NMR parameter contrast in abundant-spin images of solids

Solid state nuclear magnetic resonance imaging (NMRI) techniques have been steadily improving over the years. Today high-resolution images of rigid solids are now accomplished by many different means. For abundant nuclei, the combination of multiple-pulse line narrowing and pulsed field gradients have greatly improved both the resolution and sensitivity of the imaging experiment, but often at the expense of the chemical information in the material. In this paper we discuss means of incorporating NMR parameters in the imaging experiment to generate image contrast which provides information about local variations in the chemistry of the material.     

93Nb NMR spin echo spectroscopy in single crystal NbSe3

We report electric field induced phase displacements of the charge density wave (CDW) in a single crystal of NbSe3 using 93Nb NMR spin-echo spectroscopy. CDW polarizations in the pinned state induced by unipolar and bipolar pulses are linear and reversible up to at least E = (0.96)ET. The polarizations have a broad distribution extending up to phase angles of order 60 degrees for electric fields close to threshold. No evidence for polarizations in excess of a CDW wavelength or for a divergence in polarization near ET are observed. The results are consistent with elastic depinning models, provided that the critical regime expected in large systems is not observable.     

自旋回波的简易观测方法及共振弛豫分析

在脉冲核磁共振实验中,通常采用自旋回波法测量共振弛豫时间,但模拟示波器观测难以获得准确的实验数据.通过对计算机标准配置资源声卡的性能检测及标定,使其达到物理实验测量数据定量分析的教学要求,同时利用免费的简易程序实现多通道数字信号采集功能并用于观测记录脉冲核磁共振信号.配合实验操作技术改进,既准确地测量了横向弛豫时间,又展现了符合物理实验教学的计算机应用方法.     

Self-diffusion measurements in EuB6 using the pulsed gradient spin echo NMR technique

Self-diffusion process in EuB₆ was studied using the pulsed gradient spin echo NMR technique. At 1.7 K, zero-field value of the self-diffusion parameter, k, was found to be (66 ± 4) × 10^-6 (μs)^-3, while that with 5 kG polarizing field turned out to be (3 ± 0.6) × 10^-6 (μs)^-3.     

Selective assignments in 13C NMR spectra of chloroalkanes by J-Modulated 13C spin echo

The application of the J_CH-modulated ¹³C spin echo is extended to chloroalkanes. One-bond coupling constants of protonated carbons are increased significantly by chlorine substitution. Nulling of resonance signals from CH_x and chlorine substituted carbons is observed at selected echo delays. Assignments in ¹³C spectra of mixtures of chloroalkanes are derived from subspectra recorded at suitably chosen echo delays.     

Dephasing of spin echoes by multiple heteronuclear dipolar interactions in rotational echo double resonance NMR experiments

The application of rotational echo double resonance (REDOR) nuclear magnetic resonance (NMR) for accurate distance measurements has thus far been largely restricted to isolated heteronuclear two-spin systems. In the present paper, the informational content of REDOR curves is explored for systems characterized by multi-spin interactions. To this end, numerical REDOR simulations are presented for cases in which single observe spins S are dipolarly coupled to groups of spins I in distinct geometries. To develop the utility of REDOR for characterizing dipolar couplings in unknown and/or ill-defined geometries, the validity ranges and systematic errors of certain analytical approximations are studied. In the limit of short dipolar evolution times where 0 < deltaS/S0 < or = 0.2 to 0.3, the REDOR difference signal intensity increases approximately proportional to the square of the dipolar evolution time. Here, the curvature depends simply on the second moment M2 characterizing the overall strength of the heterodipolar coupling, irrespective of specific molecular geometries. Fitting experimental REDOR data in this manner produces slight systematic underestimates of M2. However, these errors tend to be counterbalanced by additional systematic errors made by neglecting weak couplings to more remote spins and distribution effects caused by disorder. Based on these findings, the results suggest a convenient method of obtaining site-resolved second moment information in disordered materials.     

Properties of the HYSCORE spin echo signal

In hyperfine sublevel correlation spectroscopy (HYSCORE), the finite duration of the microwave pulses leads to an incomplete inversion of the electron spin magnetization by the third pulse, which results in a significant admixture of stimulated ESEEM to HYSCORE ESEEM. This virtually unavoidable contribution of stimulated ESEEM seriously hampers the analysis of the modulation amplitudes in HYSCORE. In this work, we analyze the properties of the spin echo signals contributing to the composite HYSCORE signal. Based on this analysis, we propose the strategies of HYSCORE data acquisition and processing that allow one to practically eliminate the contribution of the stimulated echo and make the HYSCORE ESEEM analyzable in quantitative terms.     

Motions of the flux lines in high-T c superconductors from NMR linewidth,relaxation and spin echo decay

The motion of the flux lines (FL) in high temperature superconductors and their relationship with the NMR quantities are reviewed and discussed in the light of recent⁸⁹Y NMR experiments in YBCO-type compounds. In particular measurements involving the⁸⁹Y spin echo attenuation induced both by the thermal excitation of the FL’s and by motions driven by DC current and pulsed magnetic fields are presented, with preliminary results and lines of interpretation. Flux line motion as observed with¹⁹⁹Hg NMR in HgBa₂CuO_4+δ high temperature superconductor is discussed.     

An NMR investigation of naphthalene nanostructures

The molecular mobility of naphthalene molecules in porous silica has been studied over the temperature range 223?K to 363?K using NMR relaxation times T ₂, T ₁ and T _1ρ. The investigations were conducted in silicas with nominal pore diameters of 4?nm, 6?nm, 10?nm, 20?nm and 50?nm. The confined solid behaved in a way that indicated it formed a dual phase system consisting of a solid core in the centre of the pores surrounded by a mobile surface layer. The core naphthalene had the same line width as the bulk. The surface layer exhibited a narrower line of a width that suggested the onset of motional narrowing. This behaviour was characteristic of a plastic crystal phase for naphthalene that does not exist in the bulk. The T ₁ and T _1ρ results were dominated by surface interactions between the confined naphthalene and the pore wall. Magnetization transfer experiments showed that enhanced relaxation occurred throughout the confined material in a time long compared to T ₂ but short compared to T ₁ and T _1ρ. Since the line shape ruled out diffusional motion through the rigid lattice naphthalene core, the magnetization transfer must have occurred via spin diffusion.     

Homonuclear vanadium-51 dipolar couplings in inorganic solids obtained via hahn spin echo decay NMR spectroscopy

Dipolar dephasing of the magnetization following a Hahn spin echo pulse sequence potentially provides a quantitative means for determining the dipolar second moment in solids. In this work, the possibility of employing Hahn spin echo decay spectroscopy to obtain quantitative ⁵¹V–⁵¹V dipolar second moments is explored. Theoretical spin echo response curves are compared to experimental ones for a collection of crystalline vanadium-containing compounds. This work suggests that ⁵¹V dipolar second moments can be obtained by selectively exciting the central m = 1/2 → −1/2 by a Hahn echo sequence for vanadate compounds with line broadening no greater than approximately 220 ppm. For vanadates with greater broadening of the central transition due to chemical shift, second-order quadrupolar, and dipolar interactions, off-resonance effects lead to an oscillatory time dependence of the spin echo. Experimentally determined second moments of the normalized echo decay intensities lie within 10–33% of the calculated values if the second moments are extrapolated to zero evolution time due to the time scale dependence of spin exchange among neighboring vanadium nuclei. Alternatively, the second moments can be obtained to within 10–25% of the calculated values if the broadening of the central transition due to chemical shift and second-order quadrupolar effects can be estimated.     

Physical mechanisms of spin echoes. I. Two-pulse locked echo

The locked echo is the result of the application, to an inhomogeneously-broadened system, of a pulse sequence consisting of a short π/2 pulse, a free evolution period of length r and a long pulse with high turning angle (HTA). In this paper the nature and features of the signal detected after such a pulse sequence are reported upon by analysing the underlying physical mechanisms in the framework of the density operator formalism. It is found that the total signal does not contain any contribution from Free Induction Decay (FID)-like or anti-echo signals but comprises a group of several distinct echoes: the true simultaneous locked echo, arising from locked magnetization, and two or three non-simultaneous, oscillatory echoes similar to those observed in the single-pulse experiment. Properties of these echoes are deduced from the structure of the density operator at the end of the experiment. Extensive numerical simulations provide independent evidence of the correctness of the developed theory, display the variety of patterns shown by the locked echo when experimental conditions are changed, and also permit one to investigate the locked echo shape beyond the approximations introduced in the theory.     

On the three-pulse excitation of spin echo signals

Three variants of three-pulse excitation of the quadrupole spin echo are considered. The specific features are determined for each variant of the excitation.     

On the nature of the amplified spin echo in garnets

Experimental investigations of the amplified echo in the case of a homogeneously magnetized sphere have shown that the natural oscillations of a ferrite with wave number k between 10 and 7–10⁴ cm^–1 take part in the amplification process. In this connection, the production of an appropriate inhomogeneity of the internal magnetic field is necessary for the effective excitation of such oscillations, i. e., to obtain high gain coefficients. Thus, a system of two contiguous ferrite spheres with optimal orientation yield a 40 dB gain in the three centimeter waveband. A theory considering the parametric interaction between the magnetic moment vibrations in the echo mode and taking account of relaxation processes yields good agreement with experiment.Translated from Izvestiya VUZ, Fizika, No. 3, pp. 37–40, March, 1973.     

Basic principles of static proton low-resolution spin diffusion NMR in nanophase-separated materials with mobility contrast

We review basic principles of low-resolution proton NMR spin diffusion experiments, relying on mobility differences in nm-sized phases of inhomogeneous organic materials such as block-co- or semicrystalline polymers. They are of use for estimates of domain sizes and insights into nanometric dynamic inhomogeneities. Experimental procedures and limitations of mobility-based signal decomposition/filtering prior to spin diffusion are addressed on the example of as yet unpublished data on semicrystalline poly(ϵ-caprolactone), PCL. Specifically, we discuss technical aspects of the quantitative, dead-time free detection of rigid-domain signals by aid of the magic-sandwich echo (MSE), and magic-and-polarization-echo (MAPE) and double-quantum (DQ) magnetization filters to select rigid and mobile components, respectively. Such filters are of general use in reliable fitting approaches for phase composition determinations. Spin diffusion studies at low field using benchtop instruments are challenged by rather short ¹H T₁ relaxation times, which calls for simulation-based analyses. Applying these, in combination with domain sizes as determined by small-angle X-ray scattering, we have determined spin diffusion coefficients D for PCL (0.34, 0.19 and 0.032 nm²/ms for crystalline, interphase and amorphous parts, respectively). We further address thermal-history effects related to secondary crystallization. Finally, the state of knowledge concerning the connection between D values determined locally at the atomic level, using ¹³C detection and CP- or REDOR-based “¹H hole burning” procedures, and those obtained by calibration experiments, is summarized. Specifically, the non-trivial dependence of D on the magic-angle spinning (MAS) frequency, with a minimum under static and a local maximum under moderate-MAS conditions, is highlighted.     

A modified fuzzy clustering algorithm for operator independent brain tissue classification of dual echo MR images.

Methods for brain tissue classification or segmentation of structural magnetic resonance imaging (MRI) data should ideally be independent of human operators for reasons of reliability and tractability. An algorithm is described for fully automated segmentation of dual echo, fast spin-echo MRI data. The method is used to assign fuzzy-membership values for each of four tissue classes (gray matter, white matter, cerebrospinal fluid and dura) to each voxel based on partition of a two dimensional feature space. Fuzzy clustering is modified for this application in two ways. First, a two component normal mixture model is initially fitted to the thresholded feature space to identify exemplary gray and white matter voxels. These exemplary data protect subsequently estimated cluster means against the tendency of unmodified fuzzy clustering to equalize the number of voxels in each class. Second, fuzzy clustering is implemented in a moving window scheme that accommodates reduced image contrast at the axial extremes of the transmitting/receiving coil. MRI data acquired from 5 normal volunteers were used to identify stable values for three arbitrary parameters of the algorithm: feature space threshold, relative weight of exemplary gray and white matter voxels, and moving window size. The modified algorithm incorporating these parameter values was then used to classify data from simulated images of the brain, validating the use of fuzzy-membership values as estimates of partial volume. Gray:white matter ratios were estimated from 20 twenty normal volunteers (mean age 32.8 years). Processing time for each three-dimensional image was approximately 30 min on a 170 MHz workstation. Mean cerebral gray and white matter volumes estimated from these automatically segmented images were very similar to comparable results previously obtained by operator dependent methods, but without their inherent unreliability.     

Investigation of domain walls in BaFe12O19 by the NMR spin echo technique

Due to the anisotropy of the hyperfine interactions and dipolar fields in hexagonal BaFe₁₂O₁₉, the nuclear magnetic resonance frequency of ⁵⁷Fe-nuclei at different positions in domain walls situated well within the bulk samples can be detected. The dependence of the enhancement factor η on the position in the wall was measured at 4.2 K and found in good agreement with the dependence expected for a 180°-Bloch-wall. For nuclei in the wall center, a nearly uniform enhancement factor of η = 6.3 × 10³ is found. There is no evidence for a broad distribution of η due to wall pinning effects.