Recovery Of Nuclear Magnetization Research Articles

Reduction of the Spin–Phonon Coupling of Quadrupole Nuclei in NaF Crystals under Magnetic Saturation

The rate of nuclear spin-lattice relaxation is determined by the efficiency of interaction between thermal phonons and nuclear spins. The results on reducing the efficiency of spin–phonon coupling by suppressing the contribution from paramagnetic centers to quadrupole nucleus relaxation are presented. The suppression has been performed by stationary magnetic effect at the Larmor frequency. It is shown that, as in the presence of an acoustic field, the rate of spin-lattice relaxation of 23Na nuclei in a sodium fluoride crystal at magnetic saturation of the NMR signal does not change in the region of a negative average spin temperature.In the region of positive spin temperature, the rate of relaxation of 23Na spins significantly decreases and nuclear magnetization recovery with time is described by the sum of two exponentials. The contribution from nuclear spins with a lower efficiency of spin–phonon coupling, corresponding to the exponential with a long relaxation time, increases with increasing saturating field intensity. It is demonstrated that the efficiency of spin–phonon coupling for 19F nuclei without quadrupole moment does not change under the saturation conditions.The results obtained can be used for analyzing the structure of real crystals.

Magnetic inhomogeneity in charge-ordered La1.885Sr0.115CuO4 studied by NMR

We report the inverse Laplace transform (ILT) analysis of the $^{139}$La nuclear spin-lattice relaxation rate $1/T_1$ in charge ordered La$_{1.885}$Sr$_{0.115}$CuO$_4$ ($T_{charge} \sim 80$K, $T_{c} \simeq T_{spin}^{neutron}=30$K), and shed new light on its magnetic inhomogeneity. We deduce the probability density function $P(1/T_{1})$ of the distributed $1/T_1$ (i.e. the histogram of distributed $1/T_1$) by taking the inverse Laplace transform of the experimentally observed nuclear magnetization recovery curve $M(t)$. We demonstrate that spin freezing sets in in some domains precisely below the onset of charge order at $T_{charge}$, but their volume fraction grows only gradually toward $T_{c}$. Nearly a half of the sample volume exhibits properties expected for canonical high $T_c$ cuprates without charge order even near $T_c$. Our findings explain why charge order does not suppress $T_c$ of La$_{1.885}$Sr$_{0.115}$CuO$_4$ as significantly as in La$_{1.875}$Ba$_{0.125}$CuO$_4$.

NMR relaxometry in the investigation of the kinetics of the recrystallization of felodipine

The method of Nuclear Magnetic Resonance (NMR) relaxometry is proposed to be applied to investigate the kinetics of the recrystallization of a substance from the amorphous phase. The method is based on the determination and analysis of nuclear magnetization recovery Mz obtained from spin-lattice T1 relaxation time measurements as a function of time since sample amorphization. The formation of a biphasic (crystalline and amorphous) system in the process of recrystallization implies that the magnetization recovery Mz is biexponential and that the amplitudes of its components are proportional to the number of spins (protons) in the crystalline and amorphous phases, and, thus, also to the volume occupied by the phases. The method was tested for amorphous felodipine samples with grain sizes close to 0.1 or 1 mm, and the recrystallization process was confirmed by using Powder X-ray Diffraction (PXRD) and Fourier-Transform Infrared (FTIR) methods. Qualitative comparison of the results obtained with the data obtained earlier by PXRD and Raman methods for felodipine samples with grain sizes of 0.2 and 0.6 mm [U. S. Kestur, I. Ivanesivic, D. E. Alonzo, L. S. Taylor, Influence of particle size on the crystallization kinetics of amorphous felodipine powders, Powder Technology 236 (2013) 197–204] showed reasonable agreement. The method is fully extensive and can be applied to amorphous samples prepared in different ways, such as grinding, milling or melting, as well as for co-amorphous or amorphous solid dispersion samples.

Power-law versus exponential relaxation of 29Si nucleus spins in Si:B crystals

The Si:B micro-crystals enriched with 29Si isotope have been studied by high resolution nuclear magnetic resonance (NMR) in the 300–800K temperature range. The recovery of nuclear magnetization saturated by radiofrequency impulses follows pure power-law kinetics at 300K, while admixture of exponential relaxation takes place at 500K. The power-law relaxation corresponds to direct electron–nuclear relaxation due to the inhomogeneous distribution of paramagnetic centers, while exponential kinetics corresponds to the nuclear spin diffusion mechanism. The inhomogeneous distribution of deformation defects is a most probable reason of the power-law kinetics of nuclear spin relaxation.

Single-shot nuclear magnetization recovery curves with force-gradient detection.

We measure the spin-lattice relaxation time as a function of sample temperature in GaAs in a real-time single-shot inversion recovery experiment using spin force gradients acting on a magnetic tipped cantilever. After inverting 69Ga spins localized near the magnet with a single 20 ms adiabatic rapid passage sweep, the spins' magnetization recovery was passively tracked by recording the cantilever's frequency change, which is proportional to the longitudinal component of the spins' magnetization. The cantilever's frequency was recorded for a time 3*T1 for sample temperatures ranging from 4.8 to 25 K. The temperature dependence was observed for the 69Ga quadrupolar relaxation interaction.

Coexistence of antiferromagnetism and superconductivity in CeCo(In 0.9Cd 0.1) 5: A spin lattice relaxation study

We present an In(1) NQR study on the heavy-fermion (HF) compound CeCo(In 1− x Cd x ) 5 ( x = 0.10 ) . Bulk measurements indicate that Cd doping acts as an electronic tuning agent in CeCoIn 5, and that superconductivity (SC) and antiferromagnetism (AFM) may coexist at ambient pressure for 0.05 < x < 0.15 . For x = 0.10 , the nuclear spin lattice relaxation rate shows a broad peak at the Néel temperature T N = 2.8 K , with a subsequent onset of SC at T c ≃ 1.2 K . Our results provide strong evidence for microscopic coexistence of these two ground states. The nuclear magnetization recovery curves in both the ordered and mixed states reveal a single T 1 -component, which suggests a homogeneous nature of the Ce-4f 1 electronic state.

Spin dynamics inLa1−xSrxCoO3

The spin dynamics of Sr-doped cobaltite La 1 - x Sr x CoO 3 (x=0.14 and x=0.4) has been investigated in both zero magnetic field and high field by NMR. The results are consistent with microscopically phase-separated regions of ferromagnetic and nonferromagnetic materials. Nuclear spin-lattice and spin-spin relaxation in the ferromagnetic regions is attributed to fluctuating hyperfine fields produced by double exchange between Co ions. The linear temperature dependence of the correlation time, obtained from the data analysis, suggests that lattice excitations modify the double-exchange process as the temperature is raised. In the nonferromagnetic regions, a distribution of nuclear spin-lattice relaxation times is found. It is likely that low-frequency fluctuating localized moments, such as small spin clusters, in spin-glass regions provide the relaxation mechanism for both the spin-glass and low-spin (S=0) regions. A simple model involving these ideas can account for the stretched exponential nuclear magnetization recovery in the nonferromagnetic regions, and permits an estimate to be made of the mean size of low-spin regions.

NMR studies of two unusual f-electron metals

We present the results of nuclear magnetic resonance (NMR) studies down to very low temperatures for two U compounds, UCu3.5Pd1.5 and U0.8Y0.2Pd3, where the conduction electrons experience strong interactions and for which the temperature dependences of the thermal and transport properties do not obey the expectations of a simple Fermi-liquid model. Also the temperature and field dependences of the nuclear magnetization recovery of UCu3.5Pd1.5 in Cu nuclear quadrupole resonance show unusual features that cannot be reconciled with common expectations for a simple metal, but they are well accounted for by the Kondo disorder model. For U0.8Y0.2Pd3, our Y NMR results indicate a distribution of internal static magnetic fields at the Y sites and a small temperature-dependent enhancement of the spin-lattice relaxation rateT 1 − with respect to YPd3. The NMR spectra are consistent with the presence of very small frozen U moments, but the temperature dependence of the spin-lattice relaxation rate indicates a more complicated situation.

NMR evidence for Kondo disorder inUCu3.5Pd1.5

We report on Cu nuclear quadrupole resonance (NQR) and nuclear magnetic resonance experiments probing the cubic non-Fermi-liquid compound ${\mathrm{UCu}}_{3.5}{\mathrm{Pd}}_{1.5}.$ The NQR spectrum indicates a partial disorder in the occupation of Cu and Pd sites, with a preferential occupation of the cubic c site by Pd. Our results for the temperature dependence of the nuclear magnetization recovery show unusual features that cannot be reconciled with common expectations for a simple metal, but they are well accounted for in a description using the Kondo disorder model.

Mixed magnetic and quadrupolar relaxation in the presence of a dominant static Zeeman Hamiltonian

We discuss the multi-exponential nuclear magnetization recovery which occurs in spin-lattice relaxation when NMR lines are split by quadrupole interaction. We treat the general situation in which both magnetic and quadrupolar fluctuations are present and consider three cases differing in their initial conditions: (1) a short radio-frequency pulse is applied selectively to one of the transitions; (2) all lines are saturated at once; (3) a selected line is saturated by continuous waves or by means of a long comb of pulses. Exact solutions are presented for spin I = 1 and I = 3/2, whereas for spin I = 5/2, exact solutions are given for special cases and approximate solutions for the general case. The spin I = 7/2 case is treated for magnetic fluctuations only. The detailed analysis reveals that the form of the recovery law is surprisingly insensitive to an additional relaxation channel, e.g. quadrupolar fluctuations in the presence of predominantly magnetic fluctuations or vice versa.

13C NMR Investigations of Spin–Lattice Relaxation in 99%13C-Enriched Diamonds

The paper is devoted to investigations of spin–lattice relaxation processes in 99%13C-enriched diamonds. Relaxation time measurements were performed as a function of orientation, magnetic field, and temperature. Both experimental results and theoretical discussion are presented. Multiexponential behavior of nuclear magnetization recovery was observed. There was found no significant influence of the diamond orientation on the nuclear spin–lattice relaxation. The field dependence of the spin–lattice relaxation time was found to be proportional to the second power of the magnetic field. The temperature measurements showed a weak increase of the spin–lattice relaxation time with decreasing temperature. Possible mechanisms of impurity relaxation are considered and compared with the experimental data.

Nuclear Relaxation of Hydrogen-Doped Antiferromagnetic YBa2Cu3O6+xHy

Cu(1) NQR, Cu(2) zero field NMR, and proton NMR have been measured in the antiferromagnetic (AF) phase of a powdered sample of hydrogen-doped YBa 2 Cu 3 O 6.1 H 0.14 from 4.2 to 300 K. The line width of 1 H NMR increases abruptly below 20 K. The enhancements of T 2 -1 and T 1 -1 of 63 Cu(1) NQR occur around 20 and 40 K, respectively. The relaxation mechanism of T 1 of NQR changes at T rm =80 K and the time dependence of its nuclear magnetization recovery shows a nonexponential type for T T rm . The predominant source for T 1 below T rm is found to be the fluctuating magnetic fields due to the staggered Cu 2+ moments induced by the hole doping effect. Above T rm , T 1 -1 of NQR increases gradually with temperature but it is almost temperature independent above 150 K, what can not be explained by the effect of the ordered AF Cu 2+ moments in the Cu(2)O 2 plane and also by the Raman process of the lattice vibrations.

Recovery of nuclear magnetization under extreme inhomogeneous broadening.

A quantitative model is presented for the transient recovery of nuclear magnetization under conditions where nuclear spin dipolar relaxation to dilute relaxation centers proceeds without the intermediary of nuclear spin diffusion. The model is developed for rigid arrays in three, two, and one dimensions. Comparison with experimental results yields measures of effective relaxation rates and relaxation center concentrations. {copyright} {ital 1996 The American Physical Society.}

87,85Rb NMR spectra and relaxation in Rb 3C 60

85,87Rb NMR spectra and relaxation are reported in the temperature range 4–300 K, for samples of C 60 intercalated with Rb through a direct-reaction technique at the nominal composition Rb 3C 60. The NMR spectrum is characteristic of the second-order quadrupole powder distribution of the central line, indicating a local non-cubic symmetry at the Rb site and a very small Knight shift. The relaxation rate is found to follow a T 2 law above 30 K. This result, together with the ratio of the relaxation for the two Rb isotopes, points towards a quadrupole relaxation mechanism by phonons. Two contributions were found in the nuclear magnetization recovery which have been attributed to the Rb nuclei in the tetrahedral site and in the octahedral site, respectively, with the nucleus in the T-site coupled more strongly to the lattice dynamics. No detectable change in the NMR spectrum and/or relaxation rates was found at the superconducting transition located at T c = 27.5 K.

NMR excitation and relaxation in a “single domain” spin glass

Zero field NMR measurements of the spin lattice relaxation time T 1 on copper satellite nuclei in a frozen Cu Mn “single domain” spin glass show that the longitudinal nuclear magnetization recovery is not purely exponential. Three possible causes for the observed nonexponentiality have been considered: The distribution of enhancement factors, the differences in neighbourhood experienced by the satellite nuclei and a small quadrupole splitting of Zeeman levels. The latter is shown to be the suitable mechanism. The longitudinal magnetization recovery of silver satellite nuclei in Ag Mn, i.e. in the absence of a quadrupole splitting, was purely exponential. Applying the Redfield theory of relaxation times a comparison of T 1 and T 2 shows that the components of the spectral density function of the fluctuating magnetic field are anisotropic.

Nuclear-spin-lattice relaxation in rhenium metal.

Nuclear-spin-lattice relaxation measurements are presented for $^{187}\mathrm{Re}$ in Re metal as a function of temperature. The relaxation transition probabilities were extracted from the nuclear magnetization recovery curves both in high magnetic field (${H}_{0}$=8 T) nuclear-magnetic-resonance experiments and in nuclear-quadrupole-resonance (${\mathrm{H}}_{0}$=0) experiments. It is found that the dominant relaxation mechanisms is due to magnetic rather then quadrupolar hyperfine interaction with ${W}_{M}$=1.32 T. The data are analyzed in terms of the electronic structure of Re metal. The analysis confirms that Re is a ``weakly enhanced'' transition metal with a nuclear relaxation rate dominated by the s-contact hyperfine interaction.

Stretched-exponential nuclear magnetization recovery in the proton pseudo-spin-glass Rb1-x(NH4)xH2AsO4.

The proton spin-lattice recovery function at high magnetic fields and the nuclear magnetization decay in the rotating frame are nonexponential in the incipient proton glass ${\mathrm{Rb}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$(${\mathrm{NH}}_{4}$${)}_{\mathrm{x}}$${\mathrm{H}}_{2}$${\mathrm{AsO}}_{4}$. The results can be described by a stretched-exponential correlation and magnetization-recovery function with the distribution parameter n in \ensuremath{\alpha}=1-n varying from zero at room temperature to 0.25 at 100 K and 0.4 at temperatures below 12 K. The same values of n are obtained at high magnetic fields and in the rotating frame.

Mean-field theory of nuclear-spin relaxation in the spin-glass phase

Based on Edwards-Anderson mean-field theory a calculation of nuclear-spin relaxation in the spin-glass phase is presented. It is shown that most of the features of the available data can be understood within this mean-field theory. In particular, a nonexponential form of nuclear magnetization recovery is obtained, in agreement with experiment. Further, it is shown that the impurity spin-correlation time ${\ensuremath{\tau}}_{e}$ changes rapidly from $\ensuremath{\hbar}{\ensuremath{\tau}}_{e}^{\ensuremath{-}1}\ensuremath{\gg}{k}_{B}{T}_{g}$ to $\ensuremath{\hbar}{\ensuremath{\tau}}_{e}^{\ensuremath{-}1}\ensuremath{\gg}{k}_{B}{T}_{g}$ around ${T}_{g}$ (${T}_{g}$ is the spin-glass transition temperature), which agrees, qualitatively, with the result obtained from recent ${\ensuremath{\mu}}^{+}$-meson depolarization experiments.

Proton magnetic relaxation in solid poly(L‐alanine), poly(L‐leucine), poly(L‐valine), and polyglycine

AbstractMolecular motion in solid poly(L‐alanine), Poly(L‐leucine), poly(L‐valine), and polyglycine has been investigated through measurement of the portion spin‐lattice relaxation time at 30 and 60 MHz between 110 and 350°K. Rapid random reoriention of sied‐chain methyl groups provides the dominent source of relaxation in the first three; activation energies are 10.5 ± 1 1, 8.5 ± 1 kJ/mol, respectively, significantly lower than in the monomeric crystals. Relaxation times in poyglycine are two orders of magnitude longer than in the monomeric crystals. Relaxation times in polyglycine, significantly lower than in the monomeric crystals. Relaxation times in polyglycine are two orders of magnitude longer and are attributed mainly to segmental motions of the polymer chains. Evidence of nonexponential recovery of nuclear magnetization was encountered in the first three homopolyamino acids but not in polylycine, and was attributed to the correlated time to characterize these motions gave quite good agreement with the data; some improvement was obtained for two polymers using a Cole‐Davidson distribution of correlation times. For biopolymers using a Cole‐Davidson distribution of correlation times. For biopolymers generally it is concluded that rapid methyl group reorientation is a common dynamical feature and an important source of nuclear magnetic relaxation.

Nuclear magnetic resonance of ortho-H2 impurities in solid para-H2 at high pressures

(Received December 2, 1974) We have observed an anomalous broadening of the NMR absorption line of diluted ortho-H z (with rotational angular momentum J :- 1) in solid para-H 2 (J = O) as pressure is applied. Samples with 0.015 < x < 0.06, where x is the ortho mole fraction, were studied over the temperature range 1.2 < T < 4.2 K. The density range covered was Po < P < 1.7po, Po being the density at zero pressure. The observed broadening was much larger than that estimated from intermolecular and intramolecular nuclear dipolar interaction, the latter resulting from short-range orientational molecular ordering caused by electric quadrupole-quadrupole (EQQ) interactions. Furthermore, the "average" longitudinal relaxation time T~ measured from the initial nuclear magnetization recovery after saturation is found to decrease with increasing density, contrary to what is predicted from the relaxation mechanism based on EQQ interaction. Arguments based on NMR measurements taken in solid Dz show that any orientational ordering of the J = 0 molecules caused by an admixture of J = 2 states is too small to lead to polarization of the J = 1 molecules large enough to account for the anomalous line shape. Another possible explanation of the anomaly--rapid clustering of the J = 1 impurities--is also Jound to be unlikely. The anomalously broadened line, with a width proportional to T-1 gt a given density, is discussed in terms of local molecular alignment of the 'J = 1 impurities as produced by a crystalline field V c and that causes an increase In the intramolecular dipolar feld. From the NMR line shape, we can get information on the distribution of such fields throughout the lattice. A crude analysis indicates that the distribution is approximately Gaussian around a characteristic value V,/k e where the width of this distribution is of the order of Vc/k B. At p = 1.7po, [V,l/k n is found to have a value of ~0.6 K, which is about 30 times larger than previous determinations at p = Po. The origins of the anomalously large crystalline field are discussed.