Magnetic Relaxation Experiments Research Articles

Functional behavior of the anomalous magnetic relaxation observed in melt-textured YBa2Cu3O7-δ samples showing the paramagnetic Meissner effect

We have studied the functional behavior of the field-cooled (FC) magnetic relaxation observed in melt-textured YBa2Cu3O7-δ (Y123) samples with 30 wt% of Y2Ba1Cu1O5 (Y211) phase, in order to investigate anomalous paramagnetic moments observed during the experiments. FC magnetic relaxation experiments were performed under controlled conditions, such as cooling rate and temperature. Magnetic fields up to 5T were applied parallel to the ab plane and along the c-axis. Our results are associated with the paramagnetic Meissner effect (PME), characterized by positive moments during FC experiments, and related to the magnetic flux compression into the samples. After different attempts our experimental data could be adequately fitted by an exponential decay function with different relaxation times. We discuss our results suggesting the existence of different and preferential flux dynamics governing the anomalous FC paramagnetic relaxation in different time intervals. This work is one of the first attempts to interpret this controversial effect in a simple analysis of the pinning mechanisms and flux dynamics acting during the time evolution of the magnetic moment. However, the results may be useful to develop models to explain this interesting and still misunderstood feature of the paramagnetic Meissner effect.

Noninvasive Experimental Evidence of the Linear Pore Size Dependence of Water Diffusion in Nanoconfinement.

We show that nuclear magnetic relaxation experiments at variable magnetic fields (NMRD) provide noninvasive means for probing the spatial dependence of liquid diffusion close to solid interfaces. These experiments performed on samples of cylindrical and spherical nanopore geometries demonstrate that the average diffusion coefficient parallel to the interface is proportional to the pore radii in different dynamics regimes. A master curve method allows extraction of gradients of diffusion coefficients in proximity of the pore surfaces, indicative of the efficiency of coupling between liquid layers. Due to their selectivity in frequency, NMRD experiments are able to differentiate the different water dynamical events induced by heterogeneous surfaces or composed dynamical processes. This analysis relevant in physical and biological confinements highlights the interplay between the molecular and continuous description of fluid dynamics near interfaces.

Methyl and t-butyl group rotation in a molecular solid: (1)H NMR spin-lattice relaxation and X-ray diffraction.

We report solid state (1)H nuclear magnetic resonance spin-lattice relaxation experiments and X-ray diffractometry in 2-t-butyldimethylsilyloxy-6-bromonaphthalene. This compound offers an opportunity to simultaneously investigate, and differentiate between, the rotations of a t-butyl group [C(CH3)3] and its three constituent methyl groups (CH3) and, simultaneously, a pair of 'lone' methyl groups (attached to the Si atom). The solid state (1)H relaxation experiments determine activation energies for these rotations. We review the models for the dynamics of both 'lone' methyl groups (ones whose rotation axes do not move on the NMR time scale) and models for the dynamics of the t-butyl group and its constituent methyl groups (whose rotation axes reorient on the NMR time scale as the t-butyl group rotates).

Unconventional superconductivity in Y5Rh6Sn18 probed by muon spin relaxation.

Conventional superconductors are robust diamagnets that expel magnetic fields through the Meissner effect. It would therefore be unexpected if a superconducting ground state would support spontaneous magnetics fields. Such broken time-reversal symmetry states have been suggested for the high—temperature superconductors, but their identification remains experimentally controversial. We present magnetization, heat capacity, zero field and transverse field muon spin relaxation experiments on the recently discovered caged type superconductor Y5Rh6Sn18 ( TC= 3.0 K). The electronic heat capacity of Y5Rh6Sn18 shows a T3 dependence below Tc indicating an anisotropic superconducting gap with a point node. This result is in sharp contrast to that observed in the isostructural Lu5Rh6Sn18 which is a strong coupling s—wave superconductor. The temperature dependence of the deduced superfluid in density Y5Rh6Sn18 is consistent with a BCS s—wave gap function, while the zero-field muon spin relaxation measurements strongly evidences unconventional superconductivity through a spontaneous appearance of an internal magnetic field below the superconducting transition temperature, signifying that the superconducting state is categorized by the broken time-reversal symmetry.

Magnetic Order, Spin Waves and Fluctuations in the Triangular Antiferromagnet La2Ca2MnO7

We report magnetic susceptibility, specific heat and muon spin relaxation (μSR) experiments on the triangular antiferromagnet La 2 Ca 2 MnO 7 which develops a genuine two-dimensional (2D), three-sublattice [Formula: see text] magnetic order below T N = 2.8 K. From the susceptibility and specific heat an estimate of the exchange interaction is derived. A value for the spin-wave gap is obtained from the latter data. The analysis of a previously reported inelastic neutron scattering data yields values for the exchange and spin-wave gap compatible with the results obtained from macroscopic measurements. An appreciable entropy is still missing at 10 K that may be ascribed to intense short-range correlations. The critical paramagnetic fluctuations extend far above T N , and can be partly understood in terms of two-dimensional spin-wave excitations. While no spontaneous μSR field is observed below T N , persistent spin dynamics is found. Short-range correlations are detected in this temperature range. Their relation to a possible molecular spin substructure and the observed exotic spin fluctuations is discussed.

Slowdown of Interhelical Motions Induces a Glass Transition in RNA

RNA function depends crucially on the details of its dynamics. The simplest RNA dynamical unit is a two-way interhelical junction. Here, for such a unit—the transactivation response RNA element—we present evidence from molecular dynamics simulations, supported by nuclear magnetic resonance relaxation experiments, for a dynamical transition near 230 K. This glass transition arises from the freezing out of collective interhelical motional modes. The motions, resolved with site-specificity, are dynamically heterogeneous and exhibit non-Arrhenius relaxation. The microscopic origin of the glass transition is a low-dimensional, slow manifold consisting largely of the Euler angles describing interhelical reorientation. Principal component analysis over a range of temperatures covering the glass transition shows that the abrupt slowdown of motion finds its explanation in a localization transition that traps probability density into several disconnected conformational pools over the low-dimensional energy landscape. Upon temperature increase, the probability density pools then flood a larger basin, akin to a lakes-to-sea transition. Simulations on transactivation response RNA are also used to backcalculate inelastic neutron scattering data that match previous inelastic neutron scattering measurements on larger and more complex RNA structures and which, upon normalization, give temperature-dependent fluctuation profiles that overlap onto a glass transition curve that is quasi-universal over a range of systems and techniques.

Paramagnetic moments and time effects in melt-textured NdBaCuO system with Nd422 inclusions

We have performed magnetic measurements in two melt-textured NdBa2Cu3O7-δ samples with Nd422 inclusions under magnetic fields from 0.05 up to 14 T, applied parallel to the ab planes. The measurements were made with a superconducting quantum interference device (SQUID) and a vibrating sample magnetometer (VSM). Paramagnetic moments could be observed during FCC (field-cooled cooling) and FCW (field-cooled warming) experiments. This effect, known as Paramagnetic Meissner Effect (PME), persisted up to 14 T and strong irreversibilities were observed among FCC and FCW experiments, revealing the presence of time effects. These time effects were confirmed by specific magnetic relaxation experiments in different cooling rates and temperatures, showing an anomalous and curious paramagnetic behavior. We explain our results based on the flux-compressed state generated within nonsuperconducting regions of the sample, such as the Nd422 inclusions dispersed into the superconducting matrix. These inclusions may produce a strong vortex pinning that stabilize the paramagnetic state, allowing the admission of extra vortices into the sample responsible for the positive moments during the relaxation experiments.

Magnetic relaxation behavior in the Bi2Sr2Ca2Cu3−xMoxO10+δ system fabricated by glass-ceramic technique

In this paper, we present a study on the critical current density and magnetic relaxation behavior of the Mo-substituted BiSrCaCuO system. The Bi2Sr2Ca2Cu3−xMoxO10+y, where x=0, 0.5, 1.0 and 1.5 have been fabricated using glass-ceramic technique. It was found that the Mo-substitution to the BiSrCaCuO system lead to the formation of pinning centers and hence an increase of critical current density. The magnetic relaxation experiments were performed to understand physical phenomena behind of flux motion. Time dependence of the magnetization exhibited thermally activation flux motion in the samples. It was found that the normalized magnetic relaxation rate, S, did not vary linearly with temperature, especially at high temperatures, which is attributed to the collective creep theory. In addition, the characteristic pinning energy, Ue/KB, in the samples was calculated using Maley's method. Correlation between the pinning energy and the magnetization showed that obtained experimental results can be fitted to the collective creep theory.

A Coarse-Grained Langevin Equation for Protein Dynamics: Global Anisotropy and a Mode Approach to Local Complexity

We utilize a multi-scale approach where molecular dynamic simulations are performed to obtain quantitative structural averages used as input to a coarse-grained Langevin Equation for Protein Dynamics, which can be solved analytically. The approach describes proteins as fundamentally semiflexible objects collapsed into the free energy well representing the folded state. The normal mode analytical solution to this Langevin equation naturally separates into global modes describing the fully anisotropic tumbling of the macromolecule as a whole, and internal modes which describe local fluctuations about the folded structure. Complexity in the configurational free energy landscape around the folded state of the macromolecule leads to a renormalization of the internal modes, while the global modes provide a basis set in which the dipolar orientation and global anisotropy can be accounted for when comparing to experiments. Fundamental to this approach is the inclusion of internal dissipation which is absent in any rigid-body hydrodynamical modeling scheme. This simple approach predicts the dynamics of both global rotational diffusion and internal motion from the picosecond to the nanosecond regime, and is quantitative when compared to time correlation functions calculated from molecular dynamic simulations and in good agreement with Nuclear Magnetic Resonance relaxation experiments. Results for several well-characterized globular proteins are presented, suggesting our method describes the relevant dynamics around the global minimum well. Use of non-equilibrium simulation techniques such as metadynamics to sample the full free-energy landscape of the protein, and extension of the theoretical treatment to describe the dynamics into the biologically interesting microsecond to millisecond regime, will be discussed.

Coarse-Grained Langevin Equation for Protein Dynamics: Global Anisotropy and a Mode Approach to Local Complexity.

We utilize a multiscale approach where molecular dynamic simulations are performed to obtain quantitative structural averages used as input to a coarse-grained Langevin equation for protein dynamics, which can be solved analytically. The approach describes proteins as fundamentally semiflexible objects collapsed into the free energy well representing the folded state. The normal-mode analytical solution to this Langevin equation naturally separates into global modes describing the fully anisotropic tumbling of the macromolecule as a whole and internal modes which describe local fluctuations about the folded structure. Complexity in the configurational free-energy landscape of the macromolecule leads to a renormalization of the internal modes, while the global modes provide a basis set in which the dipolar orientation and global anisotropy can be accounted for when comparing to experiments. This simple approach predicts the dynamics of both global rotational diffusion and internal motion from the picosecond to the nanosecond regime and is quantitative when compared to time correlation functions calculated from molecular dynamic simulations and in good agreement with nuclear magnetic resonance relaxation experiments. Fundamental to this approach is the inclusion of internal dissipation, which is absent in any rigid-body hydrodynamical modeling scheme.

The effect of annealing of linear and branched polyamide 46 on the phase composition, molecular mobility and water absorption as studied by DSC, 1H and 2H solid-state NMR

The effect of annealing on the phase composition, molecular mobility and water absorption in linear and branched polyamide 46, poly(tetramethylene adipamide) – PA46, was studied by DSC, 1H and 2H solid-state NMR. A series of samples with varying amount of long-chain branches was synthesized for this study. The branches as well as a higher molecular weight of branched PA46 cause a decrease in crystallization rate as it follows from a lower crystallization temperature. 1H NMR transverse magnetization relaxation (T2 relaxation) experiments show that, at temperatures well above Tg, the amount of low mobile chain segments (rigid fraction) in all injection-molded samples equals 60–63%. This value is close to the crystallinity of linear PA46 as measured by WAXD. Although chain branches hardly affect the amount of the rigid fraction in injection-molded samples, molecular mobility in the crystalline phase of branched PA46 is lower than that in linear PA46, suggesting a more perfect crystalline structure in branched PA46 due to slower crystallization. 12.5 h annealing at 260 °C causes a large increase in the amount of the rigid fraction in linear PA46, whereas WAXD crystallinity only slightly increases upon annealing. The increase in the rigid fraction is largely caused by immobilization of the amorphous phase due to chains rearrangements resulting in stronger hydrogen bonds between amide groups in the amorphous phase. The amount of the rigid fraction in annealed samples is smaller in samples with higher amount of branches, and molecular mobility in the rigid and soft fractions is larger. This suggests that chain branches hamper chain rearrangements both in the crystalline and the amorphous phases during annealing of PA46. This results in a less ordered structure in the crystalline and the amorphous phases in annealed branched PA46. The amount of absorbed water decreases upon increasing amount of branches in injection-molded samples and upon annealing. 2H NMR experiments for PA46 saturated with D2O show that the mobility of absorbed water is strongly hindered as compared to pure water. Below ambient temperatures, mobility of water in PA46 is characterized by a very broad distribution of molecular motions. The mobility of water molecules gradually decreases upon sample cooling from ambient temperature to −40 °C. The majority of water molecules is immobilized below −40 °C. The results of the present study suggest that water uptake by PA46 is mainly determined by the strength of hydrogen bonds between amide groups in the amorphous phase, which is largely affected by the crystallization rate and annealing at elevated temperatures.

Ageing dynamics of a superspin glass

Magnetization dynamics of a model superspin glass system consisting of nearly monodispersed close-packed maghemite particles of diameter 8 nm is investigated. The observed non-equilibrium features of the dynamics are qualitatively similar to those of atomic spin glass systems. The intrinsic relaxation function, as observed in zero-field–cooled magnetization relaxation experiments, depends on the time the sample has been kept at constant temperature (ageing). Accompanying low-field experiments show that the archetypal spin glass characteristics —ageing, memory and rejuvenation— are reproduced in this dense system of dipolar-dipolar interacting superspins.

Nonequilibrium magnetic properties in Nd5Ge3

Using the field cooled (FC) and zero field cooled (ZFC) magnetization of Nd5Ge3 measured with intermittent stops and wait periods, we vividly demonstrate that it exhibits history dependent phenomena such as aging, memory effects and rejuvenation at temperatures below the spin glass freezing temperature (Tg). Memory effects are also observed in magnetization relaxation experiments with temporary cooling cycle. However, a small positive temperature cycle destroys the effect of previous aging and rejuvenates the system. Memory effects observed in different experimental protocols below Tg give a strong confirmation of the existence of spin glass like phase in the system. Asymmetric behavior of the relaxation data during hetaing and cooling cycles suggests that the hierarchical model for spin glasses to be valid in this compound. A comparison of different members of R5Ge3 series shows that Nd5Ge3 possesses special magnetic and related properties that make it unique among the other members of the series.

Correspondence between neutron depolarization and higher order magnetic susceptibility to investigate ferromagnetic clusters in phase separated systems

It is a tough task to distinguish a short-range ferromagnetically correlated cluster-glass phase from a canonical spin-glass-like phase in many magnetic oxide systems using conventional magnetometry measurements. As a case study, we investigate the magnetic ground state of La0.85Sr0.15CoO3, which is often debated based on phase separation issues. We report the results of two samples of La0.85Sr0.15CoO3 (S-1 and S-2) prepared under different conditions. Neutron depolarization, higher harmonic ac susceptibility and magnetic relaxation studies were carried out along with conventional magnetometry measurements to differentiate subtle changes at the microscopic level. There is no evidence of ferromagnetic correlation in the sample S-2 attributed to a spin-glass phase, and this is compounded by the lack of existence of a second order component of higher harmonic ac susceptibility and neutron depolarization. A magnetic relaxation experiment at different temperatures complements the spin glass characteristic in S-2. All these signal a sharp variance when we consider the cluster-glass-like phase (phase separated) in S-1, especially when prepared from an improper chemical synthesis process. This shows that the nonlinear ac susceptibility is a viable tool to detect ferromagnetic clusters such as those the neutron depolarization study can reveal.

A Distal Mutation Perturbs Dynamic Amino Acid Networks in Dihydrofolate Reductase

Correlated networks of amino acids have been proposed to play a fundamental role in allostery and enzyme catalysis. These networks of amino acids can be traced from surface-exposed residues all the way into the active site, and disruption of these networks can decrease enzyme activity. Substitution of the distal Gly121 residue in Escherichia coli dihydrofolate reductase results in an up to 200-fold decrease in the hydride transfer rate despite the fact that the residue is located 15 Å from the active-site center. In this study, nuclear magnetic resonance relaxation experiments are used to demonstrate that dynamics on the picosecond to nanosecond and microsecond to millisecond time scales are changed significantly in the G121V mutant of dihydrofolate reductase. In particular, picosecond to nanosecond time scale dynamics are decreased in the FG loop (containing the mutated residue at position 121) and the neighboring active-site loop (the Met20 loop) in the mutant compared to those of the wild-type enzyme, suggesting that these loops are dynamically coupled. Changes in methyl order parameters reveal a pathway by which dynamic perturbations can be propagated more than 25 Å across the protein from the site of mutation. All of the enzyme complexes, including the model Michaelis complex with folate and nicotinamide adenine dinucleotide phosphate bound, assume an occluded ground-state conformation, and we do not observe sampling of a higher-energy closed conformation by (15)N R2 relaxation dispersion experiments. This is highly significant, because it is only in the closed conformation that the cofactor and substrate reactive centers are positioned for reaction. The mutation also impairs microsecond to millisecond time scale fluctuations that have been implicated in the release of product from the wild-type enzyme. Our results are consistent with an important role for Gly121 in controlling protein dynamics critical for enzyme function and further validate the dynamic energy landscape hypothesis of enzyme catalysis.

High Vortex Depinning Temperatures in YBCO Films with BZO Nanorods

The Bose glass theory for the vortex matter in superconductors with correlated disorder predicts the depinning of vortices due to the renormalization of the vortex pinning barriers by thermal fluctuations. For YB2Cu3O7 (YBCO) in external magnetic fields H oriented along the columnar pins generated by various techniques theoretical estimates give a depinning temperature Tdp very close to the critical temperature Tc (Tdp∼0.95Tc), whereas the results of standard magnetization relaxation experiments are repeatedly interpreted in terms of a much lower Tdp (∼0.5Tc). We investigated the temperature T variation of the normalized magnetization relaxation rate S for YBCO thin films containing BaZrO3 (BZO) nanorods preferentially oriented along the c axis, with H along the nanorods. The nonmonotonous S(T) variation below the matching field observed up to close to Tc does not support a low Tdp. The often considered S(T) maximum occurring at relatively low T (which was connected to a disappointing Tdp) is related to the occurrence of thermomagnetic instabilities. We show that the accommodation of vortices to the columnar pins in the presence of the T dependent macroscopic currents induced in the sample is signaled by a pronounced S(T) deep located at high T, in agreement with a Tdp close to Tc. By increasing the film thickness and using the substrate decoration the BZO nanorods splay out, leading to the inhibition of (detrimental) vortex excitations involving double vortex kink or superkink formation, characteristic for high-quality thin films and single crystals with columnar pins along the c axis.

Dynamic correlation length growth in superspin glass: Bridging experiments and simulations

Interacting magnetic nanoparticles display a wide variety of magnetic behaviors that are now being gathered in the emerging field of “supermagnetism.” We have investigated how the out-of-equilibrium dynamics in the disordered superspin glass (SSG) state of a frozen ferrofluid sample is affected by texturation. Via magnetization relaxation experiments at low temperatures, we were able to estimate superspin correlation lengths for both textured and non-textured samples. The comparison with simulations and experiments on atomic spin glasses shows that the dynamic correlations in SSG's appear to develop in a way reminiscent to those in atomic spin glasses at intermediate time/length scales.

Coupled Flexibility Change in Cytochrome P450cam Substrate Binding Determined by Neutron Scattering, NMR, and Molecular Dynamics Simulation

Neutron scattering and nuclear magnetic resonance relaxation experiments are combined with molecular dynamics (MD) simulations in a novel, to our knowledge, approach to investigate the change in internal dynamics on substrate (camphor) binding to a protein (cytochrome P450cam). The MD simulations agree well with both the neutron scattering, which furnishes information on global flexibility, and the nuclear magnetic resonance data, which provides residue-specific order parameters. Decreased fluctuations are seen in the camphor-bound form using all three techniques, dominated by changes in specific regions of the protein. The combined experimental and simulation results permit a detailed description of the dynamical change, which involves modifications in the coupling between the dominant regions and concomitant substrate access channel closing, via specific salt-bridge, hydrogen-bonding, and hydrophobic interactions. The work demonstrates how the combination of complementary experimental spectroscopies with MD simulation can provide an in-depth description of functional dynamical protein changes.

Exchange bias and memory effect in double perovskite Sr2FeCoO6

We report on the observation of exchange bias (EB) and memory effect in double perovskite Sr2FeCoO6. Antiphase boundaries between the ferromagnetic and the antiferromagnetic regions in the disordered glassy phase are assumed as responsible for the observed effect, which reflects in the cooling field dependence and temperature evolution of exchange bias field and in training effect. The spin glass (SG) phase itself is characterized through memory, ageing, and magnetic relaxation experiments. The spin glass transition temperature, Tg, versus Hdc2/3 follows the Almeida-Thouless line yielding a freezing temperature, Tf=73K. Time-dependent magnetic relaxation studies reveal the magnetization dynamics of the underlying glassy phase in this double perovskite.

Monitoring the Air Influence on Cement–Lime Mortar Hydration Using Low-Field Nuclear Magnetic Resonance Relaxometry

In the present study, we investigate the relationship between the nuclear magnetic resonance relaxation rate and the hydration time in two types of masonry cement-lime mortar. The studies are performed with the mortars both in an enclosed and a standard atmosphere to monitor the air influence on cement-lime mortar hydration and setting. The constituents of the investigated mortar samples are: cement, slaked lime, sand and water. They were mixed to achieve a flow spread of 10 cm. These types of mortars are usually suitable for historical masonry maintenance, but they can also be used for modern buildings, or even for concrete structures coatings to prevent concrete carbonation. All nuclear magnetic resonance relaxation experiments were performed at 20 °C using a low-field nuclear magnetic resonance instrument operable at 20 MHz proton resonance frequency. A slowing down of the hydration kinetics is demonstrated for the samples kept in closed atmosphere conditions. The results contribute to the understanding of cement–lime mortar hydration, carbonation and setting under closed atmosphere conditions.