Neutron Spin Echo Technique Research Articles

Study of dynamics of microemulsion droplets by neutron spin-echo spectroscopy.

We have employed neutron spin-echo techniques to study the dynamics of the shape fluctuations of microemulsion droplets. It is found that the scattering spectrum of a droplet system with a labelled layer of surfactants exhibits a sharp peak in the relaxation frequency spectrum. We have determined that the fluctuations are driven mainly by the elastic modes of the surface, and the height of the peak allows us to estimate the surface elastic bending coefficient to be of the order of 5 kT.

Neutron spin echo and neutron spin depolarisation studies in Y9Co7

High-resolution, neutron spin echo and neutron spin depolarisation techniques have been used to study the dependence of the local fields on temperature and low applied magnetic field (up to 55 Oe) in both the 'hybrid' and superconducting states of Y9Co7. In zero field neither experiment shows neutron spin rotation effects in either state, whereas considerable effects have been observed in a small field (a few oersteds). For the hybrid state such effects can be reconciled with strong spin fluctuations of a kind that can easily be affected by even small fields. A comparison of the observed effects in the presence of small fields in the superconducting state with the theoretical expression has yielded information on the temperature dependence of the penetration depth lambda (T). In this state, the effects of spin fluctuations seem not to show up dramatically.

Dynamics of polymer molecules using neutron spin-echo

The dynamics of single polymer chains have been investigated in dilute solution and in the melt using the neutron spin—echo technique. In dilute solution the intramolecular motion is described to a first approximation by that for a Rouse chain incorporating hydrodynamic interactions (Zimm). It is characterised by an inverse correlation time which may be normalised by temperature, solvent viscosity and segment size, and which for long chains varies as Q3 at small scattering angles (Q is the change in wavevector on scattering). At very low Q vectors the predicted ‘universal’ regime is observed, but over most of the accessible range chemical structure also becomes important. For short chains, deviations from this Q3 behaviour are associated with overall molecular diffusion. In the melt, chain entanglements come into play and modify the simple Rouse-type motion. The correlation functions are described by a combination of Rouse motion over short distances and times and a long-time slow-motion predicted by the reptation model of melt dynamics.

Incommensurate phase transition in sodium nitrite: A study of the dynamics by neutron spin echo technique

NaNO2 is a model system for crystals undergoing an incommensurate order-disorder phase transition. In this case, the incommensurate phase transition is charaterized by a relaxational mode which exhibits critical slowing-down. We have performed neutron experiments to study the pretransitional fluctuations and the dynamics in the modulated phase. The order of magnitude of relaxation times being 109 s, we used a high resolution technique: neutron spin-echo. Surprisingly, it has been found that the relaxation process contains a fast and a "central peak"-type slow component. It appears that previous macroscopic measurements, interpreted in terms of a single relaxation time model, produced intermediate effective relaxation rates.

Study of magnon dynamics in Fe near Tc by modified neutron spin echo techniques

The principle of a new variant in Neutron Spin Echo (NSE) spectroscopy (called Intensity Modulation NSE) has been established, which extends the capabilities of NSE to the study of samples which fully depolarize the beam and also to the possibility of separate, independent investigation of spin-flip and non-spin-flip scattering processes. The method has been first used in the investigation of critical magnon dynamics in Fe at Tc − 3 K ⩽ T < Tc and 0.016 Å−1 ⩽ q ⩽ 0.022 Å−1. Taking advantage of the very high resolution of NSE it has been established, that there is no apparent ω = 0 peak between the magnon lines. Furthermore, in the magnon dynamics the observed substantial deviations from extrapolated previous results were found to be consistent with dipolar effects as predicted by standard magnon theory.

Incommensurate phase transition in sodium nitrite: A study of the dynamics by neutron spin echo technique

Abstract NaNO 2 is a model system for crystals undergoing an incommensurate order-disorder phase transition. In this case, the incommensurate phase transition is charaterized by a relaxational mode which exhibits critical slowing-down. We have performed neutron experiments to study the pretransitional fluctuations and the dynamics in the modulated phase. The order of magnitude of relaxation times being 10 9 s, we used a high resolution technique: neutron spin-echo. Surprisingly, it has been found that the relaxation process contains a fast and a central peak-type slow component. It appears that previous macroscopic measurements, interpreted in terms of a single relaxation time model, produced intermediate effective relaxation rates.

Dynamics of polymer molecules

The dynamics of single polymer chains have been investigated in dilute solution and in the melt using the neutron spin-echo technique. In dilute solution the intra-molecular motion is described to a first approximation by that for a Rouse chain incorporating hydrodynamic interactions (Zimm). It is characterised by an inverse correlation time which may be normalised by temperature, solvent viscosity and segment size, and which for long chains varies as Q3 at small scattering angles; (Q is the change in wavevector on scattering). At very low Q vectors the predicted 'universal' regime is observed but over most of the accessible range chemical structure also becomes important. For short chains, deviations from this Q3 behaviour are associated with overall molecular diffusion. In the melt, chain entanglements come into play and modify the simple Rouse-type motion. The correlation functions are described by a combination of Rouse motion over short distances and times and a long time slow motion predicted by the reptation model of melt dynamics.

Study of magnon dynamics in Fe near Tc by modified neutron spin echo techniques

The principle of a new variant in Neutron Spin Echo (NSE) spectroscopy (called Intensity Modulation NSE) has been established, which extends the capabilities of NSE to the study of samples which fully depolarize the beam and also to the possibility of separate, independent investigation of spin-flip and non-spin-flip scattering processes. The method has been first used in the investigation of critical magnon dynamics in Fe at Tc − 3 K ⩽ T

Effects of entanglements on the single-chain motion of polymer molecules in melt samples observed by neutron scattering

The motion of single molecules of polytetrahydrofuran in melt samples of deuterated polytetrahydrofuran has been observed using high-resolution quasi-elastic neutron scattering (the neutron spin-echo technique). Comparison of results for two systems in which the matrix of deuterated molecules surrounding the observed molecules is either well above or well below the entanglement molecular weight shows clearly the effects of molecular entanglements on the observed correlation functions. The form of these correlation functions agrees with the theoretically predicted behaviour for entangled systems and allows a value for the average distance between entanglements of 30 A to be extracted.

Comparison of spin-glass dynamics determined by the muon-spin-relaxation and neutron spin-echo techniques

Impurity-spin-correlation functions obtained from muon-spin-relaxation and neutron spin-echo experiments are found to agree quantitatively above and below the "glass" temperature ${T}_{g}$. The agreement confirms the power-law decay of correlations in time near and below ${T}_{g}$. Features of the experimental results which depend on (quasi)static impurity-spin correlations suggest that the spatial distribution of thermally averaged impurity-spin magnitudes is quite uniform below ${T}_{g}$. The stochastic behavior of dipolar fields at muon sites is essentially the same as that of the impurity spins themselves at all temperatures.

Thermal neutron polarisation

The basic principle for the production of polarised thermal neutrons is discussed and the choice of various crystal monochromators surveyed. Brief mention of broad-spectrum polarisers is made. The application of polarised neutrons to the study of magnetisation density distributions in magnetic crystals, the dynamic concept of polarisation, principle and use of polarisation analysis, the neutron spin-echo technique are discussed.

The dynamics of spin glass freezing in La 0.7Er 0.3Al 2

The spin relaxation dynamics and the evolution of ferromagnetic short range order has been investigated in a La 0.7Er 0.3Al 2 spin glass alloy by neutron spin echo and polarization analysis techniques. The relaxation behaviour found in other spin glasses such as Cu -Mn alloys characterized by a broad distribution of relaxation times is observed for this system also. It is shown that the slowing down of relaxation times above the freezing temperature can be described by the Arrhenius law with a broad distribution of activation energies. This description however, breaks down around and below the freezing temperature, and in the present system it coincides with the dramatic build-up of ferromagnetic short range correlations.

Neutron spin echo study of spin glass dynamics (invited)

The neutron spin echo technique allows us to obtain direct, model independent information on spin relaxation in the time domain of 3×10−12 to 3×10−9 sec. The experimental results on a variety of spin glass samples reveal a number of common characteristic features. Namely (a) the relaxation kinetics correspond to a broad distribution of relaxation times; (b) the slowing down of the relaxation can be described by an Arrhenius law above the freezing temperature, but not around and below; (c) the absence of momentum dependence in the relaxation dynamics indicates that nonexchange type interactions play a fundamental role in this dynamics.

Neutron spin echo, polarisation analysis and A.C. susceptibility measurements of spin glass dynamics

We have studied several Cu  Mn alloys with Mn concentrations between 1 and 10 at%, using the neutron spin echo technique combined with three dimensional polarisation analysis as well as ac suceptibility measurements to determine the time dependent spin-spin correlation function over a wide time range between 10 -12 and 10 -12 sec with, however, a time gap between 10 -8 and 10 -4 sec. The results show a progressive evolution of longer relaxation times in the spin dynamics of a metallic spin glass with increasing solute concentration.

Dynamics of Collective Fluctuations and Brownian Motion in Polymer Melts

The dynamic structure factor of entangled polymer chains in a melt is investigated by neutron spin-echo techniques and by computer simulations of related models. Over a wide range of wave vectors $q$ the structure factor decays with time $t$ exponentially with a rate $\ensuremath{\propto}{q}^{2}{t}^{\frac{1}{2}}$, in agreement with the simple Rouse model. Reptation is found to be effective only for simulations of chains in a frozen-in environment, under otherwise identical conditions.

Combined three-dimensional polarization analysis and spin echo study of spin glass dynamics

We report direct measurements of the time-dependent spin correlation function S( k, t) for a Cu -Mn spin glass alloy over nearly 3 decades of time in a single scan in the range 10 -12 < t < 10 -9 s. This was achieved by using, for the first time, a combination of neutron spin echo and polarization analysis techniques. The results show most clearly the particular dynamics of spin glasses, namely a behaviour that can be described by a spectrum of relaxation times spreading over a wide range.

Dynamical Scaling in Polymer Solutions Investigated by the Neutron Spin Echo Technique

AbstractChain dynamics in polymer solutions was investigated by means of the recently developed neutron spin echo spectroscopy. — By this technique, it was possible for the first time to verify unambiguously the scaling predictions of the Zimm model in the case of single chain behaviour and to observe the cross over to many chain behaviour. The segmental diffusion of single chains exhibits deviations from a simple exponential law, indicating the importance of memory effects.

Dynamical Scaling in Polymer Solutions Investigated by the Neutron Spin-Echo Technique

Applying the recently developed technique of neutron spin-echo spectroscopy, we found it possible to verify unambiguously the scaling predictions of the Zimm model: $\ensuremath{\Delta}\ensuremath{\omega}\ensuremath{\sim}{q}^{3}$ for the dynamics of a single chain. At higher polymer concentrations, experimental indications for the predicted crossover to many-chain behavior, which is accompanied by a change in exponent from 3 to 2, were observed. Line-shape analysis rules out a simple exponential decay, indicating the importance of memory effects.