Influence Of Paramagnetic Impurities Research Articles

Nuclear-magnetic-resonance relaxation due to the translational diffusion of fluid confined to quasi-two-dimensional pores.

Nuclear-magnetic-resonance (NMR) relaxation experimentation is an effective technique for nondestructively probing the dynamics of proton-bearing fluids in porous media. The frequency-dependent relaxation rate T_{1}^{-1} can yield a wealth of information on the fluid dynamics within the pore provided data can be fit to a suitable spin diffusion model. A spin diffusion model yields the dipolar correlation function G(t) describing the relative translational motion of pairs of ^{1}H spins which then can be Fourier transformed to yield T_{1}^{-1}. G(t) for spins confined to a quasi-two-dimensional (Q2D) pore of thickness h is determined using theoretical and Monte Carlo techniques. G(t) shows a transition from three- to two-dimensional motion with the transition time proportional to h^{2}. T_{1}^{-1} is found to be independent of frequency over the range 0.01-100 MHz provided h≳5 nm and increases with decreasing frequency and decreasing h for pores of thickness h<3 nm. T_{1}^{-1} increases linearly with the bulk water diffusion correlation time τ_{b} allowing a simple and direct estimate of the bulk water diffusion coefficient from the high-frequency limit of T_{1}^{-1} dispersion measurements in systems where the influence of paramagnetic impurities is negligible. Monte Carlo simulations of hydrated Q2D pores are executed for a range of surface-to-bulk desorption rates for a thin pore. G(t) is found to decorrelate when spins move from the surface to the bulk, display three-dimensional properties at intermediate times, and finally show a bulk-mediated surface diffusion (Lévy) mechanism at longer times. The results may be used to interpret NMR relaxation rates in hydrated porous systems in which the paramagnetic impurity density is negligible.

Anomalous isotope effect in high-temperature superconductor LSCO with 3d-impurities

The influence of transition metal substitutions (Ni, Fe, Co) and Zn at the Cu sites on T c and the oxygen isotope effect α o has been studied in the system La 2− y Sr y Cu 1− x M x O 4 with the optimum ( y = 0.15) and overdoped ( y = 0.20) compositions. Essential distinctions in the behavior of parameter α o for systems with heterovalent Fe and Co substitutions and isovalent Ni and Zn substitutions have been revealed. For Fe doped compounds with y = 0.15 the isotope effect was found to be very large: α o ≈ 1.3 at x = 0.011. The dependence of the isotope effect on the substituted atom concentration changes substantially when the degree of strontium doping varies. The experimental data were theoretically analyzed on the basis of the phonon mechanism of superconductivity and the concept of the existence of a singularity in the electron density of state N( E F). It is shown that the influence of paramagnetic impurities on the T c isotope shift of superconductors is enhanced when there is a sharp peak in N( E F). The difference in these effects for a system with the optimum strontium composition and for an overdoped system is also explained.

Surface NMR measurement of proton relaxation times in medium to coarse-grained sand aquifer

A surface NMR investigation of groundwater in the geomagnetic field is under study. To detect the surface NMR a wire loop with a diameter of about 100 m, being an antenna for both an exciting field source and the NMR signal receiver, is laid out on the ground. A sinusoidal current pulse with a rectangular envelope is passed through the loop to excite the NMR signal. The carrier frequency of the oscillating current in this pulse is equal to the Larmor frequency of protons in the Earth's magnetic field. The current amplitude is changed up to 200 amps and the pulse duration is fixed and is equal to 40 ms. The exciting pulse is followed by an induction emf signal caused by the Larmor nuclear precession in geomagnetic field. The relaxation times T 1, T 2, and T 2 ∗ were measured by the surface NMR for both groundwater in medium to coarse-grained sand at borehole and for bulk water under the ice surface of frozen lake. To determine T 1, a longitudinal interference in experiments with repeated pulses was measured. A sequence with equal period between equal excitation pulses was used. The relaxation times T 1, T 2, measured for bulk water under the ice of the Ob reservoir were 1.0 s and 0.7 s, respectively. To estimate an influence of dissolved oxygen T 1 of the same water at the same temperature was measured by lab NMR with and without pumping of oxygen. The relaxation time T 1 measured for water in the medium to coarse-grained sand is 0.65 s. The relaxation time T 2 estimated by spin echo sequence is found to be equal to 0.15 s. The relaxation time T 2 ∗ is found to be about 80 ms. This result contradicts published earlier phenomenological correlation between relaxation time T 2 ∗ and grain size of water-bearing rock. This could be as a result of unsound approach based on grain size or influence of paramagnetic impurities.

The influence of paramagnetic impurities on magnetic effects in radical reactions

The influence of paramagnetic impurities on magnetic and spin effects in radical reactions in liquid solutions is treated. The recombination probability of a radical pair is obtained. It is shown, that at high impurity concentration or in high-viscosity solvents the effect of the magnetic field vanishes.

The nuclear spin interaction with paramagnetic impurities in antiferromagnets

The influence of paramagnetic impurities on the relaxation frequencies of the nuclear subsystem in antiferromagnets is investigated. In the framework of the spin operator diagram technique the nuclear spin-spin as well as spin-lattice relaxation rates are calculated. The correlation effects due to indirect spin-spin interaction via magnons both in nuclear and impurity subsystems are taken into account.

Influence of paramagnetic impurities on molecular self-diffusion as seen by nuclear spin-lattice relaxation and dynamic polarization

Measurements, near the melting point, of nuclear spin-lattice relaxation and dynamic polarization in plastic cyclohexane doped with a nitroxyde free radical show deviations from the expected spin-diffusion theory. These deviations are explained by the quenching of the molecular self-diffusion around the radical and interpreted semiquantitatively in terms of a two zone model. It is shown that the quenching extends up to more than four intermolecular distances and the spin-diffusion coefficient in this region is shown to be around 0.3× 10−12 cm2 sec−1. It seems that similar measurements would be of interest in any crystal which exhibits NMR motional narrowing due to fast molecular self-diffusion.

Absorption lineshape of highly polarized nuclear spin systems

We have studied both theoretically and experimentally the variation of NMR absorption lineshape as a function of nuclear polarization in systems with one spin species. General expressions are given for the first and second moments M 1 and M 1, as well as the values of M 3 and M 4 in the special case of spherical samples of spins 1 2 forming a simple cubic lattice. The influence of paramagnetic impurities on the line shape is analyzed, in detail. Measurements of M 3 in spherical samples of CaF 2 polarized up to 90% by the solid effect confirm the theory. In such samples the second moment, corrected from the effect of paramagnetic impurities, follows the simple law: M 2( p) = M 2(0) (1− p 2). This has been used to perform an absolute calibration of fluorine polarization with an accuracy between 3 and 5%.

Paramagnetic Impurities in Superconductors with Overlapping Energy Bands

AbstractThe influence of paramagnetic impurities on the thermodynamic properties of two‐band superconductors at zero and near zero temperatures is investigated. It is shown that due to interband scattering of electrons there is one gap in two‐band superconductors with impurities. The extreme cases of low and large impurity concentrations are investigated. In the first limiting case an increase in the concentration is accompanied by a linear decrease of order parameters. The expression for the energy gap, densities of states near the energy gap, and the specific heat as functions of impurity concentration are given.

Verschiebungen der magnetischen Kernresonanzsignale adsorbierter Moleküle

Proton resonance shifts of molecules adsorbed on the surface of silicagels are investigated. In order to explain these shifts which are in the direction to higher fields (compared with the resonances of free liquids) the influence of paramagnetic impurities in the silicagel surface, the different diamagnetic volume susceptibilities, as well as the influence of the relaxation process and of bonding effects are discussed. It is pointed out that the observed resonance shifts are mainly caused by bonding effects in our case. An inhomogeneous broadening of the resonance lines (by a factor F up to 12), found by comparison with spinecho experiments, is shown to be correlated with the shifts. Using this fact the dependence of the factor F on the surface coverage can be explained qualitatively.

The influence of paramagnetic impurities on the dynamic polarization of protons in (La, Nd)2Mg3(NO3)12.24H2O

In single crystals of La2Mg3(NO3)12.24H2O containing 2.0% Nd and 0.02% to 0.50% Ce the proton dynamic polarization factor and the polarization time have been measured. The results are compared with the predictions on the basis of Borghini's spin temperature theory including the influence of the factor f. Most of the results can be explained by assuming that nuclear spin diffusion is very fast, so that the main part of the proton spin system is in internal equilibrium. Only when the leakage of polarization due to the Ce ions is very important, the dependence of the enhancement factor on microwave power indicates that spin diffusion is not fast enough to maintain equilibrium in the proton spin system.

Influence of Paramagnetic Impurities on Nuclear-Magnetic-Resonance Lines in Dielectric Solids

The nuclear magnetic resonance lineshape problem is analyzed for a dielectric solid which has a nuclear constituent of nonzero spin and contains, in addition, a low concentration of paramagnetic impurity ions. A model is defined and used as a basis to obtain the resonance line as a convolution of independent resonanceline components. An effective mean-square linewidth is defined and calculated as a function of the macroscopic parameters of the sample.

Influence of Paramagnetic Impurities on the Temperature Dependence of Nuclear Magnetic Resonance Lines

Influence of Paramagnetic Impurities on the Temperature Dependence of Nuclear Magnetic Resonance Lines