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The divergences, on cooling, of NMR-NQR relaxation rates and muon spin rotation in underdoped cuprates, until now interpreted in terms of glassy spin freezing of magnetic moments resulting from charge inhomogeneities, are critically reconsidered. The relaxation data are analyzed in the light of the lack of dependence from an external magnetic field, of the strength of the effective field driving the divergences and of the stretched exponential character of the recovery processes. By resorting also to the theoretical conclusions of the extended t-J model including the intersite Coulomb interactions, it is argued that the divergences of the relaxation rates, rather than arising from magnetic moments could be related to sliding motions of vortex-antivortex orbital currents coexisting with d-wave superconducting state.

The spin dynamics of the molecular magnetic chain [Dy(hfac)(3){NIT(C6H4OPh)}] were investigated by means of the Muon Spin Relaxation (mu+SR) technique. This system consists of a magnetic lattice of alternating Dy(III) ions and radical spins, and exhibits single-chain-magnet behavior. The magnetic properties of [Dy(hfac)(3){NIT(C6H4OPh)}] have been studied by measuring the magnetization vs. temperature at different applied magnetic fields (H - 5, 3500, and 16500 Oe) and by performing mu+SR experiments vs. temperature in zero field and in a longitudinal applied magnetic field H - 3500 Oe. The muon asymmetry P(t) was fitted by the sum of three components, two stretched-exponential decays with fast and intermediate relaxation times, and a third slow exponential decay. The temperature dependence of the spin dynamics has been determined by analyzing the muon longitudinal relaxation rate lambda(interm)(T), associated with the intermediate relaxing component. The experimental lambda(interm)(T) data were fitted with a corrected phenomenological Bloembergen-Purcell-Pound law by using a distribution of thermally activated correlation times, which average to tau - tau(0) exp(Delta/k(B)T), corresponding to a distribution of energy barriers Delta. The correlation times can be associated with the spin freezing that occurs when the system condenses in the ground state. (C) 2015 AIP Publishing LLC.

89 Y NMR relaxation has been used to study the spin freezing process in Y 1-x Ca x Ba 2 Cu 3 O 6.1. Substituting Ca 2+ for Y 3+ in the parent antiferromagnetic YBa 2 Cu 3 O 6.1 leads the system in the underdoped superconducting phase. In the normal state of the sample at x=0.15 the spin-lattice relaxation rate, which is not affected by the antiferromagnetic correlation of the Cu 2+ magnetic moments, still evidences a decrease of (1/ T 1 T ) on cooling, indicative of a pseudo-gap opening in the density of states around the Fermi energy. The most relevant result is found in the superconducting phase, where the recovery of the 89 Y NMR signal displays two distinct laws, pointing out different relaxation mechanisms without common spin temperature. One exponential recovery law yields a decay rate which decreases on cooling, as expected in the superconducting phase. The second relaxation process turns out to be described by a stretched exponential, with fast relaxation rates, strongly increasing on cooling, with a characteristic spin fluctuations frequency reaching the 20 MHz range at about 8 K. This observation is interpreted as the direct experimental evidence of the coexistence, at mesoscopic level, of superconductive and spin-glass like phases, the spin freezing process being described by the fast rate of the nuclear relaxation.

The anharmonic vibrational dynamics in nearly stoichiometric ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4+\ensuremath{\delta}}$ is studied by means of anelastic and ${}^{139}\mathrm{La}$ NQR relaxation. In the absorption component of the elastic susceptibility as well as in the nuclear relaxation rate a peak is detected as a function of temperature, and a relaxation time $\ensuremath{\tau}=1.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}\mathrm{exp}[(2800\phantom{\rule{0ex}{0ex}}\mathrm{K})/T]$ s is derived. The relaxation processes are attributed to tilt motion of the ${\mathrm{CuO}}_{6}$ octahedra in double-well potentials, whose cooperative character increases the effective energy barrier to the observed value. The analysis of the relaxation mechanisms has been carried out by reducing the dynamics of the interacting octahedra to a one-dimensional equation of motion. The solitonlike solutions correspond to parallel walls separating domains of different tilt patterns and give rise to pseudodiffusive modes which appear as a central component in the spectral density of the motion of the octahedra. The tilt waves may be considered to correspond to the dynamical lattice stripes observed in La-based and Bi-based high-${T}_{\mathrm{c}}$ superconductors.

The study of thermal motion of flux lines as evidenced in NMR measurements is reviewed in the light of our recent results on the nuclear spin-lattice and spin-spin relaxation rates in anisotropic HT c SC YBa 2 Cu 4 O 8 and HgBa 2 CuO 4+δ

The spin-lattice relaxation time T1, the spin echo dephasing time T2 and the NMR linewidth δv are used to study the spontaneous fluctuations under thermal excitation of the flux lines (FL) in YBa2Cu4O8. It is shown how the relaxation rate T1-1 is sensitive to the fluctuations of the transverse components of the local field at a frequency around ω = γH0, while T2-1 and δv probe the longitudinal components at frequencies smaller by a factor 103-105. Thus, from a combination of T1 T2 andδv measurements one derives microscopic insights on the degree of correlation in the motion of 2D pancakes belonging to adjacent superconducting planes, on the barriers for the pseudo-diffusion of the FL’s and on their field and temperature dependence. 2D NMR spectroscopy can also be used to evidence the diffusion of the FL’s and a comparison between YBa2Cu3O7 (YBCO123) and YBa2Cu48 (YBCO124) is briefly discussed. Finally for temperatures below Tirr(H the evolution of the amplitude of the63Cu NQR echo after sudden application and/or rotation of a magnetic field is used to investigate the FL flow in the critical state.

Measurements of the $^{89}\mathrm{Y}$ NMR linewidth \ensuremath{\Delta}\ensuremath{\nu} and spin-lattice relaxation rate ${\mathit{T}}_{1}^{\mathrm{\ensuremath{-}}1}$ have been performed in an oriented powder sample of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{4}$${\mathrm{O}}_{8}$ (${\mathit{T}}_{\mathit{c}}$ = 81 K) for both ${\mathbf{H}}_{0}$\ensuremath{\parallel}c and ${\mathbf{H}}_{0}$\ensuremath{\perp}c at ${\mathit{H}}_{0}$ = 9.4 and 5.9 T. Evidence for the effects of thermal motion of flux lines (FL's) below ${\mathit{T}}_{\mathit{c}}$ is presented and the microscopic correlation times of the FL motion are derived by using a simple model. From the temperature and field dependence of ${\mathit{T}}_{1}^{\mathrm{\ensuremath{-}}1}$ an effective correlation time ${\mathrm{\ensuremath{\tau}}}_{\mathit{e}}$ describing the pseudodiffusion of two-dimensional (2D) pancake vortices is extracted. The effective activation energy of the FL motion is found to be inversely proportional to the magnetic field, supporting the similar conclusion recently obtained in ${\mathrm{HgBa}}_{2}$${\mathrm{CuO}}_{4+\mathrm{\ensuremath{\delta}}}$. From the temperature dependence of \ensuremath{\Delta}\ensuremath{\nu} another correlation time \ensuremath{\tau}, different from ${\mathrm{\ensuremath{\tau}}}_{\mathit{e}}$, is obtained. It is argued that the correlation times derived from \ensuremath{\Delta}\ensuremath{\nu} and ${\mathit{T}}_{1}^{\mathrm{\ensuremath{-}}1}$, respectively, refer to different motional regimes below and above the irreversibility temperature ${\mathit{T}}_{\mathrm{irr}}$. \textcopyright{} 1996 The American Physical Society.

{sup 199}Hg NMR spin-lattice relaxation rates in an aligned powder sample of the anisotropic high-{ital T}{sub {ital c}} superconductor HgBa{sub 2}CuO{sub 4+{delta}} ({ital T}{sub {ital c}}=96 K) show anisotropy in the temperature interval 10{endash}50 K, with an extra contribution present for the external magnetic field parallel to the {ital c} axis. The extra contribution shifts with magnetic field in a way related to the irreversibility temperature {ital T}{sub irr}, indicating that the thermal motion of flux lines (FL{close_quote}s) is the cause of the extra relaxation. A simple model is presented to analyze the relaxation data and to extract values for the correlation time {tau}{sub {ital c}} and the effective activation energy {ital U} of the FL motion. {copyright} {ital 1996 The American Physical Society.}

{sup 35}Cl nuclear spin-lattice relaxation rate (NSLR) and magnetic susceptibility ({chi}) measurements are reported for Sr{sub 2}CuO{sub 2}Cl{sub 2} single crystals as functions of temperature and of magnetic field intensity and orientation. From the anisotropy of the {sup 35}Cl NSLR and {chi} we obtained evidence for a crossover of the Cu{sup 2+} spin correlations from Heisenberg to {ital XY}-like at {ital T}{approx}290 K. For H{parallel}{ital c} the 3D antiferromagnetic transition is at {ital T}{sub {ital N}}=257 K and is field independent while for H{parallel}{ital ab} {ital T}{sub {ital N}} increases to 265 K at {ital H}=8.2 T. The field-dependent {ital T}{sub {ital N}} for H{parallel}{ital ab} is explained by a field-induced Ising-like anisotropy in the {ital ab} plane.

$^{139}\mathrm{La}$ NQR and zero-field ${\mathrm{\ensuremath{\mu}}}^{+}$SR in antiferromagnetic (AF) ${\mathrm{La}}_{2}$${\mathrm{Cu}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Zn}}_{\mathit{x}}$${\mathrm{O}}_{4}$, for x up to 0.13 and in the temperature range 1.6--350 K, are used to study the effects related to the substitution of magnetic ${\mathrm{Cu}}^{2+}$ S=1/2 with homovalent diamagnetic S=0 ${\mathrm{Zn}}^{2+}$ in ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$. We report measurements both of static magnetic properties, such as N\'eel temperatures ${\mathit{T}}_{\mathit{N}}$, sublattice magnetization and field \ensuremath{\Vert}h\ensuremath{\Vert} at the La nucleus or at the ${\mathrm{\ensuremath{\mu}}}^{+}$ site, as well as of NQR relaxation rates W. These quantities are used to study the effects of Zn doping on the low-energy ${\mathrm{Cu}}^{2+}$ spin excitations. It is found that ${\mathit{T}}_{\mathit{N}}$ decreases with x in a way close to the one expected by diluting quasi-two-dimensional Heisenberg magnets on square lattice, while the sublattice magnetization is slightly affected by Zn doping. Mean-field arguments based on the dilution model for the interplanar interactions allow one to conclude that the in-plane magnetic correlation length is little sensitive to the Zn presence. Up to x\ensuremath{\simeq}0.08 the temperature dependence of the AF field \ensuremath{\Vert}h\ensuremath{\Vert} is close to the one in pure ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$, with a sharp decrease for T\ensuremath{\rightarrow}${\mathit{T}}_{\mathit{N}}^{\mathrm{\ensuremath{-}}}$ indicative of a continuous transition with a small critical exponent \ensuremath{\beta}.For strong doping the low-temperature dependence of \ensuremath{\Vert}h\ensuremath{\Vert} appears to depart from the one in pure ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$. For x\ensuremath{\ge}0.05 both NQR spectra and \ensuremath{\mu}SR reveal the presence of regions where the long-range AF order is suppressed. For temperature above 100 K up to ${\mathit{T}}_{\mathit{N}}$ the $^{139}\mathrm{La}$ relaxation rate W due to the ${\mathrm{Cu}}^{2+}$ spin fluctuations shows only slight corrections with respect to pure ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ which are possibly related to the disorder in the AF interactions or to finite-size effects. A novel and remarkable effect of Zn doping is the appearance in W, for T\ensuremath{\le}100 K, of large and marked maxima, which are x dependent. This phenomenon is attributed to the cooperative freezing of local magnetic moments induced by Zn on Cu orbitals, interacting via the underlying AF matrix. The maxima in W occur when the fluctuation frequencies of the anomalous spins become of the order of the NQR frequency, thus driving the system to a spin-glass state superimposed to the AF matrix.

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 HgBa2CuO4+δ high temperature superconductor is discussed.

[sup 63]Cu nuclear quadrupole resonance (NQR), nuclear antiferromagnetic resonance (AFNMR), and spin-lattice relaxation, as well as [sup 7]Li NMR and relaxation measurements in CuO and in Cu[sub 1[minus][ital x]]Li[sub [ital x]]O, are used to study the spin dynamics in the paramagnetic and in the antiferromagnetic (AF) phase, and in particular the effects induced by lithium doping. It is argued that Li enters in CuO lattice up to a doping [ital x] around 4%; from a comparison of the electric-field gradient (EFG) and of the dipolar field at the nucleus with theoretical estimates, it appears that Li[sup +] is slightly displaced from the Cu lattice site. From NQR, NMR, and AFNMR spectra, information on the Cu hyperfine interaction and the EFG are derived. The Neel temperature [ital T][sub [ital N]] is affected by Li doping to a moderate extent, decreasing from 226 K in CuO to [ital T][sub [ital N]]=183 K for [ital x]=3.7%. The relaxation rates, driven by the time-dependent part of the magnetic electron-nucleus Hamiltonians, indicate that the Cu[sup 2+] spin dynamics is almost unaffected by Li doping for [ital T][much gt][ital T][sub [ital N]] and possibly controlled by valence fluctuations. The Li doping modifies in a dramatic waymore » the spin fluctuations in the AF phase.« less

We study the phonon-induced relaxation dynamics in the heterometallic ${\text{Cr}}_{7}\text{Ni}$ ring. By assuming that spin-phonon coupling occurs through the modulation of local crystal fields, we investigate the decay of time correlations of molecular observables, such as fluctuations of the cluster magnetization. Single-crystal $^{1}\text{H}$ nuclear magnetic resonance measurements have been performed for two orthogonal orientations of the applied magnetic field. The nuclear spin-lattice relaxation rate $1/{T}_{1}$ probes fluctuations of specific molecular observables, thus allowing to determine the magnetoelastic coupling strength and to calculate the magnetization relaxation time as well as phonon-induced level lifetimes. The knowledge of these time scales is crucial to control the spin dynamics of the magnetic cluster in view of the potential technological applications.

NMR and NQR spectra and spin-lattice relaxation measurements carried out in LASCO and YBCO-type crystals are presented and analyzed in order to derive insights on the correlations and spin-dynamics of the Cu2+ ions and on the microscopic mechanisms of high-T c superconductivity. As an illustrative example on how the magnetic correlation length and spin dynamics properties can be extracted from the relaxation rateW, the35Cl NMR data in the two-dimensional Heisenberg system Sr2CuCl2O2, around the paramagnetic-antiferromagnetic (PA-AF) transition are first considered. Then the139La NQR relaxation measurements in La2−xSrxCuO4 are briefly reviewed and it is shown how a simple picture of localized Cu2+ magnetic moments, whose spin fluctuation times are controlled by the charge defects induced by the doping, leads in a direct way to quantitative estimates for the progressive shift, on cooling, of the spectral density of the low-frequency spin excitations towards the high frequency range. This phenomenon can be described in terms of effective spin at the Cu2+ ions, and its similarities with the analogous effect of progressive delocalization in Heavy Fermions systems are pointed out. Thus, the superconducting transition appears to occur in an unconventional Fermi liquid with AF correlations among itinerant pseudoparticles, possibly involving a mechanism not directly related to the magnetic correlated dynamics. In fact, a universal behavior of the relaxation rates as a function of temperature is observed, regardless of the transition temperatureT c. The independence ofT c from the low frequency static and dynamical spin properties is also indicated by89Y Knight shifts and from63Cu relaxation rates in systems like YBa2Cu4O8 (Y124), whereT c can be changed by atomic substitutions and by controlling the oxygen stoichiometry. The effect of an external magnetic field on the correlated spin dynamics of the AF Fermi liquid is investigated and from a comparison of Cu NQR relaxation and NMR relaxation in oriented powder of YBCO and LASCO it is shown that the external field has the small but unambiguous effect of depressing the relaxation rates aboveT c, besides strongly enhancing them in the superconducting phase. A maximum in the ratio $${{W\left( {NQR} \right)} \mathord{\left/ {\vphantom {{W\left( {NQR} \right)} {W\left( {\vec H\left\| {\vec c} \right.} \right)}}} \right. \kern-\nulldelimiterspace} {W\left( {\vec H\left\| {\vec c} \right.} \right)}}$$ is thus observed around 80 K, either in LASCO or in YBCO, again indicating that the transition could be driven by a mechanism not directly involving the spin dynamic properties. To study the role of the fluxions belowT c 89Y NMR shifts and spectra in oriented powders of YBCO are analyzed. Information on the spin susceptibility and on the structure of the vortex lattice is obtained. In addition, from the temperature behavior of the linewidth a motional narrowing related to flux melting is evidenced. The effective correlation time for the vortex motion is derived and it is discussed why μ+SR cannot detect it in view of the different rigid-lattice line broadening.

Measurements of the $^{35}\mathrm{Cl}$ nuclear-spin\char21{}lattice relaxation rate (NSLR) are reported for ${\mathrm{Sr}}_{2}$${\mathrm{CuO}}_{2}$${\mathrm{Cl}}_{2}$ around the antiferromagnetic transition temperature ${\mathit{T}}_{\mathit{N}}$=260\ifmmode\pm\else\textpm\fi{}0.5 K. The temperature dependence of the ${\mathrm{Cu}}^{2+}$ spin correlation length \ensuremath{\xi}(T) is obtained indirectly by using the conventional description of NSLR and scaling arguments and compared with the \ensuremath{\xi}(T) obtained in ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ by neutron scattering. In both systems \ensuremath{\xi}(T) follows the two-dimensional Heisenberg behavior renormalized by quantum fluctuations except close to ${\mathit{T}}_{\mathit{N}}$ where a crossover to a power-law T dependence is observed in ${\mathrm{Sr}}_{2}$${\mathrm{CuO}}_{2}$${\mathrm{Cl}}_{2}$.

Nuclear electric quadrupole measurements of {sup 63}Cu nuclear spin-lattice relaxation rates ({ital R}{sub 1}) in zero applied magnetic field and nuclear magnetic resonance {ital R}{sub 1} in 8.2 T are reported for YBa{sub 2}Cu{sub 3}O{sub 7} and La{sub 1.85}Sr{sub 0.15}CuO{sub 4}. A direct effect of the magnetic field is seen for the first time both above and below {ital T}{sub {ital c}}. Above {ital T}{sub {ital c}} the effect is ascribed to the partial suppression of antiferromagnetic (AF) spin fluctuations. For {ital T}{lt}{ital T}{sub {ital c}} the effect goes in the opposite sense and may be related to the spin-temperature diffusion for the core of the fluxons or to the enhancement of AF spin fluctuations persisting in the superconducting phase.

Primarily motivated by the similarities between the underdoped superconducting cuprates and the granular systems in regards of electric conductivity, phase fluctuations of the order parameter, and nuclear spin-lattice relaxation, a study has been carried out in a NbN(111) textured film at controlled granularity by means of superconducting quantum interference device magnetization and $^{93}\text{N}\text{b}$ NMR measurements. The Meissner diamagnetism in zero-field-cooling and field-cooling conditions and for different orientation of the magnetic field and the isothermal magnetization curves around the superconducting transition temperature ${T}_{\text{c}}$, are studied. $^{93}\text{N}\text{b}$ spectra and relaxation measurements have been performed for two values of the external magnetic field in parallel and perpendicular geometry, in the temperature range 4\char21{}300 K. In the superconducting phase the experimental findings for the textured film are similar to the one in bulk NbN. The nuclear spin-lattice relaxation process is the same as in bulk NbN in the temperature range 50\char21{}300 K, confirming a dominant contribution to the density of states at the Fermi energy arising from the Nb $4d$ band. At variance, on cooling from about 40 K down to ${T}_{\text{c}}$ $(H)$, the $^{93}\text{N}\text{b}$ relaxation rate in the film dramatically departs from the expected behavior for the Fermi gas and mimics the opening of a spin gap. The interpretation of the spin-gap opening in terms of depletion in the density of states at the Fermi energy can justify the anomalous temperature behavior of the $^{93}\text{N}\text{b}$ relaxation rate on approaching ${T}_{\text{c}}$ $(H)$ from above. The experimental findings suggest the occurrence of superconducting fluctuations (density-of-states term) in one-dimensional regime, coupled to a reduction in the time of flight of the electrons, both effects being related to the granularity. The data also suggest that the spin-gap phase in underdoped cuprates could be connected more to granularity, rather than to exotic mechanisms of magnetic origin.

The temperature and frequency dependence of the $^{11}\mathrm{B}$ nuclear spin-lattice-relaxation rate has been investigated in a number of ionic glassy conductors of general composition (AgI${)}_{\mathit{x}}$(${\mathrm{Ag}}_{2}$O\ensuremath{\cdot}n${\mathrm{B}}_{2}$${\mathrm{O}}_{3}$${)}_{1\mathrm{\ensuremath{-}}\mathit{x}}$, with n=1, 2 and variable x. At Tg100 K a BPP-type maximum in ${\mathit{T}}_{1}^{\mathrm{\ensuremath{-}}1}$ is found that is consistent with a thermally activated reorientational motion of ${\mathrm{BO}}_{4}$ groups. [BPP indicates Bloembergen, Percell, and Pound, Phys. Rev. 73, 679 (1948).] It is found that the sum of the activation energy for ${\mathrm{BO}}_{4}$ motion with the activation energy recently measured for ${\mathrm{Ag}}^{+}$ local motion is equal to the activation energy derived from conductivity measurements. At low temperature one observes the usual quasilinear temperature dependence of ${\mathit{T}}_{1}^{\mathrm{\ensuremath{-}}1}$ and an unusual frequency dependence not described by a power law. An interpretative model of nuclear relaxation due to disorder modes is developed to explain the frequency dependence. The analysis of the data indicates the presence of a relatively large number of slowly fluctuating two-level systems, formed by coupled ${\mathrm{BO}}_{4}$ units, responsible for the frequency dependence of the experimental relaxation rate.

We present muon spin relaxation and $^{1}H$ NMR results on the magnetically frustrated keplerate molecule ${{\mathrm{Mo}}_{72}{\mathrm{Fe}}_{30}}$, aimed at studying the local spin dynamics as a function of temperature. We find that, in common with other molecular magnets, the relaxation spectrum of this material is characterized by a single dominating electronic correlation time $\ensuremath{\tau}$. Experiments and theory show that $\ensuremath{\tau}$ has a thermally activated behavior with an activation gap of the order of the energy difference between the lowest-lying rotational bands in this material. This shows that, in the intermediate temperature range just above $1\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the relaxation of the electronic spin system occurs via Orbach processes involving energy levels belonging to the two lowest-lying rotational bands. Our data thus provide an experimental estimate of the first interband gap.

$^{29}\mathrm{Si}$ NMR spectra and nuclear spin-lattice relaxation rate measurements in $\mathrm{MnSi}$ paramagnetic phase are presented. The experimental results are analyzed in the framework of the self-consistent renormalization theory for spin fluctuations, and detailed estimates of microscopic parameters describing the $\stackrel{P\vec}{q}$ and frequency dependence of the dynamical spin susceptibility are obtained. It is shown, in the light of a comparison with neutron scattering results, that some early determined parameters need to be revised. Accordingly, also the derivation of Curie-Weiss law within this model should be critically reconsidered.

NMR and $\ensuremath{\mu}\mathrm{SR}$ relaxation measurements in the ${\mathrm{Mn}}_{9}{({\mathrm{C}}_{19}{\mathrm{H}}_{15}{\mathrm{N}}_{9}{\mathrm{O}}_{2})}_{6}{({\mathrm{ClO}}_{4})}_{6}.18{\mathrm{H}}_{2}\mathrm{O}({\mathrm{Mn}}_{3\ifmmode\times\else\texttimes\fi{}3})$ molecular grid are presented. The temperature dependence of the relaxation rates can be conveniently described by a single correlation time for the spin fluctuations at temperatures of the order of the exchange coupling among ${\mathrm{Mn}}^{2+}$ ions. The inverse of the correlation time is observed to reach the MHz range at low temperature and its temperature dependence can be justified on the basis of a theoretical model where the electron spin fluctuations are driven by spin-phonon coupling. Finally, it is pointed out that the magnetic field dependence of the $\ensuremath{\mu}\mathrm{SR}$ relaxation rate allows one to derive the Zeeman splitting of the low-energy spin levels.

Anelastic spectra and 139 La NQR relaxation measurements have been carried out in La 2 - x Sr x CuO 4 (LSCO) for x around the critical value xc = 0.02 separating the antiferromagnetic (AF) from the cluster spin glass (CSG) phase for T → 0. The anelastic spectra around 10 K show step-like features common to samples pertaining to both AF and CSG phases. In the light of recent neutron scattering measurements suggesting that the spin-glass phase of LSCO with x < xc corresponds to a microscopic phase separation between regions with x ~ 0 and x ~ 0.02, the rise of the elastic energy dissipation has been attributed to the movement of the domain walls within the x ≃ xc CSG regions. The spin dynamics driving the AF transition has been investigated by means of 139 La NQR relaxation rate. The in-plane magnetic correlation length ξ2D has been extracted. In spite of the vicinity to the percolation threshold, ξ2D(x = 0.016, T) follows the temperature behavior expected in the renormalized classical regime, with spin stiffness reduced to about 0.4 of the one in pure La 2 CuO 4. Still, the 3D transition occurs when ξ2D is of the order of about 150 lattice steps, as in pure La 2 CuO 4, possibly related to the presence of AF regions unaffected by the holes.

The relationship between the NMR parameters and the motion of the flux lines (FL) in high-T c superconductors are discussed in the light of recent 89 Y NMR experiments in YBa 2 Cu 3 O 7 . In particular, novel experiments involving echo dephasing due to both thermal fluctuations of FL's and motion driven by pulsed magnetic fields are presented with preliminary results.