about 21 meV; its intensity increases with decreasing temperature. In the same temperature range (T<50 K) the quasielastic linewidth decreases rapidly and atT=5 K the quasielastic response has been apparently transformed to a second inelastic feature at about 4.7 meV. The width of this low-energy excitation fits well to the temperature dependence of the quasielastic linewidth forT>5 K.     

Muon spin relaxation studies in frustrated and/or low-dimensional spin systems

An overview is given on muon spin relaxation (SR) measurements in frustrated and/or low dimensional spin systems. In the frustrated Kagomé lattice system SrCr₈Ga₄O₁₉, we observed dynamic spin fluctuations of 30 GHz, without any static frozen component even atT=0.1 K, much below the susceptibility-cusp temperatureT _g=3.5 K. This is in clear contrast with the case in dilute-alloy spin glassesCuMn andAuFe, where static order develops belowT _g. We also present the dimensionality dependence of the sub-lattice magnetization curves in 2-d Heisenberg systems, the remarkable suppression of the ordering temperature in a 1-d system Sr₂CuO₃, the observation of activation type spin dynamics in a 1-d Ising ferromagnet (DMeFc)(TCNE) aboveT _c, slow spin fluctuations ( 60 MHz) in Haldane-gap systems at low temperatures, and some results from organic 1-d and 2-d magnetic systems.     

Confinement of molecular liquids: Consequences on thermodynamic,static and dynamical properties of benzene and toluene

We relate the dynamical behavior of molecular liquids confined in mesoscopic cylindrical pores to the thermodynamic properties, heat capacity and density and to the static structure by combining different experimental methods (H-NMR, calorimetry, elastic and inelastic neutron scattering, numerical simulations). The crystallization process is greatly reduced or avoided by confinement under standard cooling conditions, instead a glass transition temperature T _g at the 1000s time scale can be observed. The pore averaged local structure of the confined liquid is not noticeably affected when excluded-volume corrections are carefully applied, but follows the density changes reflected by the Bragg peak intensities of the porous matrices. The pore size dependence of T _g is dominated by two factors, surface interaction and finite-size effect. For the smallest pores ( , being the van der Waals radius of a molecule), one observes an increase of T _g and a broadening of the transition region, related to the interaction with the surface that induces a slowing-down of the molecules close to the wall. This is confirmed by neutron scattering experiments and molecular-dynamics simulations at shorter time scales and higher temperatures, which indicate a remaining fraction of frozen molecules. For larger pore sizes, taking the decrease of density under confinement conditions into account, a decrease of T _g is observed. This could be related to finite-size effects onto the putative cooperativity length that is often invoked to explain glass formation. However, no quantitative determination of this length (not to mention its T-dependence) can be extracted, since the interaction with the wall itself introduces an additional length that adds to the complexity of the problem.Received: 1 January 2003, Published online: 30 October 2003PACS: 64.70.Pf Glass transitions - 65.20. + w Thermal properties of liquids: heat capacity, thermal expansion, etc. - 61.12.-q Neutron diffraction and scatteringD. Morineau: Present address: Groupe Matiére Condensée et Matériaux, CNRS-UMR 6626, Bâtiment 11A, Université de Rennes 1, F-35042 Rennes, France.V. Teboul: Present address: Laboratoire des Propriétés Optiques des Matériaux et Applications, CNRS-UMR 6136, Université dAngers, F-49045 Angers, France.Y. Xia: Present address: School of Chemistry, The University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom.     

Effect of structural relaxation on low energy excitations in amorphous Zr x Cu1−x

We report on an investigation of the liquid-quenched metallic glass Zr _x Cu_1?x (0.6≦x≦0.74) subjected to heat treatments below the glass transition temperatureT _g. Annealing temperatures up to 200°C (<0.8T _g) were chosen as to achieve topological relaxation only. The superconducting transition temperaturesT _c are lowered, as already observed for other metallic glasses. Low temperature measurements of the thermal conductivity (0.5 K≦T≦15 K) and of the specific heat (0.1 K≦T≦3 K) were carried out in order to determine the effect of structural relaxation on the low energy configurational excitations characteristic of the amorphous state. The annealed samples show no detectable (<20%) change in the specific heat forT?T _c, but an increase of the thermal conductivity by a factor of 2 forT?T _c is observed. Within the tunneling model of two level systems (TLS) for the low energy excitations, this behavior can be qualitatively understood in terms of a change of the TLS relaxation time distribution upon annealing. This distribution differs from that of the commonly used standard tunneling model. The change of the phonon scattering by TLS directly observed forT?T_c is largely responsible for the enhancement of the thermal conductivity found also aboveT _c.     

Anomalies in the scattering of gamma-rays from single crystals of HF-doped ice-I h around 100 K

Using the high energy resolution of the Mössbauer effect, the elastic and inelastic scattering of gamma-rays from single crystals of HF-doped IceI _h has been distinguished. The crystals were studied in the region of 100 K, since measurements by previous workers have suggested the possibility of an order-disorder transition at this temperature. Marked anomalies in the scattered intensity were observed between 106 and 125 K, and these were shown to depend on the thermal history of the crystals.     

Crystal-field splitting and temperature dependence of two-dimensional antiferromagnetism in the high-Tc compound DyBa2Cu4O8

By means of a quantitative analysis of extensive elastic neutron scattering experiments performed on theT _c =78 K superconductor DyBa₂Cu₄O₈ it is shown that two-dimensional (2D) antiferromagnetic Dy ordering in the (a,b)-plane remains stable in the whole temperature range fromT _N =(1.10±0.02) K down to 7 mK. The magnetic difference pattern with good angular resolution fits well to the Warren equation for scattering on 2D systems with powder averaging. Consequently, the fitted sublattice magnetisation is found to have a 2D Ising character. Moreover, the Dy³⁺ crystal-field levels were determined by inelastic neutron scattering. From the resulting crystal-field parameters we calculate the sublattice magnetisation of Dy³⁺ at saturation to be _Dy,CEF=6.0 _B in good agreement with _Dy,obs=(5.9±0.5) _B as measured by neutron diffraction.     

Hyperfine interaction in Co2SiO4 investigated by high resolution neutron spectroscopy

We have investigated the hyperfine interaction in Co₂SiO₄ by inelastic neutron scattering with a high resolution back-scattering neutron spectrometer. The energy spectrum measured from a Co₂SiO₄ powder sample revealed inelastic peaks at at T=3.5 K on both energy gain and energy loss sides. The inelastic peaks move gradually towards lower energy with increasing temperature and finally merge with the elastic peak at the electronic magnetic ordering temperature . The inelastic peaks have been interpreted to be due to the transition between hyperfine-split nuclear level of the ⁵⁹Co isotopes with spin . The temperature dependence of the energy of the inelastic peak in Co₂SiO₄ showed that this energy can be considered to be the order parameter of the antiferromagnetic phase transition. The determined hyperfine splitting in Co₂SiO₄ deviates from the linear relationship between the ordered electronic magnetic moment and the hyperfine splitting in Co, Co-P amorphous alloys and CoO presumably due to the presence of unquenched orbital moment. These results are very similar to those of CoF₂ recently reported by Chatterji and Schneider [7].     

The magnetic behavior of CeB6: Comparison between elastic and inelastic neutron scattering,initial susceptibility and high-field magnetization

We report measurements of the elastic and inelastic neutron scattering, initial susceptibility and high-field magnetization on thoroughly prepared poly- and single crystalline samples of CeB₆. Part of these experiments have been performed at temperatures down to 60 mK and magnetic fields up to 70 kØe. Our neutron-diffraction data provide the first proof that CeB₆ is an antiferromagnet belowT _N2K as has been suggested by previous bulk experiments. The reduced value of the low-temperature magnetic moment both below and aboveT _N points to the existence of a Kondo effect of the ₇ crystal-field (CF) ground state of Ce³⁺. From the low-temperature width of the quasielastic neutron line, the Kondo temperature is inferred to beT _K3 K. The thermal variation of the initial susceptibility (forT>20K) is semiquantitatively explained invoking, besides the Kondo effect, a ₇- ₈ CF splitting of 70 K and magnetic interactions, which are about 10 times stronger between ₈ states than those between ₇ states. This large ₈- ₈ exchange interaction is also assumed to account for the most striking result of this work, i.e. the lack of any CF-transition peak up to 44 meV in our inelastic neutron-scattering spectra.     

Determination of inelastic critical scattering at a first order phase transition in the CDW structure of 1TTaS2 using Mössbauer diffraction

We report Mössbauer diffraction measurements of the temperature dependence of the elastic and inelastic intensities at the (100) Bragg reflection in 1TTaS₂. These measurements use a newly developed microfoil conversion electron (MICE) spectrometry. They cover the temperature range from 19°C to 100°C, bracketing the first order 1T₁ to 1T₂ phase transition in the charge density wave structure at 79°C. The elastic Bragg peak shows a discontinuity at the phase transition as reported by Moret and Colella. The inelastic scattering shows a significant peak near the phase transition. This peak is interpreted as inelastic critical scattering at this first order phase transition.     

Calorimetric fragility of maltitol

The heat capacity of maltitol has been measured with an adiabatic calorimeter for the crystal from 100 K to 425 K (T _m = 420 K), for the glass from 249 K to T _g (around 311 K) and for the liquid from T _g to 400 K. The heat of melting is 55.068 kJ/mol. The calorimetric glass transition occurs at about T _g = 311 K with a sudden jump of the heat capacity ΔC _p (T _g ) of about 243.6 J/(K mol). The excess entropy between the undercooled liquid and the crystal was calculated from the heat capacity data and was used to estimate the Kauzmann temperature T _K which was found 50 K below T _g . ΔC _p (T _g ) and T _K for maltitol were compared to other compounds like sugars, polyol and hydrogen bonded liquids. It has been found that the glass former maltitol is a "fragile" liquid on the thermodynamic point of view.     

Mössbauer studies of the layered compound 1T-TAS2

We report the temperature dependence in Tantalum Disulfide 1T-TaS₂ of the elastic and inelastic intensities of the first order satellite (1.285, 0, 0.333) and (3, 0, 0.21) near the Bragg peaks (1, 0, 0) and (3, 0, 0), respectively. The phason temperature factor has been measured as a function of temperature from 70 to 295 K using Mössbauer gamma-ray scattering. The high-energy resolution provided by this technique allowed experimental separation of the elastic scattering from the inelastic thermal diffuse scattering. The first order satellite is found to be 15% inelastic. The results were compared with those found by Chapman and Colella obtained by X-ray method (Moret and Colella, Phys. Rev. Lett. 52:652, 1984).     

Molecular dynamics in thin films of isotactic poly(methyl methacrylate)

The molecular dynamics in thin films (18 nm-137 nm) of isotactic poly(methyl methacrylate) (i-PMMA) of two molecular weights embedded between aluminium electrodes are measured by means of dielectric spectroscopy in the frequency range from 50 mHz to 10 MHz at temperatures between 273 K and 392 K. The observed dynamics is characterized by two relaxation processes: the dynamic glass transition (α-relaxation) and a (local) secondary β-relaxation. While the latter does not depend on the dimensions of the sample, the dynamic glass transition becomes faster (≤2 decades) with decreasing film thickness. This results in a shift of the glass transition temperature T _g to lower values compared to the bulk. With decreasing film thickness a broadening of the relaxation time distribution and a decrease of the dielectric strength is observed for the α-relaxation. This enables to deduce a model based on immobilized boundary layers and on a region displaying a dynamics faster than in the bulk. Additionally, T _g was determined by temperature-dependent ellipsometric measurements of the thickness of films prepared on silica. These measurements yield a gradual increase of T _g with decreasing film thickness. The findings concerning the different thickness dependences of T _g are explained by changes of the interaction between the polymer and the substrates. A quantitative analysis of the T _g shifts incorporates recently developed models to describe the glass transition in thin polymer films. Received 12 August 2001 and Received in final form 16 November 2001     

Oxygen diffusion in C60 films

We have performed detailed resistivity measurements as a function of temperature in the range from 12 to 300 K on oxygen loaded C₆₀ films. We observe that two ordering phase transitions (i.e.,T ₀=260 K andT _g =90 K) are present in (T), which, in addition, strongly depends on the oxygen content. We find a decrease of both ordering temperatures with increasing oxygen concentrations. The mechanisms of oxygen diffusion are greatly enhanced in the ordered phase on heating. Finally, the transition to a glassy state atT _g is detected as a point of reversibility of the resistivity curve as a function of temperature.     

The thermal conductivity of disordered and amorphous metal films

The thermal conductivity of quenched condensed polycrystalline and amorphous Pb and Pb_0.9Cu_0.1 films has been measured between 0.5 and 11 K, i.e. in the superconducting (T7 K) and in the normal state (T7 K). Whereas, in agreement with previous results, phonon heat transport is very small for crystalline films, a considerable portion of heat is carried by phonons in amorphous films, owing to the absence of extended lattice defects. Phonon scattering in these latter films is analyzed in terms of scattering from conduction electrons aboveT _c, whereas well belowT _c it is very likely due to low energy excitations inherent in the amorphous structure.Work performed within the research program of the Sonderforschungsbereich 125 — Aachen/Jülich/Köln     

Muon spin relaxation measurements in kagomé lattice system SrCr8Ga4O19

Dynamical spin fluctuations in SrCr_8–xGa_4+xO₁₉ a frustrated spin system on a kagomé lattice, is examined by the longitudinal field muon spin relaxation technique. This system shows a spin-glass (SG)-like cusp in the susceptibility atT _g=3.5(2) K. The slowing down of Cr spin fluctuations is found to occur over a very wide temperature rangeT _g<T<30T _g. AsT/T _g 0 these fluctuations remain without static polarization (order parameter). Such strong fluctuations belowT _g have not been observed before in a conventional SG system.     

Metal-induced effects on the glass transition kinetics of glassy Se70Te30 alloy

The calorimetric measurements have been made in glassy Se₇₀Te₃₀ and Se₇₀Te₂₈M₂ (M?=?Ag, Cd, and Zn) alloys using non-isothermal differential scanning calorimetry technique to see the effects of Ag, Cd, and Zn additives on the glass transition kinetics of binary Se₇₀Te₃₀. From the heating rate dependence of glass transition temperature, T _g, different kinetic parameters of glass transition have been evaluated. The composition dependence of glass transition temperature T _g and the related activation energy (E_t ) is also discussed.     

Temperature range of the liquid–glass transition

It has been shown that the currently used method for calculating the temperature range of δT_g in the glass transition equation qτ_g = δT_g as the difference δT_g = (T₁₂–T₁₃) results in overestimated values, which is explained by the assumption of a constant activation energy of glass transition in deriving the calculation equation (T₁₂ and T₁₃ are the temperatures corresponding to the logarithmic viscosity values of logη = 12 and logη = 13). The methods for the evaluation of δT_g using the Williams–Landel–Ferry equation and the model of delocalized atoms are considered, the results of which are in satisfactory agreement with the product qτ_g (q is the cooling rate of the melt and τ_g is the structural relaxation time at the glass transition temperature). The calculation of τ_g for inorganic glasses and amorphous organic polymers is proposed.     

Methyl rotation in acetamide: the transition from quantum mechanical tunneling to classical reorientation studied by inelastic neutron scattering

Quasielastic and inelastic incoherent neutron scattering has been used to study in detail the transition from quantum mechanical tunneling motion to classical reorientation of the methyl groups in rhombohedral acetamide CH₃CONH₂. The temperature dependence of the low temperature quasielastic and inelastic scattering due toE ^a E ^b and AE transitions of the tunneling methyl groups has been investigated with eV resolution and — together with the higher temperature quasielastic scattering-compared with theoretical predictions. Microscopic theories are capable to describe most of the experimental observations at low temperatures. A heuristic theoretical approach accounts well for the high temperature results.     

Ferroelastic phase transition in CaCl2 studied by Raman spectroscopy

CaCl₂ undergoes a second-order proper ferroelastic phase transition from the tetragonal rutile type to the orthorhombic calcium-chloride type structure atT _c 490 K. The transition is of the optical type and induced by an order parameter ofB _1g symmetry. An underdamped soft mode exists above and belowT _c , the frequency and intensity of which have been measured by Raman spectroscopy. Due to a strong coupling to strain the softening is incomplete and a transverse acoustical mode is predicted to become soft. The frequency of the optical soft mode, _s, exhibits a classical Landau behavior in a remarkably large temperature range. The unusual value of the ratio of _s ² / below and aboveT _c of about –6.5 can be accounted for by appropriate terms of the thermodynamical potential. The Raman active hard modes show no significant anomaly atT _c .