Spin-spin Relaxation Rate Research Articles

Effect of the 16O → 18O isotopic substitution and a magnetic field on the spin dynamics in the (La0.25Pr0.75)0.7Ca0.3MnO3 manganite according to the 139La NMR data

139La nuclear magnetic resonance spectra have been recorded and the spin-spin relaxation times in the paramagnetic region in external magnetic fields of 5 and 9.4 T have been measured for two types of manganites: initial (La0.25Pr0.75)0.7Ca0.3Mn16O3 and enriched with the 18O isotope ((La0.25Pr0.75)0.7Ca0.3Mn18O3). The manganite enriched with the heavier oxygen isotope exhibits disappearance of a signal in the charge-ordering region (T < T co) in an external field of 5 T. This effect is related to the anomalous increase in the spin-spin relaxation rate. With an increase in the external magnetic field to 9.4 T, the difference in the behavior of the relaxation rate for the two samples in the charge-ordering region remains pronounced, although the magnitude of this effect becomes much smaller. The observed giant isotope effect is a bright evidence of the important role of oxygen motion in the formation of long-range magnetic order in the manganites under consideration.

Utilisation des gels de polymères MAGIC pour la dosimétrie d’une source non scellée d’yttrium 90

Polymer gels are relative chemical dosimeters. They allow to access to three-dimensional dose distribution. The aim of this study has been to investigate the preparation and the use of a polymer gel with a tissue equivalent density known as MAGIC gel from magnetic resonance imaging and x-ray computed tomography for non-sealed source dosimetry. This kind of gel is “normoxic” because it can be manufactured and used in normal room atmosphere. In the first part of this study, its accuracy and sensibility were studied using external beam irradiation by photons. Spin-spin relaxation rate ( R 2) and Computed Tomography (CT) number had been used to record gel responses. Using the same manufacture process, radiolabelled gels composed of 95% MAGIC gel and 5% of 90Y termed 90Y-MAGIC 95, with varying activity ranged from 0 to 30 MBq were made. In case of photon external beam irradiation, a linear response is observed whatever the calibration method and the imaging system used (the correlation coefficient r 2 > 0.98 in all cases). 90Y-MAGIC 95 radiolabelled gel responses were recorded after 28, 76 and 124 h. The R 2/dose curves are not linear; three phases can be described, the first being linear with a slow slope (0.14 s −1 Gy −1 instead of 0.41 s −1 Gy −1 for external beam irradiation of the same gel batch). This study shows safety of radiolabelled MAGIC gels manufacturing process and their large dosimetric feasibility. 90Y-MAGIC 95 gel response appears to be reproducible and related to the absorbed dose, thus this gel is a promising tool for non-sealed source dosimetry.

Proton NMR study in hexanuclear manganese single-molecule magnets

We report a detailed proton NMR study, as a function of temperature and external magnetic field, of two hexanuclear manganese magnetic molecule clusters with chemical formula $[{\mathrm{Mn}}_{6}{\mathrm{O}}_{2}{({\mathrm{O}}_{2}\mathrm{C}\mathrm{Me})}_{2}{(\text{salox})}_{6}{(\mathrm{Et}\mathrm{O}\mathrm{H})}_{4}]∙4\mathrm{Et}\mathrm{O}\mathrm{H}$ (in short ${\mathrm{Mn}}_{6}$ acetate) and $[{\mathrm{Mn}}_{6}{\mathrm{O}}_{2}{({\mathrm{O}}_{2}\mathrm{C}\mathrm{Ph})}_{2}{(\text{salox})}_{6}{(\mathrm{Et}\mathrm{O}\mathrm{H})}_{4}]∙4\mathrm{Et}\mathrm{O}\mathrm{H}$ (henceforth ${\mathrm{Mn}}_{6}$ benzoate). Both clusters are characterized by a ferrimagnetic ground state with total spin ${S}_{T}=4$ and a large uniaxial anisotropy, which gives rise to an effective energy barrier for the relaxation of the magnetization of the order of ${U}_{\mathrm{eff}}\ensuremath{\sim}28\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The main characteristics of the $^{1}\mathrm{H}$ NMR spectra (measured between $1.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and room temperature for different fields) are explained in terms of the dipolar hyperfine interaction of the proton nuclei with the adjacent magnetic ions. At low temperatures $(Tl3.5\phantom{\rule{0.3em}{0ex}}\mathrm{K})$, the spectra broaden significantly and become structured due to the slowing down of the local fluctuating fields at the proton sites, caused by the gradual freezing of the ${\mathrm{Mn}}^{3+}$ moments into the ${S}_{T}=4$ collective ground state. The spin dynamics of the exchange coupled magnetic ions was also probed by proton spin-spin relaxation rate ${T}_{2}^{\ensuremath{-}1}$ and spin-lattice relaxation rate ${T}_{1}^{\ensuremath{-}1}$ measurements. On decreasing the temperature, a gradual enhancement of both relaxation rates is observed, followed by a significant decrease of the signal intensity (wipe-out effect). The low frequency regime of the spin fluctuations as probed by ${T}_{1}^{\ensuremath{-}1}$, can be described and analyzed in terms of a single characteristic correlation frequency ${\ensuremath{\omega}}_{c}(T)$, which is interpreted as the lifetime broadening of the discrete magnetic energy levels due to spin-phonon interactions.

Furosemide increases water content in renal tissue

The present study was designed to evaluate the short-term effects of intravenous administration of furosemide on key functions in the kidney cortex and the outer and inner medulla of rats by using magnetic resonance imaging (MRI). Renal tissue water content, renal tissue oxygenation (in relation to the magnetic resonance spin-spin relaxation rate), the apparent diffusion coefficient (ADC) of water, and volume of renal blood flow were measured. Furosemide administration resulted in an increased water content in all regions of the kidney. In parallel with this, we found a significant reduction in ADC in the cortex (2.7 +/- 0.1 x 10(-3) to 2.3 +/- 0.1 x 10(-3) mm(2)/s; P < 0.01) and in the outer medulla (2.3 +/- 0.1 x 10(-3) to 2.0 +/- 0.1 x 10(-3) mm(2)/s; P < 0.01), indicating that the intra- to extracellular volume fraction of water increased in response to furosemide administration. Furosemide also decreased the blood oxygenation in the cortex (49.1 +/- 2.9 to 40.9 +/- 2.0 s(-1); P < 0.01), outer medulla (41.9 +/- 2.8 to 33.2 +/- 1.6 s(-1); P < 0.01) and in the inner medulla (37.1 +/- 2.9 to 26.7 +/- 1.8 s(-1); P < 0.01), indicating an increased amount of oxygenated Hb in the renal tissue. Moreover, renal blood flow decreased in response to furosemide (6.9 +/- 0.2 to 4.4 +/- 0.2 ml/min; P < 0.001). In conclusion, furosemide administration was associated with increased renal water content, an increase in the intra- to extracellular volume fraction of water, an increased oxygen tension, and a decrease in the renal blood flow. Thus MRI provides an integrated evaluation of changes in renal function, leading to decreased renal water and solute reabsorption in response to furosemide, and, in addition, MRI provides an alternative tool to monitor noninvasively changes at the cellular level.

A Model for Agonism and Antagonism in an Ancient and Ubiquitous cAMP-binding Domain

The cAMP-binding domain (CBD) is an ancient and conserved regulatory motif that allosterically modulates the function of a group of diverse proteins, thereby translating the cAMP signal into a controlled biological response. The main receptor for cAMP in mammals is the ubiquitous regulatory (R) subunit of protein kinase A. Despite the recognized significant potential for pharmacological applications of CBDs, currently only one group of competitive inhibitor antagonists is known: the (R(p))-cAMPS family of phosphorothioate cAMP analogs, in which the equatorial exocyclic oxygen of cAMP is replaced by sulfur. It is also known that the diastereoisomer (S(p))-cAMPS with opposite phosphorous chirality is a cAMP agonist, but the molecular mechanism of action of these analogs is currently not fully understood. Previous crystallographic and unfolding investigations point to the enhanced CBD dynamics as a key determinant of antagonism. Here, we investigate the (R(p))- and (S(p))-cAMPS-bound states of R(CBD-A) using a comparative NMR approach that reveals a clear chemical shift and dynamic NMR signature, differentiating the (S(p))-cAMPS agonist from the (R(p))-cAMPS antagonist. Based on these data, we have proposed a model for the (R(p)/S(p))-cAMPS antagonism and agonism in terms of steric and electronic effects on two main allosteric relay sites, Ile(163) and Asp(170), respectively, affecting the stability of a ternary inhibitory complex formed by the effector ligand, the regulatory and the catalytic subunits of protein kinase A. The proposed model not only rationalizes the existing data on the phosphorothioate analogs, but it will also facilitate the design of novel cAMP antagonists and agonists.

Optimization of the T2 parametric image map calculation in MRI polymer gel dosimetry

In polymer gel dosimetry, Magnetic Resonance Imaging (MRI) provides the spin-spin relaxation rate R2=1/T2 image data, which in turn is correlated with the absorbed dose in the irradiated gel dosimeter. The aim of this work is to compare two different mathematical models in determining R2, in terms of precision namely: i) the routinely applied conventional linear regression fitting model, and ii) a new weighted fitting model, which is based on the random noise figure applied for each signal of the T2 decay curve. To investigate the precision of the above models over a wide range of T2 values we have manufactured VIPAR gels with varying gelatine content and irradiated them to known doses.

Three dimensional radiation dosimetry in lung‐equivalent regions by use of a radiation sensitive gel foam: Proof of principle

A polymer hydrogel foam is proposed as a potential three dimensional experimental dosimeter for radiation treatment verification in low-density tissue such as the lung. A gel foam is created by beating a radiation sensitive polymer gel mixture in an anoxic atmosphere. The mass density of the gel foam is in the order of 0.25-0.35 kg/dm3. Both nuclear magnetic resonance (NMR) spin-spin relaxation rate (R2) and magnetization transfer ratio (MTR) have been used to map the dose distribution from the gel dosimeter. It is found that MTR has significant advantages compared to R2 for mapping the dose distribution in the polymer gel foam dosimeters. The magnetization transfer ratio is found to be less dependent on the density and microstructure of the gel foam dosimeter while spin-spin relaxation dispersion has been observed making the spin-spin relaxation rate dependent on the interecho time interval. Optical microscopy reveals a microstructure that shows great similarity with human lung tissue. It is also shown how NMR hydrogen proton density measurements can be used to map the density distributions in gel dosimeters.

Spin and orbital frustration inFeSc2S4probed bySc45NMR

We report on $^{45}\mathrm{Sc}$ NMR experiments in the geometrically frustrated spinels ${\mathrm{FeSc}}_{2}{\mathrm{S}}_{4}$ and ${\mathrm{MnSc}}_{2}{\mathrm{S}}_{4}$. The temperature dependences of line shift $K(T)$, linewidth $\ensuremath{\Delta}(T)$, spin-spin relaxation rate $1∕{T}_{2}(T)$, and spin-lattice relaxation rate $1∕{T}_{1}(T)$ are deduced. The manganese compound reveals the behavior of a long-range ordered antiferromagnet, but with the onset of critical spin fluctuations at unusually high temperatures of 40 ${T}_{N}$. $K(T)$ of ${\mathrm{FeSc}}_{2}{\mathrm{S}}_{4}$ exhibits a cusplike maximum, a hallmark of spin frustration. Orbital fluctuations strongly enhance $1∕{T}_{1}(T)$ at elevated temperatures. Toward lowest temperatures these fluctuations slow down resulting in an increase of $1∕{T}_{2}(T)$. The exponential decrease of $1∕{T}_{1}(T)$ results from the opening of a spin gap $\ensuremath{\Delta}=0.2\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, a characteristic feature of a spin liquid.

Features of low-frequency spin dynamics in manganite LaMnO3 according to 139La NMR data

The spin-lattice and spin-spin relaxation times of 139La are measured in manganite LaMnO3. Analysis of the frequency dependence of the spin-lattice relaxation rate in the paramagnetic temperature range shows that this quantity is determined by magnetic fluctuations. The magnitude of the fluctuating field is estimated. It is shown that the correlation time for spin fluctuations varies with temperature in accordance with the Arrhenius law. The high value of the spin-spin relaxation rate in the paramagnetic region can be due to strong anisotropy of fluctuating magnetic fields at La nuclei.

NMR study of lineshifts and relaxation rates of the one-dimensional antiferromagnetLiCuVO4

NMR measurements were performed on single-crystalline samples of the one-dimensional Heisenberg antiferromagnet (1D HAF) ${\mathrm{LiCuVO}}_{4}$. We investigated $^{7}\mathrm{Li}$ and $^{51}\mathrm{V}$ NMR spectra, deduced the respective lineshifts $K(T)$, spin-lattice relaxation rates $1∕{T}_{1}$, and spin-spin relaxation rates $1∕{T}_{2}$ and $1∕{T}_{2G}$. The results are compared to theoretical predictions for the relaxation rates $1∕{T}_{1}(T)$ and $1∕{T}_{2G}(T)$ deduced from the dynamic susceptibility $\ensuremath{\chi}(\mathbf{q},\ensuremath{\omega})$ of a 1D HAF spin chain as proposed by Sachdev [Phys. Rev. B 50, 13006 (1994)]. We document a correspondence between the anisotropic relaxation rates derived from electron paramagnetic resonance linewidth $\mathrm{\ensuremath{\Delta}}H(T)$ and nuclear magnetic resonance $1∕{T}_{1}(T)$, respectively, the former being well described in the framework of symmetric anisotropic exchange interactions for ${\mathrm{LiCuVO}}_{4}$ by Krug von Nidda et al. [Phys. Rev. B 65, 134445 (2002)].

A Study of H1 NMR Signal from Hydrated Synthetic Sands

Magnetic resonance imaging (MRI) has recently been proposed as an in situ subsurface imaging technique for locating buried objects in fully hydrated soils. This paper explores the available hydrogen [Formula: see text] nuclear magnetic resonance (NMR) signal from water in fully hydrated monodispersed synthetic sands (glass beads) as a predictor of the utility of subsurface MRI. The MRI signal from hydrated soils is a function of three main properties of water in the voids between the soil particles: the spin-lattice relaxation rate [Formula: see text], the spin-spin relaxation rate [Formula: see text], and the spin density [Formula: see text]. The [Formula: see text] values were measured for random close packed synthetic sands as a function of particle diameter. The [Formula: see text] and [Formula: see text], values were measured at a magnetic field strength of [Formula: see text] as a function of particle diameter. [Formula: see text] was also measured as a function of magnetic field strength at 0.235, 2.35, 23.5, and [Formula: see text]. The synthetic sands studied have a predictable [Formula: see text] and [Formula: see text], and a constant [Formula: see text]. The water in these sands also has a predictable [Formula: see text] as a function of applied magnetic field. Therefore, fully hydrated monodispersed real sands are expected to have a predictable NMR signal.

Phase Separation inA-Site-Ordered Perovskite ManganiteLaBaMn2O6Probed byLa139andMn55NMR

$^{139}$La- and $^{55}$Mn-NMR spectra demonstrate that the ground state of the A-site ordered perovskite manganite LaBaMn$_2$O$_6$ is a spatial mixture of the ferromagnetic (FM) and antiferromagnetic (AFI(CE)) regions, which are assigned to the metallic and the insulating charge ordered state, respectively. This exotic coexisting state appears below 200 K via a first-order-like formation of the AFI(CE) state inside the FM one. Mn spin-spin relaxation rate indicates that the FM region coexisting with the AFI(CE) one in LaBaMn$_2$O$_6$ is identical to the bulk FM phase of the disordered form La$_{0.5}$Ba$_{0.5}$MnO$_3$ in spite of the absence of A-site disorder. This suggests mesoscopic rather than nanoscopic nature of FM region in LaBaMn$_2$O$_6$\@.

R2 relaxometry with MRI for the quantification of tissue iron overload in β‐thalassemic patients

To evaluate the usefulness of a time-efficient MRI method for the quantitative determination of tissue iron in the liver and heart of beta-thalassemic patients using spin-spin relaxation rate, R2, measurements. Images were obtained at 1.5 T from aqueous Gd-DTPA solutions (0.106-8 mM) and from the liver and heart of 46 beta-thalassemic patients and 10 controls. The imaging sequence used was a respiratory-triggered 16-echo Carr-Purcell-Meiboom-Gill (CPMG) spin-echo (SE) pulse sequence (TR = 2000 msec, TE(min) = 5 msec, echo spacing (ES) = 5 msec, matrix = 192 x 256, slice thickness = 10 mm). Liver iron concentration (LIC) measurements were obtained for 22 patients through biopsy specimens excised from the relevant liver segment. Biopsy specimens were also evaluated regarding iron grade and fibrosis. Serum ferritin (SF) measurements were obtained in all patients. A statistically significant difference was found between patients and healthy controls in mean liver (P < 0.004) and myocardium (P < 0.004) R2 values. The R2 values correlated well with Gd DTPA concentration (r = 0.996, P < 0.0001) and LIC (r = 0.874, P < 0.0001). A less significant relationship (r = 0.791, P < 0.0001) was found between LIC measurements and SF levels. R2 measurements appear to be significantly affected (P = 0.04) by different degrees of hepatic fibrosis. The patients' liver R2 values did not correlate with myocardial R2 values (r = 0.038, P < 0.21). Tissue iron deposition in beta-thalassemic patients may be adequately quantified using R2 measurements obtained with a 16-echo MRI sequence with short ES (5 msec), even in patients with a relatively increased iron burden.

Magnetic phase transition at88KinNa0.5CoO2revealed byNa23NMR investigations

Na0.5CoO2 exhibits a metal-insulator transition at 53 K upon cooling. The nature of another transition at 88 K has not been fully clarified yet. We report the results of measurements of the electrical conductivity, the magnetic susceptibility and 23Na NMR on a powder sample of Na0.5CoO2, including the mapping of NMR spectra, as well as probing the spin-lattice relaxation rate and the spin-spin relaxation rate, in the temperature range between 30 K and 305 K. The NMR data reflect the transition at T_X very well but provide less evidence for the metal-insulator transition at T_MI. The temperature evolution of the shape of the spectra implies the formation of a staggered internal field below T_X, not accompained by a rearrangement of the electric charge distribution. Our results thus indicate that in Na0.5CoO2, an unusual type of magnetic ordering in the metallic phase precedes the onset of charge ordering, which finally induces an insulating ground state.

NMR evidence of hidden order in the high-temperature phase of (TaSe4)2I

(TaSe 4 ) 2 I has a quasi-one-dimensional electronic structure with charge-density wave (CDW) ground state forming in a phase transition at Tp = 265 K. Here we report on 77 Se nuclear magnetic resonance (NMR) spectrum and spin-spin relaxation time (T 2 ) measurements in the temperature range of 100 to 380 K. The line positions are shown to couple to the CDW order parameter. The spin-spin relaxation rate displays a sharp minimum at Tp; signs of critical slowing down are absent. Together with the fast increase of the order parameter, these findings are consistent with a true gap due to hidden order in the high-temperature phase.

Non-Arrhenius conductivity in the fast lithium conductorLi1.2Ti1.8Al0.2(PO4)3: ALi7NMR and electric impedance study

Lithium ion motion has been investigated in the ${\mathrm{Li}}_{1.2}{\mathrm{Ti}}_{1.8}{\mathrm{Al}}_{0.2}{(\mathrm{P}{\mathrm{O}}_{4})}_{3}$ compound with NMR and impedance spectroscopies. From the analysis of the bulk dc conductivity $({\ensuremath{\sigma}}_{b,\mathrm{dc}})$ and the frequency at the maximum $({\ensuremath{\omega}}_{bp})$ of the imaginary electric modulus, two regimes for lithium motion have been identified. From the analysis of the temperature dependence of $^{7}\mathrm{Li}$ NMR quadrupole constant $({C}_{Q})$ and spin-spin relaxation rate $({T}_{2}^{\ensuremath{-}1})$, the evolution of ${M}_{1}$ and ${M}_{2}$ sites occupancy in the conduction network of the fast ion conductor has been deduced. At low temperatures $(T&lt;250\phantom{\rule{0.3em}{0ex}}\mathrm{K})$, differences in activation energies deduced from NMR $({E}_{m}^{R}=0.19\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ and conductivity $({E}_{M}^{C}=0.31\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ are discussed in terms of a correlated lithium motion. In this regime, the $\ensuremath{\beta}$ parameter that relates both energies $({E}_{m}=\ensuremath{\beta}{E}_{M})$ takes the value 0.61. In the high temperature regime $(T&gt;250\phantom{\rule{0.3em}{0ex}}\mathrm{K})$, the activation energy ${E}_{M}$ deduced by the two techniques show similar values $(0.22\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$. In this case the $\ensuremath{\beta}$ parameter is nearly equal to 0.85. From the NMR results, it has been concluded that the partial cancellation of the correlation in lithium motion is associated with the creation of vacancies at ${M}_{1}$ sites. On the other hand, some differences in $\ensuremath{\beta}$ and ${\ensuremath{\tau}}_{0}$ parameters have been detected, indicating that relaxation functions are slightly different in two techniques. In particular, the NMR ${\ensuremath{\beta}}^{R}$ correlation factor is slightly higher and the NMR ${\ensuremath{\tau}}_{0}^{R}$ parameter is almost one order of magnitude higher than that deduced from electric measurements.

NMR Studies of Exotic Members of Bechgaard Salt with Non- centrosymmetric Anions under Pressure

The electronic properties of (TMTSF) 2FSO 3 have been investigated under pressure of 0.65 GPa by measuring 77Se-NMR lineshape, spin-lattice relaxation rate ( T 1 − 1 ) and spin-spin relaxation rate ( T 1 − 1 ) measurements on a single crystal sample. The NMR signal was found abnormally broadened below 90 K. Below 40 K, a narrow component appears and coexists with the broader one. The temperature dependence of T 1 − 1 suggests that the broader component comes from a metallic phase and the narrow one from an insulating phase. The activation energy determined for the insulating component was found three times lower than the value found at ambient pressure. From the T 1 − 1 measurements, the broadening of the metallic component was confirmed as inhomogeneous. An unusual double peak anomaly of T 1 − 1 suggests the existence of extremely slow fluctuations of the inhomogeneous local fields below 90 K at this pressure.

Proton NMR wipeout effect due to slow fluctuations of the magnetization in single molecule magnets

We report a detailed $^{1}\mathrm{H}$ NMR study on the spin dynamics of single molecule magnets as a function of temperature and external magnetic field. A gradual loss of the $^{1}\mathrm{H}$ NMR signal intensity (wipeout effect) is observed on decreasing the temperature for all the investigated ferromagnetic clusters. This effect is accompanied by a simultaneous enhancement of the spin-spin and spin-lattice relaxation rate ${T}_{2}^{\ensuremath{-}1}$ and ${T}_{1}^{\ensuremath{-}1}$, respectively. The complications entered in the interpretation of the signal loss by the wipeout effect are overcome, and the information about the spin dynamics is retrieved, by implementing a simple and intuitive model that captures the main physical characteristics of the problem and reveals a universal behavior of the spin dynamics for all the clusters. According to our analysis the origin of the wipeout effect as well as the enhancement of the relaxation rates ${T}_{1}^{\ensuremath{-}1}$ and ${T}_{2}^{\ensuremath{-}1}$ in the FM clusters is related to a decrease of the lifetime broadening parameter of the magnetic energy levels, down to the range of the $^{1}\mathrm{H}$ Larmor frequency. The temperature dependence of the lifetime broadening can be described at intermediate temperatures by a power law dependence on $T$ similar to that observed in antifferomagnetic rings [S. H. Baek et al., Phys. Rev. B 70, 134434 (2004)].

NMR relaxation parameters of a Lennard-Jones fluid from molecular-dynamics simulations

Ensembles of soft spheres or of Lennard-Jones atoms were studied by molecular dynamics at reduced temperatures from 0.8 to 3, and radial distribution functions, diffusion coefficients, and magnetic dipole-dipole correlation functions were measured as functions of system size. The expected relation between the values of the correlation functions at zero lag time and the integrals of the radial distribution was verified for each system. The measured correlation functions were compared with theoretical expressions derived by [Ayant et al., J. Phys. (Paris) 36, 991 (1975)] and by [Hwang and Freed, J. Chem. Phys. 63, 4017 (1975)]. It was shown that, in order to recover the long-time behavior characteristic of diffusion-controlled relaxation processes, the simulation must comprise at least 10 000 particles. By fitting the simulation results to the Hwang-Freed function, independent values of the diffusion coefficient were obtained, similar but not identical to those computed using the Green-Kubo formalism. The spectral densities of the dipole-dipole interaction were computed as Fourier transforms of the correlation functions. These quantities are less sensitive to model imperfections and reproduce quite well the values derived from theory. The dimensionless spin-lattice and spin-spin relaxation rates were derived from the spectral densities. It was shown that the spin-lattice (longitudinal) relaxation rate goes through a maximum as the temperature increases, while the spin-spin (transverse) rate decreases monotonously.

Fe57NMR and relaxation by strong collision in the tunneling regime in the molecular nanomagnet Fe8

$^{57}\mathrm{Fe}$ NMR measurements have been performed in single crystal and oriented powder of enriched $^{57}\mathrm{Fe}8$ molecular cluster in the temperature range $0.05--1.7\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in zero external field and with small perturbing field up to $1\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ for both transverse and longitudinal orientation of $H$ with respect to the anisotropy axis. The $^{57}\mathrm{Fe}$ NMR spectrum is analyzed in terms of a dominant contribution due to the hyperfine interaction arising from core polarization. The measured temperature dependence of the resonance frequency is explained well by calculating the local average magnetic moment of the ${\mathrm{Fe}}^{3+}$ ion with a simple model which incorporates the effects of thermal average in the low lying energy states. Nuclear spin-lattice relaxation rate $(1∕{T}_{1})$ and spin-spin relaxation rate $(1∕{T}_{2})$ were investigated via temperature and field dependences. The obtained results are analyzed in terms of both intrawell thermal fluctuations of the hyperfine fields due to spin-phonon interaction, and interwell fluctuations due to phonon assisted quantum tunneling of the magnetization. It is argued that in zero external field and at low $T$ the $^{57}\mathrm{Fe}$ and the proton $1∕{T}_{1}$ is dominated by a strong collision relaxation mechanism due to the fact that phonon assisted tunneling transitions generate a sudden reversal of the local quantization field at the nuclear site. The data could be explained satisfactorily by assuming that the $^{57}\mathrm{Fe}$ $1∕{T}_{1}$ measures directly the effective tunneling rate. However, in order to fit the data we had to assume a larger in-plane anisotropy than previously reported, resulting in a bigger tunneling splitting in zero field. A comparison with published data of $^{55}\mathrm{Mn}$ in Mn12 indicates that a strong collision relaxation mechanism may apply also in Mn12. Finally the $H$ and $T$ dependence of $^{57}\mathrm{Fe}$ $1∕{T}_{2}$ is well explained simply in terms of thermal fluctuations of the magnetization without any tunneling contribution. At very low $T$ the $1∕{T}_{2}$ approaches a limiting value which can be explained in terms of the dipolar interaction between proton and $^{57}\mathrm{Fe}$ nuclei in the quasistatic regime.