Magnetic Relaxation Experiments Research Articles

Flux pinning by columnar defects in high-temperature superconducting crystals

Y-Ba-Cu-O:123 and Bi-Sr-Ca-Cu-O:2212 crystals were irradiated with 5.8 GeV Pb ions. The columnar defects, produced by this irradiation are expected to yield the strongest possible pinning energy. Indeed, there is an apparent increase in the width of the magnetization curves. However, the results of magnetic relaxation experiments indicate the energy barrier for flux creep from the columnar defects much smaller than expected pinning energy. This effect is explained in terms of nucleation of vortex loop mechanism of flux creep. The estimate of pinning energy of columnar defect can be obtained from measurement of the angular dependence of the magnetization curves.

Extension of the time window for investigation of relaxation effects in high- Tc superconductors

Measurements of the dependence of the magnetic hysteresis loop amplitude in a superconductor on the magnetic field sweep rate d H e/d t are shown to provide essentially the same information as magnetic relaxation experiments (flux-creep) at fixed external field H e. For practical reasons, however, magnetic hysteresis loop measurements are better suited to investigate fast flux-line movements than conventional flux-creep measurements. From the general flux movement differential equation it is shown that each magnetic hysteresis loop recorded at a given d H e/d t can be labelled by an effective time t M 0 so that this experimental information can be used to extend the time window of flux-creep measurements to short times. This method is applied to the experimental data obtained on a YBa 2Cu 3O 7 single crystal. In this way the time window is extended to less than 10 -2 s which is valuable for a more precise check of existing theoretical models for the activation energy of flux motion. It is found that the experimental data are well described by using an activation energy U( j)=( U c / μ)[( j c / j) μ -1] with μ=0.74 at 16 K and 1.8 T.

Critical current relaxation in epitaxial thin YBaCuO films in changing and constant field

Abstract The magnetic hysteresis loop (MHL) amplitude increases with the sweep rate of the external magnetic field Be. Each MHL recorded with a sweep rate dB e dt can be labeled with an “effective” time tmo which represents the time needed for the relaxation at constant Be of magnetic moment M from the critical state Me to the value Mo of a given MHL. The dependence of M on dB e dt provides essentially the same information as magnetic relaxation experiments at constant Be. The determination of tMo from a general dependence of M on sweep rate dB e dt or on rate of relaxation dM dt is described and applied to experimental data obtained for an epitaxial YBa2Cu3O7 thin film. In comparison with conventional relaxation at constant magnetic field MHL experiments make it possible to drastically extend the time window of relaxation processes towards shorter times (down to 10−2s or less). Combination of both methods enables more precise tests of existing models on vortex motion in layered superconductors.

U-j relationship in type-II superconductors.

The Anderson-Kim model has been modified in terms of the [ital U]-[ital j] relationship for high-[ital T][sub [ital c]] superconductors. We have obtained [ital U]-[ital j] curves by an approach based on the temperature dependence of [ital j][sub [ital c]]. Our results are consistent with the physical picture of flux creep. Our data is obtained from magnetic-relaxation experiments on YBa[sub 2]Cu[sub 3]O[sub [ital x]] single crystals with the direction of the field parallel to the [ital c] axis. The decay of magnetization was studied for both increasing and decreasing fields, used to obtain the current density.

Study on the microphase structure of sodium sulfonated polystyrene lonomer by nuclear magnetic relaxation measurements

AbstractNuclear magnetic relaxation techniques have been used to study the microphase structure of sodium sulfonated polystyrene ionomers. Microphase separation in the ionomers could be evidenced by the magnetization decays, which were resolved into two components. At lower temperature, 300K, the decay function for the ionomer was found to be a combination of two Gaussian functions that are indicative of more rapid dephasing of individual spins, while the magnetization decay at higher temperature, 413K, was composed of both a Gaussian function and a Lorentzian function. The relative portion of the ionic phase in the ionomer was seen to be significantly affected by the ion concentration or the temperature. From the comparison of the relative amounts of the ionic phases obtained from both the material's composition and the nuclear magnetic relaxation experiment, it is anticipated that the ionic domains contain a substantial amount of hydrocarbon segments.

Magnetic relaxation and intrinsic pinning in a single crystal of Bi2Sr2CaCu2Ox.

Magnetic-relaxation experiments were performed on Bi[sub 2]Sr[sub 2]CaCu[sub 2]O[sub [ital x]] single crystals with the direction of the field parallel to the [ital ab] plane. Based on the relaxation data, we have obtained relationships between the activation energy [ital U] and the current density [ital j] by an approach we developed previously. We found that the activation energy has a logarithmic dependence on [ital j] in a wide regime of driving force. It has been reported that CuO[sub 2] planes in high-[ital T][sub [ital c]] superconductors can act as strong intrinsic pinning centers and that the relation [ital U][similar to][ital U][sub 0]ln([ital j][sub [ital c]]/[ital j]) may describe such a pinning mechanism. Our experimental results have shown good agreement with such a physical model of intrinsic flux pinning.

Magnetic behavior of a reentrant Ising spin glass.

The ac susceptibility of the reentrant Ising spin glass ${\mathrm{Fe}}_{0.62}$${\mathrm{Mn}}_{0.38}$${\mathrm{TiO}}_{3}$ has been investigated in a superconducting-quantum-interference-device magnetometer. The data show a transition from a paramagnetic to an antiferromagnetic state at ${\mathit{T}}_{\mathit{N}}$=32.8 K. At lower temperatures, the in-phase component of the susceptibility, \ensuremath{\chi}'(\ensuremath{\omega}), becomes frequency dependent and, simultaneously, a finite out-of-phase component of the susceptibility, \ensuremath{\chi}''(\ensuremath{\omega}), appears. This behavior is interpreted to arise from a transition to a mixed spin-glass and antiferromagnetic state at ${\mathit{T}}_{\mathit{g}}$=25.8 K. A dynamic scaling analysis of \ensuremath{\chi}''(\ensuremath{\omega}) yields critical exponents quite different from those found for the pure Ising spin glass ${\mathrm{Fe}}_{0.5}$${\mathrm{Mn}}_{0.5}$${\mathrm{TiO}}_{3}$. However, some apparent deviations from optimal scaling behavior are observed. Magnetic relaxation experiments on this reentrant spin glass reveal the existence of an aging phenomenon similar to that observed in ordinary spin glasses. It is also found that the behavior of the magnetic relaxation can be interpreted within the context of spin-glass domain theories.

Current density dependence of the activation energy in type II superconductors

We have conducted magnetic relaxation experiments to calculate the effective activation energy, U, as a function of critical current density. We have shown that, from the nonlinearity of U, one can explain the nonlogarithmic decay of magnetization in type II superconductors. The nonlinearity of U was considered by expanding U about a current density, J0. The coefficients of expansion were determined experimentally and were used to develop the relationship between U and J for both conventional type II superconductors (Nb3Sn filamentary) and high-Tc superconductors (YBa2Cu3O7−δ and Bi2Sr2CaCu2Oy single crystals). Both types of superconductor were observed to behave in similar fashion. We have also calculated U0 using the standard formulas and the intercepts of the tangent of the U vs M curve and have found the results to be comparable.

A 500 ps molecular dynamics simulation study of interleukin-1β in water: Correlation with nuclear magnetic resonance spectroscopy and crystallography

We report the results of a 500 ps molecular dynamics simulation of the cytokine interleukin-1β, a protein of 153 amino acids, immersed in a sphere of 3783 bulk water molecules with a radius of 33 Å. The simulation reproduces the amplitudes of the fast librational motions of the backbone NH bonds determined from 15N nuclear magnetic relaxation data, as well as the crystallographic B-factors. Moreover, this study suggests a molecular picture of the nature of the slow internal motions that have been inferred from nuclear magnetic resonance relaxation experiments. These experiments indicated that, in addition to fast motions common to all residues, 32 surface residues exhibit slow motions on the 400 ps to 5 ns time-scale. While the present simulation is not sufficiently long to provide a quantitative description of events on this time-scale, it is long enough to observe several large amplitude transitions that are likely candidates for these slow motions. Specifically, in many of these 32 residues, the NH groups are hydrogen bonded and infrequent dihedral transitions cause the NH vectors to jump between states with well-defined orientations. It is shown that the time course of the angular reorientational correlation functions of these residues calculated from the trajectory is a reflection of the random times at which these infrequent jumps happen to have occurred. Thus, while the rate of these transitions cannot be quantified, the simulated decay of these correlation functions is completely consistent with the physical picture in which the NH vectors, in addition to fast librational motion, undergo large amplitude jumps between conformations stabilized by hydrogen bonds.

The temperature- and field-dependent activation energy of a proton irradiated YBCO single crystal

We have investigated the separate temperature and field dependences of the activation energy U ( T, H, J ) for pinning of magnetic flux in a high- J c , proton irradiated Y 1 Ba 2 Cu 3 O 7−δ single crystal. The work achieves two objectives: (1) to clarify arguments concerning the “anomalous” rise of U with increasing temperature and (2) to resolve differences in magnitude of U found when comparing magnetic relaxation and resistivity results. The temperature and field dependences of U for fixed J were first obtained by extrapolating U ( J ) as observed in magnetic relaxation (flux creep) experiments to lower J . For fixed current density J , peaks in U were observed with respect to both temperature and magnetic field. For H > 2T, U ( H ) varied as ∼ H −α , with α depending on temperature and current. The attempt time, treated as a fitting parameter, lies near 10 −3 ∼10 −2 s in most cases. The magnitude of U agrees well with values reported from resistivity studies on high- J c epitaxial films.

Equilibrium and nonequilibrium magnetic behaviour of a reentrant Ising spin glass

The reentrant Ising spin glass Fe 0.62Mn 0.38TiO 3 has been investigated using SQUID magnetometry. The system is antiferromagnetic below T N( H = 0) = 33 K, and enters a mixed antiferromagnetic/spin glass phase below T g( H = 0) = 25.8 K. Within the concept of a recent mean field model, a ratio T N(0)/ T g(0) close to unity indicates both intra- and interlayer frustration. The existence of an overlap length, l ΔT , below which equilibrium spin correlations at temperatures T and T + ΔT are indistinguishable has been verified by magnetic relaxation experiments on this spin glass system.

Non-linear flux-pinning energies versus current densities in high-T c superconductors: Implications for U o's derived from magnetic relaxation experiments

Magnetic relaxation rate measurements of high-T c superconductors in the critical state are often used to determine the characteristic flux-pinning energies of these materials. The relaxation-rate equations used to obtain flux-pinning energies usually assume a linear U vs. J dependence. Recent studies of the flux pinning properties of high-T c materials, however, show that the U vs. J relation is highly non-linear, the relation being more accurately described by a power-law or logarithmic function than a linear one. It is shown here that for a power-law U vs. J dependence, the U o derived from a magnetic relaxation experiment is a closer measure of U( J= J c ) than of U(J≈0). As a result, for a power-law U vs. J dependence U o is not an accurate measure of the absolute depth of the flux-pinning potential, as is commonly assumed.

Mesomorphism, molecular structure and dynamics of polydiethylsiloxane

The correlation between the phase behaviour of polydiethylsiloxane (PDES) and conformational and motional changes at the various disordering transitions has been investigated by nuclear magnetic resonance (n.m.r.), dielectric relaxation and shear experiments. Diffusive rotation of the chain segments around the long axis of the molecules is indicated by 29Si n.m.r. below the isotropization transition. The remarkably fluid character is explained in part by the coexistence of anisotropic and isotropic motional states of the -OSiEt 2- segments, which indicate microscopic domain formation. The molecular motion in the α 2- and β 2-phases is restricted but still fast with respect to the 29Si n.m.r. timescale. Transmission electron micrographs show, besides chain-folded lamellae, also extended-chain lamellae. These differences in the morphologies can explain why the interconversion of α 2-PDES into the thermodynamically more stable β 2-polymorph is slow, in spite of the pronounced mobility of the polymer segments. Long-range reorganization processes have to be considered to allow the morphological changes observed by electron microscopy.

Measuring relative acetylcholine receptor agonist binding by selective proton nuclear magnetic resonance relaxation experiments

A method is presented that uses selective proton Nuclear Magnetic Resonance (NMR) relaxation measurements of nicotine in the presence of the acetylcholine receptor to obtain relative binding constants for acetylcholine, carbamylcholine, and muscarine. For receptors from Torpedo californica the results show that (a) the binding constants are in the order acetylcholine greater than nicotine greater than carbamylcholine greater than muscarine; (b) selective NMR measurements provide a rapid and direct method for monitoring both the specific and nonspecific binding of agonists to these receptors and to the lipid; (c) alpha-bungarotoxin can be used to distinguish between specific and nonspecific binding to the receptor; (d) the receptor--substrate interaction causes a large change in the selective relaxation time of the agonists even at concentrations 100x greater than that of the receptor. This last observation means that these measurements provide a rapid method to monitor drug binding when only small amounts of receptor are available. Furthermore, the binding strategies presented here may be useful for the NMR determination of the conformation of the ligand in its bound state.

An investigation of micro-brownian motions in polydimethylsiloxane by complementary incoherent-neutron-scattering and nuclear-magnetic-resonance experiments below room temperature

Two different experimental methods, neutron scattering and nuclear magnetic resonance are used to investigate the random methyl group and segmental rotational jump motions in polydimethylsiloxane. It is shown that nuclear magnetic relaxation and line width experiments are complementary to incoherent neutron scattering “fixed-window” experiments, the principle of the “fixed-window” experiments being discussed in somewhat detail. Satisfactory agreement of these experimental techniques is achieved as to the determination of the two model parameters of the motional processes in question, i. e. the activation energy and the preexponential factor of the Arrhenius ansatz made for the correlation time or jump time, respectively.

Nuclear magnetic resonance and the coherent state of unstable-moment rare-earth compounds at low temperatures

This paper discusses nuclear magnetic resonance (NMR) and relaxation experiments in rare-earth compounds which exhibit unstable 4f magnetism and related effects. In several systems paramagnetic NMR shifts indicate modification of electronic band structure below characteristic temperatures found from “classical” experiments, e.g. susceptibility and specific heat. Shift anomalies are small or absent in the nearly integral-valent compounds CeCu 2 Si 2 and CeRu 2 Si 2 . Nuclear spin-lattice relaxation rates yield estimates of spin fluctuation rates, and are sensitive to the onset of correlated near-neighbor fluctuations. Recent NMR relaxation data for the heavy-fermion superconductors CeCu 2 Si 2 and UBe 13 will be presented.

Binding energy of hydrogen-impurity complexes in nickel

Abstract Trapping of hydrogen by substitutional impurity metal atoms (Me) has been studied by means of magnetic relaxation experiments in alloys of Ni. The measured binding energies of the diatomic MeH complexes are interpreted in terms of electronic and elastic interactions between the impurity and the hydrogen atoms.

Orientation-Dependent Friction in Liquid Crystals

Abstract The meaning of the rotational friction and diffusion tensor in Liquid crystalline phases characterized by anisotropic viscosities is discussed, by generalizing the hydrodynamic Stokes-Einstein relationships. Corrections due to the viscosity anisotropy effects to the rotational correlation times, relevant for the interpretation of magnetic and dielectric relaxation experiments, have been computed by solving the appropriate Fokker-Planck equation.

Model-free approach to the interpretation of nuclear magnetic resonance relaxation in macromolecules. 1. Theory and range of validity

A new approach to the interpretation of nuclear magnetic resonance relaxation experiments on macromolecules in solution is presented. This paper deals with the theoretical foundations and establishes the range of validity of this approach, and the accompanying paper demonstrates how a wide variety of experimental relaxation data can be successfully analyzed by using this approach. For both isotropic and anisotropic overall motion, it is shown that the unique imformation on fast internal motions contained in relaxation experiments can be completely specified by two model-independent quantities; (1) a generalized order parameter, S, which is a measure of the spatial restriction of the motion, and (2) an effective correlation time, T/sub e/, which is a measure of the rate of motion. A simple expression for the spectral density involving these two parameters is derived and is shown to be exact when the internal (but not overall) motions are in the extreme narrowing limit. The model-free approach (so called because S/sup 2/ and T/sub e/ have model-independent significance) consists of using the above spectral density to least-squares fit relaxation data by treating S/sup 2/ and T/sub e/ as adjustable parameters. The range of validity of this approach is illustrated by analyzing error-free relaxationmore » data generated by using sophisticated dynamical models. Empirical rules are presented that allow one to estimate the of S/sup 2/ and T/sub e/ extracted by using the model-free approach by considering their numerical values, the resonance frequencies, and the parameters for the overall motion. For fast internal motions, it is unnecessary to use approaches based on complicated spectral densities derived within the framework of a model because all models that can give the correct value of S/sup 2/ work equally well.« less

Ion binding to polyelectrolytes as described by the Poisson–Boltzmann equation. Comparison with 23Na nuclear magnetic resonance relaxation experiments

Download options Please wait... Article information DOI https://doi.org/10.1039/F19827802901 Article type Paper Download Citation J. Chem. Soc., Faraday Trans. 1, 1982,78, 2901-2910 BibTex EndNote MEDLINE ProCite ReferenceManager RefWorks RIS Permissions Request permissions Ion binding to polyelectrolytes as described by the Poisson–Boltzmann equation. Comparison with 23Na nuclear magnetic resonance relaxation experiments G. Gunnarsson and H. Gustavsson, J. Chem. Soc., Faraday Trans. 1, 1982, 78, 2901 DOI: 10.1039/F19827802901 To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given. If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page. Read more about how to correctly acknowledge RSC content. Search articles by author Gudmundur Gunnarsson Hans Gustavsson