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Novel systems based on suspensions or colloidal magnetic nanoparticles have been investigated as perspective super paramagnetic contrast agents (CA) for magnetic resonance imaging (MRI) The nanostructures that we have studied contain surfactant-capped magnetic (Fe(3)O(4)) inorganic cores wall different controlled sizes, ranging from 5 5 to 12 nm The as-synthesized nanostructures are passivated by hydrophobic surfactants and thus are fully dispersible in nonpolar media The magnetic nanocrystals have been transferred into aqueous meta by a procedure based on the surface intercalation and coating with an amphiphilic polymer shell The MRI efficiency in contrasting images. i.e. the NMR relaxivities r(1) and r(2), have been compared with Endorem and Sinerem, commercial super paramagnetic MRI contrast agents We found that out nanostructures exhibit r(1) and r(2) relaxivities comparable to those of commercial CA over the whole frequency range The MRI efficiency of our samples was related to their microstructural and magnetic properties. The transverse relaxivity r(2), leading the contrast in "negative" super-paramagnetic agents, was round to improve as the diameter of the inorganic core is increased. The NMR relaxometry profile confirmed the nature or the physical mechanisms inducing the increase of the nuclear relaxation rates at low (magnetic anisotropy) and high (Curie relaxation) fields.

The scientific communityis seeking to exploit the intrinsic properties of magneticnanoparticles (MNPs) to obtain medical breakthroughs indiagnosisandtherapy.OneofthemainadvantagesofmagneticNPs is that they can be visualized acting as magnetic contrastagents(CA)formagneticresonanceimaging(MRI).Heatedina high-frequency magnetic field they trigger drug release orproducehyperthermia/ablationoftissues,currentlyreportedasmagnetic fluid hyperthermia (MFH). Accordingly, the termtheranostic nanomedicine has been defined as an integratednanotherapeutic system, which can diagnose, provide targetedtherapy and monitor the response to therapy.

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.

89 Y NMR shifts Δ K and 63 Cu NQR spin lattice relaxation rate T −1 1 in YBCO-family superconductors with different transition transition temperatures T c are used to analyze the possible role of the Cu 2+ magnetic correlations and spin dynamics in driving the superconducting transition. The changes in T c are obtained by varying the oxygen stoichiometry or by Ca doping in YBa 2 Cu 4 O 8 or by Zn for Cu substitutions in YBa 2 Cu 3 O 7−δ . From 89 Y ΔK and 63 Cu(2) NQR T 1 no sizeable modifications either in the statio (spin susceptibility) or in the dynamic ( q -integrated low frequency dynamical susceptability) properties of Cu 2+ spins in the CuO 2 plane are inferred and therefore it is argued that the superconducting transition is not directly related to them.

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.

We have prepared water-soluble gadolinium oxide nanoparticles that show potential as MRI contrast agents. The particles were built into the apoferritin cavity and have an average size of 5 nm. After seven days a loss of 5% of Gd was detected compared with the as-prepared samples; after that the Gd remained constant and stabilized inside the apoferritin, indicating that the apoferritin capsid acts as a Gd store, avoiding metal delivery and consequent toxicity. The NMR longitudinal and transverse relaxivities resulted about 10 and 70 times higher than the ones of clinically approved paramagnetic Gd-chelates, thus indicating the possible route for synthesizing a novel class of MRI contrast agents.

We have developed a nonconventional broadband electron spin resonance (ESR) spectrometer operating continuously in the frequency range from 0.5 to 9 GHz. Dual antenna structure and the microwave absorbing environment differentiate the setup from the conventional one and enable broadband operation with any combination of frequency or magnetic field modulation and frequency or magnetic field sweeping. Its performance has been tested with the measurements on a 1,1-diphenyl-2-picrylhydrazyl (DPPH) sample and with the measurements on the single molecular magnet, V6, in solid state at low temperature. (C) 2008 American Institute of Physics.

Novel systems based on colloidal magnetic nanocrystals (NCs), potentially useful as superparamagnetic (SP) contrast agents for magnetic resonance imaging (MRI) have been investigated. The NCs we have studied comprise organic-capped single-crystalline maghemite (gamma-Fe(2)O(3)) cores possessing controlled sizes and shapes. We have comparatively examined spherical and tetrapod-like NCs, the latter being branched particles possessing four arms which depart out at tetrahedral angles from a central point. The as-synthesized NCs are passivated by hydrophobic surfactant molecules and thus are fully dispersible in nonpolar media only. The NCs have been made soluble in aqueous solution by applying a procedure based on the surface intercalation and coating with an amphiphilic polymer shell. NMR relaxivities R(1) and R(2) were compared with ENDOREM(R), one of the standard commercial SP-MRI contrast agent. We found that the spherical NCs exhibit R(1) and R(2) relaxivities slightly lower than those of ENDOREM(R), over the whole frequency range; on the contrary, tetrapods show relaxivities about one order of magnitude lower. The physical origin of such difference in relaxivities between tetrapod-and spheres-based nanostructures is under investigation and it is possibly related to different sources of the magnetic anisotropy. (C) 2008 Elsevier B.V. All rights reserved.

We investigated the spin dynamics of Zn(II)-doped “single-chain magnet” (SCM) Co(hfac)2NITPhOMe (in short CoPhOMe) by means of 1H NMR measurements. CoPhOMe was the first magnetic chain that showed a slowly relaxing magnetization (commonly called spins’ or magnetization “slowing down” mechanism), which follows the predictions of the Glauber model: the electronic spin freezing sets in before any transition to long-range magnetic order. The slowing down of the magnetization is also evidenced in the Zn-doped samples, which present chains that reach an average length of some tenth of spins for the highest doping, together with growing finite-size effects as the doping concentration is increased. The nuclear spin–lattice relaxation rate T1-1(T) at different applied magnetic fields, displayed a high-temperature peak and a low-temperature shoulder that become lower and shifts toward higher temperatures with increasing the field. This behaviour is qualitatively in agreement with the recent AC susceptibility data that revealed the Glauber dynamics and finite-size effects, like the collective reversal of the spins of each chain segment.

New water-soluble paramagnetic Gd-containing cyano-bridged metallic coordination polymer nanoparticles with a chitosan shell show high nuclear relaxivity in acidic water which is up to six times higher than that of the actually used Gd-chelates.

A one-pot, two-step colloidal strategy to prepare bimagnetic hybrid nanocrystals (HNCs), comprising size-tuned fcc FePt and inverse spinel cubic iron oxide domains epitaxially arranged in a heterodimer configuration, is described. The HNCs have been synthesized in a unique surfactant environment by temperature-driven sequential reactions, involving the homogeneous nucleation of FePt seeds and the subsequent heterogeneous growth of iron oxide. This self-regulated mechanism offers high versatility in the control of the geometric features of the resulting heterostructures, circumventing the use of more elaborate seeded growth techniques. It has been found that, as a consequence of the exchange coupling between the two materials, the HNCs exhibit tunable single-phase-like magnetic behavior, distinct from that of their individual components. In addition, the potential of the heterodimers as effective contrast agents for magnetic resonance imaging techniques has been examined.

Novel systems to be employed as superparamagnetic contrast agents (CA) for magnetic resonance imaging (MRI) have been synthesized. These compounds are composed of an iron oxide magnetic core coated by polyethylenimine (PEI) or carboxylated polyethylenimine (PEI-COOH). The aim of the present work was to prepare and study new nanostructured systems (with better or at least comparable relaxivities, R-1 and R-2, with respect to the commercial ones) with controlled, almost monodisperse average dimensions and shape, as candidates for molecular targeting. By means of atomic force microscopy (AFM) measurements we determined the average diameter, of the order of 200 nm, and the shape of the particles. The superparamagnetic behavior was assessed by SQUID measurements. From X-ray data the estimated average diameters of the magnetic cores were found to be similar to 5.8nm for PEI-COOH60 and similar to 20nm for the compound named PEI25. By NMR-dispersion (NMRD), we found that PEI-COOH60 presents R-1 and R-2 relaxivities slightly lower than Endorem(R). The experimental results suggest that these novel compounds can be used as MRI CA. (C) 2008 Elsevier B.V. All rights reserved.

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.

Fundamental aspects of the Magnetic Resonance Imaging (MRI) technique which is one of the preferred modalities for non-invasive clinical applications and “in vivo” medical and biological research, are presented. The physical principles of MRI and protocols recently introduced to investigate the microscopic details of pathological cerebral damages are introduced. The main results obtained from the analysis of T2 relaxation curves are briefly summarized.

Vesicles of the ganglioside GM3 form spontaneously in water. Their average diameter is about 500 A. Vesicles are in equilibrium with a very small number of much larger non-spherical aggregates. This single component vesicle system is quite “frustrated”, since the two monolayers of the bilayer have spontaneous curvatures with the same magnitude, but opposite sign. This situation is dramatically changed if a second amphiphile GM1, of the same type of the ganglioside GM3 but with larger headgroup, is added. In this case, spontaneous curvature readjustments of the two monolayers via demixing can give rise to a finite spontaneous curvature of the bilayer, energetically favoring vesicles towards other structures. Experimentally, it is observed that, by adding increasing proportions of GM1, the large aggregates gradually diminish in number until a pure vesicle solution is obtained for a GM1 to GM3 ratio of 35 to 65.

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.

Recent studies of the magnetic properties of several high superconducting transition temperature (T c ) cuprate parent compounds and related materials will be reviewed. The observations of a Heisenberg to XY-like crossover upon cooling below ∼ 300 K towards the Neel temperature T N =257 K and a subsequent magnetic field-induced XY-like to Ising-like crossover near T N in single crystals of the K2NiF4-type spin 1/2 model compound Sr2CuO2Cl2 will be described. The spin 1/2 linear chain compound Sr2CuO3, the parent of the Sr2CuO3+δ oxygen-doped superconductors, is found to exhibit classic Bonner-Fisher magnetic behavior, with a large antiferromagnetic Cu−Cu superexchange coupling constant. Studies of the evolution of La2−x SrxCuO4 with Sr doping in the insulating regime (x

Hole-doping effects in Haldane chain have been studied by means of magnetic susceptibility and 89Y nuclear magnetic resonance in Y2−xCaxBaNiO5 for x=0, x=0.06 and x=0.18. In nominally pure YBNO detailed information on this prototype of Haldane systems is extracted. In the charge-doped compounds the 89Y relaxation rates indicate that holes induce low-energy excitations with an effective spectral density having a structure characterized by a narrow central peak.

The authors report on NMR and susceptibility measurements in Sr14−xNaxCu24O41 for 0⩽x⩽2. The analysis of the susceptibility evidences that only a small fraction of Cu2+ spins are involved in the dimerization of Cu(1)O2 chains. The introduction of extra holes by Na+ doping is found to decrease the Cu(1)2+ uniform static susceptibility without affecting the amplitude of the dimerization gap. On the other hand, the amplitude of the spin gap in the Cu(2)2O3 two-leg-ladders and the resistivity are reduced by doping. The origin of the discrepancy in the amplitude of the gap estimated by means of nuclear spin-lattice relaxation and susceptibility measurements is briefly discussed.

The vibrational dynamics in nearly stoichiometric La2CuO4+δ is studied by means of anelastic and 139La NQR relaxation. After lowering δ to values of the order of 0.001, a relaxational peak remains in both the anelastic and NQR spectra with a mean relaxation time τ=1.7×10−12 exp[(2800 K)/T]s. Such a process is attributed to the motion of CuO6 octahedra in doublewell potentials whose cooperative character increases the effective energy barrier to the observed value. It is speculated that the freezing of the motion of the octahedra may correspond to the locking of the stripes.

International audience; Magnetic resonance imaging (MRI) is at the forefront of non-invasive medical imaging techniques. It provides good spatial and temporal resolution that can be further improved by the use of contrast agents (CAs), providing a valuable tool for diagnostic purposes. Ultrasmall superparamagnetic iron oxide (USPIO) nanoparticles are attractive MRI contrast agents due to their negative (T-2) contrast enhancement capability and biocompatibility. Clusters of USPIOs with polymer material are of particular interest since they can sustain additional functionalities like drug delivery and targeting. Aiming to establish a relationship between the morphology of the clusters and their efficacy as MRI contrast agents (relaxometric properties), we prepared - using three different maghemite (gamma-Fe2O3) USPIO diameters - a series of hybrid copolymer/iron oxide CAs presenting two different geometries (micellar or vesicular). The NMR relaxometry profiles confirmed the nature of the physical mechanisms inducing the increase of nuclear relaxation rates at low (magnetic anisotropy) and high (Curie relaxation) magnetic fields. A heuristic model, first proposed by Roch, Muller, Gillis, and Brooks, allowed the fitting of the whole longitudinal relaxivity r(1)(v) profile, for samples with different magnetic core sizes. We show that both types of clusters exhibit transverse relaxivity (r(2)) values comparable to or higher than those of common contrast agents, over the whole tested frequency range. Moreover, in-depth analysis revealed substantially a linear relationship between r(2) and the number of encapsulated USPIOs divided by the diameter of the clusters (N-USPIO/D-H), for each USPIO size. The cluster structure (i.e. micelle or vesicle) appeared to have a mild influence on the transverse relaxivity value. Indeed, the r(2) value was mainly governed by the individual size of the USPIOs, correlated with both the cluster external diameter and the magnetic material volume fraction.

The magnetic properties of YVO3, a prototype of highly correlated d-electron Mott–Hubbard antiferromagnetic (AF) orthovanadates, are studied by means of magnetization and susceptibility measurements, coupled to 89Y NMR spectra and relaxation. Both field-cooled and zero-field-cooled conditions are considered. The history and field-dependent properties recently reported for the AF phases below TN≃113 K and TN′≃77 K are confirmed. The behavior in the paramagnetic phase and around TN is studied and it is shown how the field-induced short range order and canted-spin weak ferromagnetism is quenched in strong field, yielding (M/H)ZFC≃(M/H)FC for H≃6 T. At variance with the susceptibility in strong field, the 89Y NMR line width shows a divergent behavior for T→TN+. The 89Y relaxation rates do not display, for T→TN+, the behavior expected for spin fluctuations of AF-ferromagnetic (FE) symmetry as the one in the ordered state, possibly in view of the quasistatic character of the short range order.

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.

Dielectric, anelastic, and $^{139}\mathrm{La}$ NMR relaxation measurements have been made on the relaxor ferroelectric ${\mathrm{Pb}}_{1\ensuremath{-}3x∕2}{\mathrm{La}}_{x}{\mathrm{Zr}}_{0.2}{\mathrm{Ti}}_{0.8}{\mathrm{O}}_{3}$ (PLZT) with $x=0.22$. The dielectric susceptibility exhibits the frequency dispersive maximum due to the freezing of the polar degrees of freedom around $T\ensuremath{\simeq}250\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The anelastic and especially NMR relaxation, besides this maximum, indicate an intense and broad component at lower temperatures, attributed to rotational modes of the O octahedra, weakly coupled to the polar modes. It is discussed why such short range rotational instabilities, known to occur in the Zr-rich rhombohedral region of the PLZT phase diagram, might appear also in the Ti-rich region.

The effects induced by charge defects on the spin dynamics and on the static properties of antiferromagnetic cuprates, as detected through NMR-NQR and μ+SR, are addressed. Information on hole localization and evidence supporting hole segregation are presented.

29Si NMR spectra and spin-lattice relaxation measurements in FeSi at different magnetic fields ranging from 1.6 to 15 T, from 1.6 K up to 900 K, are reported. The spin-lattice relaxation rate T - 1 1 and the Knight shift K S display very interesting temperature dependences, with three different regimes. At low temperatures (T ≤ 100 K) T - 1 1 increases linearly on increasing temperature while K S is practically constant. In the intermediate temperature range (100≤T≤350 K) T - 1 1 shows a behavior typical of a gapped system, with an energy gap Δ≃560 K and tracking the homogeneous static susceptibility Χ(T). For T≥350 K the temperature dependence of T - 1 1 departs from that of the susceptibility. While Χ(T) levels around 500 K, T - 1 1 continues to increase. From the experimental findings, insights on the spin dynamics of Fermi-like carriers with strong correlation are obtained.

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.

139La nuclear quadrupole resonance and relaxation measurements, aimed at the study of static magnetic properties and of the dynamical generalized susceptibility in La2Cu1−xZnxO4, upon substitution of Zn2+ (S=0) for Cu2+ (S=1/2) inducing a spin vacancy in the antiferromagnetic matrix, are presented. From the doublet resulting from the mixed Zeeman‐quadrupolar eigenstates, it is shown how the sublattice magnetization is only slightly x dependent, while its temperature dependence changes with respect to pure La2CuO4. The relaxation rates show moderate (compared to the case of itinerant spin vacancies, as in La2−xSrxCuO4) effects, which are related to the decrease of the correlation length and to the modifications induced in the spin‐wave spectrum.

35Cl nuclear‐magnetic‐resonance (NMR) and relaxation measurements have been performed in a Sr2CuO2Cl2 single crystal as a function of orientation between the external magnetic field H0 and the tetragonal c axis. The onset is found of an anisotropic inhomogeneous broadening of the NMR line without paramagnetic shift starting below about 320 K. The broadening is strongly temperature dependent and it increases with increasing H0. The relaxation rate T−11, on the other hand, diverges at TN≂260 K in agreement with previous results in powder sample. It is suggested that the observed broadening is due to the onset at T≳TN of a spontaneous nonzero thermal average of the Cu2+ moment lying in the a‐b plane.

Cu‐nuclear quadrupole resonance (NQR) frequency and relaxation rates measurements in CuO, in the vicinity of the antiferromagnetic ordering temperature TN, are presented. As T→T+N divergent behaviors of the relaxation rates T−11,2 and an anomalous decrease in the NQR frequency are observed. These effects are discussed in light of the temperature dependence expected for conventional one‐dimensional Heisenberg paramagnet and some anomalies are interpreted as possible results of valence fluctuations in the CuO bond character.

The evolution of the magnetic properties of the insulating parent cuprate compounds as they are doped into the metallic and superconducting state continues to be an important area of research. Understanding this evolution and its relationship to the microscopic distribution of the doped holes may offer insights into interactions important in the superconducting materials. In addition, the nature of this evolution in strongly correlated electron systems is interesting in its own right. In this paper, we will review some of the progress made in attempting to address this issue in the La2CuO4-type and related systems.

We utilize a nonconventional (i.e., without resonance cavity) broadband electron spin resonance (ESR) spectrometer operating continuously in the frequency range 0.3–9 GHz to investigate the ground state of the magnetic molecule {V6} at low temperature. There exist two different varieties of {V6} molecules with small differences in the organic part and in the crystal packing. We find differences in the width and hyperfine structure of the ESR line in the two compounds. However, the central frequency of the broad ESR line measured vs H down to 0.3 GHz shows a linear behavior in both compounds indicating a g value close to 2 and a zero field gap no larger than 30 × 10−3 K. The result is in contrast with a level scheme for the ground state previously reported for one of the two varieties of {V6}. We propose an alternative scenario for the structure of the magnetic ground state.

"We present the magnetic properties and the H-1 nuclear magnetic resonance dispersion profiles of Mn-ferrites-based compounds, as possible novel contrast agents (CAs) for magnetic resonance imaging (MRI). The samples consist of nanoparticles (NPs) with the magnetic core made of Mn1+xFe2-xO4, obtained by the rapid decomposition of metalcarbonyl into a hot solvent containing an oxidizer and a coordinating surfactant; by this procedure, monodisperse capped NPs with different sizes have been obtained. We have performed structural and morphological investigation by x-ray powder diffraction and transmission electron microscopy techniques and SQUID magnetometry experiments to investigate the magnetic behaviour of the samples. As required for MRI applications using negative CAs, the samples are superparamagnetic at room temperature, having blocking temperatures in the range 14-80 K. The longitudinal r(1) and transverse r(2) nuclear relaxivities appear to vary strongly with the magnetic core size, their values being comparable to commercial compounds in the high-frequency range nu > 100 MHz. The experimental results suggest that our samples are suitable for high-frequency MRI imagers in general and in particular for the 3 T clinical imager, as indeed suggested by a recent report (Tromsdorf et al 2007 Nanoletters 7 2422)."

The effects of substituting the Cu2+ (S-l/2) with Ni2+ (S=l) in the prototype system for high-Tc superconductors, La2CuO4, is investigated by 139La NQR spectra and relaxation, for x ranging from 0 to 1.

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.

139La NQR spectra, μSR precessional frequencies and relaxation rates W driven by the Cu2+ spin fluctuations are used to study the static magnetic properties and the low energy spin-excitations upon substitution of Zn2+ (S=0) for Cu2+ in the AF phase of La2Cu1−xZnxO4. In particular, from the temperature behavior of W a cooperative freezing in a spin-glass state involving Zn-induced local magnetic moments is detected.

Temperature‐dependent (10–300 K) field‐cooled (FC) and zero‐field‐cooled (ZFC) susceptibilities, and 89Y NMR spectra and relaxation in Y1−xCaxVO3 with 0≤x≤0.9 are reported. In the paramagnetic phase (PA) of YVO3 and for T≥120 K the susceptibility χ is of antiferromagnetic (AF) type, with no field dependence. Around the Neel temperature T(1)N=113±1 K a field‐dependent ferromagnetic behavior is observed under FC condition. Below T(2)N≂77 K (AF phase with AF interplane coupling) strong difference is present between FC and ZFC χ’s. For T→T(1)+N the 89Y NMR line shows marked broadening and no measurable shift, consistent with a dominant dipolar contribution in the nucleus–electrons interaction. The 89Y relaxation rate T−11, related to the correlated spin dynamics of V3+ S(t), seems to indicate that the interplane ferromagnetic symmetry of the spin fluctuations is quenched in strong fields. The peculiar findings for χ and T1 could result from a field‐dependent weak ferromagnetism of Dzyaloshinsky–Moriya charact...

35Cl nuclear magnetic resonance (NMR) and relaxation measurements have been performed in stacked Sr2CuO2Cl2 single crystals with Neel temperature TN=257 K for two crystal orientations with respect to the applied field: H∥c and H⊥c. All the parameters of 35Cl NMR are found to exhibit a strong orientation dependence varying in temperature. In particular, the unconventional anisotropy and strong temperature dependence of the 35Cl nuclear spin–spin relaxation rate, T−12, are presented and discussed in terms of a crossover of the Cu2+ spin dynamics from Heisenberg to XY‐like correlation for T→T+N. The onset of anisotropic broadening and shift of the 35Cl NMR line was observed well above TN, which is in agreement with the previous results.

139La NQR spin-spin and spin-lattice relaxation measurements in La2−x Sr x CuO4−y are presented, for x ranging from zero to 0.3 and for variable oxygen content, in the temperature range (1.6÷450) K. The data are analysed in terms of the different possible relaxation mechanisms. The dominant relaxation mechanism over most of the x and T range is associated with the Cu++−Cu++ magnetic correlations and spin dynamics. To explain the data we introduce a heuristic model whereby the degree of magnetic correlations, that would result from the strength of the Cu−Cu exchange coupling, is reduced by mobile charge defects (solitons or holons) causing a liquidlike thermal bath of magnetic excitations described by a concentration-dependent and thermally activated correlation time τd. The activation energy for τd is inversely proportional to the concentration of mobile defects. It is shown that the T 1 data yield the x and T dependence of the correlation length, with quantitative estimate in agreement with neutron scattering results. Some implications of the model on other quantities, like magnetic susceptibility, x dependence of the Neel temperature and electric conductivity, are briefly discussed.