Local structure and magnetic properties of Mn substituted manganites studied by EXAFS and Dc magnetic measurements

We report extended X-ray absorption fine structure (EXAFS) measurements at the Mn K edge and magnetic measurements performed on (La_1−xCa_x)(Mn_1−yM_y)O₃ samples (M=Cr or Ni; x=0.37 and 0.75 and y=0.03 or 0.08). The Mn substitution produces important effects on both the sides of the LaMnO₃-CaMnO₃ phase diagram. For x<0.5 the ferromagnetic-metallic phase maintains its main character even after Mn substitution, but both the doping species (Ni or Cr) lower T_C and broaden the magnetic transition, and the EXAFS study evidences two Mn-O distances, suggesting the presence of zones of distorted insulating phase. For x>0.5, after the doping with Cr, the charge ordered phase persists but on a shorter scale, whereas the Jahn-Teller distortion is weakened as indicated by EXAFS measurements, and the formation of ferromagnetic clusters is evidenced by magnetic measurements.EXAFS and magnetic measurements are in mutual agreement, thus confirming the correlation between the local disorder determined by charge localization and magnetic degrees of freedom.     

Four-angle method for practical ultra-high-resolution magnetic resonance mapping of brain longitudinal relaxation time and apparent proton density

Changes in longitudinal relaxation time (T₁) and proton density (PD) are sensitive indicators of microstructural alterations associated with various central nervous system diseases as well as brain maturation and aging. In this work, we introduce a new approach for rapid and accurate high-resolution (HR) or ultra HR (UHR) mapping of T₁ and apparent PD (APD) of the brain with correction of radiofrequency field, B₁, inhomogeneities. The four-angle method (FAM) uses four spoiled-gradient recalled-echo (SPGR) images acquired at different flip angles (FA) and short repetition times (TRs). The first two SPGR images are acquired at low-spatial resolution and used to accurately map the active B₁⁺ field with the recently introduced steady-state double angle method (SS-DAM). The estimated B₁⁺ map is used in conjunction with the two other SPGR images, acquired at HR or UHR, to map T₁ and APD. The method is evaluated with numerical, phantom, and in-vivo imaging measurements. Furthermore, we investigated imaging acceleration methods to further shorten the acquisition time. Our results indicate that FAM provides an accurate method for simultaneous HR or UHR mapping of T₁ and APD in human brain in clinical high-field MRI. Derived parameter maps without B₁⁺correction suffer from large inaccuracies, but this issue is well-corrected through use of the SS-DAM. Furthermore, the use of SPGR imaging with short TR and phased-array coil acquisition permits substantial imaging acceleration and enables robust HR or UHR T₁ and APD mapping in a clinically acceptable time frame, with whole brain coverage obtained in less than 2 min or 5 min, respectively. The method exhibits high reproducibility and benefits from the use of the conventional SPGR sequence, available in all preclinical and clinical MRI machines, and very simple modeling to address a critical outstanding issue in neuroimaging.     

Identification of patients with hereditary haemochromatosis by magnetic resonance imaging and spectroscopic relaxation time measurements

A total of 4302 healthy blood donors were screened for elevated serum ferritin and transferrin saturation. Fifteen had increased serum ferritin at a follow-up examination. Five relatives of these donors also entered the study. Eleven patients had elevated liver iron concentrations, while five had normal liver iron concentrations. The R₂ relaxation rate in the liver was first measured with a conventional multi-spin-echo imaging sequence, and then by a volume-selective spectroscopic multi-spin-echo sequence, in order to achieve a minimum echo time of 4 msec. No correlation was found between the relaxation rate R₂ and the liver iron concentration, when R₂ was calculated from the imaging data. Multi-exponential transverse relaxation could be resolved when the spectroscopic sequence was used. A strong correlation between the initial slope of the relaxation curve and the liver iron concentration was found (r = 0.90, p < 0.001). Signal intensity ratios between liver and muscle were calculated from the first three echoes in the multi-echo imaging sequence, and from a gradient echo sequence. A strong correlation between the logarithm of the signal intensity ratios and the liver iron concentration was found. Although both spectroscopic T₂ relaxation time measurements and signal intensity ratios could be used to quantify liver iron concentration, the gradient echo imaging seemed to be the best choice. Gradient echo imaging could be performed during a single breath hold, so motion artifacts could be avoided. The accuracy of liver iron concentration estimates from signal intensity ratios in the gradient echo images was about 35%.     

Determination of blood longitudinal relaxation time (T1) at high magnetic field strengths

In this study, a circulation system was used to measure T(1) values of bovine blood under physiological conditions at field strengths of 4.7, 7 and 9.4 T. Results show that T(1) increases linearly with magnetic field B(0) and can be described with the equation T(1)=129 ms/T B(0)+1167 ms for magnetic field strengths between 1.5 and 9.4 T.     

Diffusion of gold in silicon studied by means of neutron-activation analysis and spreading-resistance measurements

In- and out-diffusion of gold in silicon were investigated with the aid of a neutronactivation analysis in combination with mechanical sectioning or by the spreadingresistance technique. In-diffusion profiles in the range 1371–1073 K show that Au diffuses in Si mainly via the so-called kick-out mechanism. From the Au diffusion and solubility measurements the interstitialcy contributionD _I ^SD to the Si self-diffusion coefficient was determined, which shows that the self-diffusion occurs to a considerable extent via selfinterstitials. Out-diffusion profiles at 1173 K were measured on wafers homogeneously supersaturated with Au. The observed decrease of the electrical activity of Au in the bulk indicates that during the out-diffusion anneal the majority of Au atoms originally dissolved substitutionally changes its configuration.     

The dependence on magnetic field strength of correlated internal gradient relaxation time distributions in heterogeneous materials

Magnetic susceptibility differences in porous media produce local gradients within the pore space. At high magnetic fields, these inhomogeneities have the potential to greatly affect nuclear magnetic resonance measurements. We undertake a study using a new NMR technique to measure the internal gradients present in highly heterogeneous samples over a wide range of magnetic field strengths. Our results show that even at ultra-high fields there can exist signal at internal gradient strengths sufficiently small that techniques for suppressing unwanted side effects have the possibility to be used. Our findings encourage the use of these high and ultra-high field strengths for a broader range of samples. Our results also give experimental evidence to support the theory of internal gradient scaling as a function of field strength within pores.     

Investigation of liquid lithium relaxation time by means of the memory function formalism

The temporal characteristics of relaxation processes in liquid lithium extracted from the results of inelastic neutron scattering experiments, are analyzed with the use of the memory function formalism.     

Molecular motions in chloramphenicol studied by35Cl nuclear quadrupole and1H nuclear magnetic resonance spectroscopy

The³⁵Cl nuclear quadrupole resonance (NQR) frequency (v_Q), nuclear quadrupole spinlattice relaxation time (T_1Q),¹H nuclear magnetic resonance second moment (M₂) and nuclear magnetic spin-lattice relaxation timeT ₁) were measured for polycrystalline chloramphenicol (drug) as a function of temperature. Hindered rotation of the CHC1₂ group and the phenyl ring was detected, the relevant activation energies were determined. The rotations are discussed in detail.     

Electron spin relaxation times and internal motions of radicals in the solid state investigated by ENDOR and pulsed EPR

The angular dependent ENDOR spectra of the radical formed by ψ-irradiated single crystal of 4-methyl-2,6-di-t-butylphenol have been studied and the full hyperfine tensors of all the t-butyl and the ring protons have been obtained atT=190 K. We found six different tensors for the t-butyl protons. This result shows that the t-butyl groups are slowly rotating on the ENDOR time scale, whereas each methyl group rotates fastly. The dynamical parameters of the motions have been determined by pulsed EPR experiments. The longitudinal relaxation and the phase memory times have been measured in the temperature range 130–290 K. Three different kinds of motions have been detected and the resulting values of the dynamical parameters have been compared with those obtained for the undamaged molecule by previous NMR studies.     

Anisotropic relaxation and motion of half-integer quadrupole nuclei studied by central transition nuclear magnetic resonance spectroscopy

An efficient theoretical formalism and advanced experimental methods are presented for studying the effects of anisotropic molecular motion and relaxation on solid-state central transition NMR spectra of half-integer quadrupole nuclei. The theoretical formalism is based on density operator algebra and involves the stochastic Liouville–von Neumann equation. In this approach the nuclear spin interactions are represented by the Hamiltonian while the motion is described by a discrete stochastic operator. The nuclear spin interactions fluctuate randomly in the presence of molecular motion. These fluctuations may stimulate the relaxation of the system and are represented by a discrete relaxation operator. This is derived from second-order perturbation theory and involves the spectral densities of the system. Although the relaxation operator is valid only for small time intervals it may be used recursively to obtain the density operator at any time. The spectral densities are allowed to be explicitly time dependent making the approach valid for all motional regimes. The formalism has been applied to simulate partially relaxed central transition ¹⁷O NMR spectra of representative model systems. The results have revealed that partially relaxed central transition lineshapes are defined not only by the nuclear spin interactions but also by anisotropic motion and relaxation. This has formed the basis for the development of central transition spin-echo and inversion-recovery NMR experiments for investigating molecular motion in solids. As an example we have acquired central transition spin-echo and inversion-recovery ¹⁷O NMR spectra of polycrystalline cristobalite (SiO₂) at temperatures both below and above the α–β phase transition. It is found that the oxygen atoms exhibit slow motion in α-cristobalite. This motion has no significant effects on the fully relaxed lineshapes but may be monitored by studying the partially relaxed spectra. The α–β phase transition is characterized by structural and motional changes involving a slight increase in the Si–O–Si bond angle and a substantial increase in the mobility of the oxygen atoms. The increase in the Si–O–Si angle is supported by the results of ¹⁷O and ²⁹Si NMR spectroscopy. The oxygen motion is shown to be orders of magnitude faster in β-cristobalite resulting in much faster relaxation and characteristic lineshapes. The measured oscillation frequencies are consistent with the rigid unit mode model. This shows that solid-state NMR and lattice dynamics simulations agree and may be used in combination to provide more detailed models of solid materials.     

Characterization of energy barrier and particle size distribution of lyophilized ferrofluids by magnetic relaxation measurements

The magnetic properties of a ferrofluid are strongly influenced by its particle size distribution. We analyzed a ferrofluid with an unknown particle size distribution as well as fractionated samples of the original material. The ferrofluid in our investigations consists of a mixture of maghemite and magnetite. We investigated these different samples using temperature-dependent magnetorelaxometry method. The evaluation of the Néel relaxation signal allows us a direct determination of the energy barrier distribution, which is one of the most important parameters of such systems of magnetic nanoparticles. The calculated particle volumes were compared with particle sizes determined by transmission electron microscopy.     

Study of activation parameters and NMR spin-lattice relaxation time in some substituted amines

The present communication reports the experimental values of NMR spin-lattice relaxation time (T₁) and dielectric relaxation time (τ) of piperidine, pyrrole, pyridine, diethylamine, triethylamine and pyrrolidine. The values of activation energy (ΔE_A) obtained using dielectric relaxation time, have been correlated with calculated values of ΔE_A obtained using Arrhenius equation of NMR relaxation time (T₁) for pyridine, diethylamine and pyrrole. Authors have also established a correlation between the experimental values of NMR spin-relaxation time (T₁) with its calculated values obtained using different equations of dielectric relaxation time (τ).     

Rapid internal conversion in a symmetric molecule LD 700 studied by means of femtosecond fluorescence depletion

The rapid internal conversion dynamics at room temperature is determined by using the femtosecond time-resolved fluorescence depletion measurements of a complex solvated molecule of LD 700 (rhodamine 700) combined with steady-state absorption and fluorescence spectroscopy, as well as quantum chemical calculation. The molecule is excited by a 50fs laser pulse at 400nm which directly populated the highly excited singlet state, the rapid internal conversions (ICs) are observed, which leads to the directional changes of the emission transition moment following photoexcitation to the highly excited singlet state S₅ of LD 700.     

GdBaCo2O5+δ体系的滞弹性内耗研究

GdBaCo2O5+δ体系的低频内耗研究表明:体系中存在一个由额外氧运动引起的弛豫内耗峰;额外氧δ对这个弛豫内耗峰的大小、峰形及峰位有较大影响,反映了额外氧状态随δ而变化.当δ=0.005,体系中额外氧含量很少而接近零时,相应的内耗峰消失;δ达到一定数量后,出现弛豫内耗峰.由δ=0.278,0.407,0.421,0.515样品的弛豫内耗峰分析可得到体系随δ不同存在着三种不同的额外氧形态.此外,δ=0.421及0.515的样品在360K附近存在一个相变内耗峰,它对应着体系中的金属-绝缘体转变.     

Cavity linewidth narrowing by means of electromagnetically induced transparency in Rb with a longitudinal magnetic field

We demonstrate a method for further reducing the cavity linewidth by the application of a small longitudinal magnetic field on the Rb cell. Because of the magnetic field multiple electromagnetically induced transparen- cies （EITs） are observed. The center EIT linewidth is measured as a function of the magnetic field. By utilizing the center EIT we narrow the cavity linewidth to 2 MHz which is half of the cavity linewidth without magnetic field.     

Study of molecular motions in liquids by electron spin-lattice relaxation measurements,I: Semiquinone ions in hydrogen bonding solvents

The electron spin-lattice relaxation times (T ₁) of a variety of semiquinone ions in hydrogen bonding solvents have been measured by the pulsed saturation recovery technique as a function of temperature (T) and viscosity (η) of the solvent. Also linewidths (ΔH) have been measured in suitable cases in such solvents at low radical concentrations (～10^?4 M). It is observed that (i) the temperature and viscosity dependence ofT ₁ can be fitted to an equation of the form 1/T ₁=A(T/η)+Bexp(-ΔE/RT) whereA andB are constants and ΔE is an activation energy of the order of 1 kcal mole^?1 for these systems; (ii)T ₁ is essentially independent of the radical concentration within the range 10^?3 to 5×10^?2 M; (iii) the concentration independent part of the linewidth (ΔH) increases linearly with (η/T) at sufficiently low temperatures, and (iv) the (η/T) dependent part ofT ₁ is sensitive to the size of the semiquinone as well as that of the solvent molecule, whereas the linewidth which is proportional to (η/T) at high viscosity, low temperature region is not sensitive to the size of the semiquinone and that of the solvent. Based on these observations, it is postulated that in hydrogen bonding solvents, three types of motion contribute significantly to electron spin relaxation:

1. A restricted small step diffusional motion, not involving large changes in the orientation of the molecule, leading to the dominant viscosity dependent contributions toT ₁ and ΔH, due to spin rotation interaction;
2. a large amplitude reorientation of the semiquinone, coupled to translational diffusion, resulting in viscosity dependent contributions toT ₁ and ΔH, throughg-modulation;
3. a hindred rotation of the semiquinone within the solvent cage, contributing toT ₁ due to spin rotation interaction.

The fact thatT ₁ is not sensitive to the concentration of the radicals, is ascribed to the formation of the solvent cage that prevents the close approach of radicals, thereby rendering radical-radical interactions to be weak mechanisms for relaxation, even at relatively high radical concentrations.     

NMR-propagator measurements in porous media in the presence of surface relaxation and internal fields

We measure the probability distribution P(zeta)--the propagator--of molecular displacements on Stokes flow through a pack of microporous glass beads and a carbonate rock. An optimized sampling of q-space is introduced for the measurement of a P(zeta) and its first moment zeta. Our results delineate and provide an understanding of the experimental regimes where background fields and surface relaxation distort the measured propagators.     

Molecular motion and internal rotation in C6H5CHCl2 as studied by 13C relaxation data

Carbon-13 spin-lattice relaxation times and nuclear Overhauser enhancement factors of C₆H₅CHCl₂ in deuteriochloroform solutions were measured at several temperatures in the range from 233 to 323 K. The dipole-dipole relaxation rates were used with the Woessner's equations to calculate the rotational diffusion coefficients for the anisotropic motion. A random jump model was used to evaluate the internal rotation rate of the CHCl₂ group, which was found to be about two times faster than the overall motion. The carbon-13 spin-spin relaxation times were also qualitatively analyzed in terms of the possible relaxation mechanisms.     

Internal evaluation of impregnation treatment of waterlogged wood; relation between concentration of internal materials and relaxation time using magnetic resonance imaging

The purpose of this study is to clarify the degree of impregnation resulting from treatment of internal waterlogged wood samples using MRI. On a 1.5 T MR scanner, T₁ and T₂ measurements were performed using inversion recovery and spin-echo sequences, respectively. The samples were cut waterlogged pieces of wood treated with various impregnation techniques which were divided into different concentrations of trehalose (C₁₂H₂₂O₁₁) and polyethylene glycol (PEG; HO-(C₂H₄O)_n-H) solutions. Then these samples underwent impregnation treatment every two weeks. From the results, we found that the slope of the T₁-concentration curve using linear fitting showed the value of the internal area for PEG to be higher than the external area; internal, − 2.73 ms/wt% (R² = 0.880); external, − 1.50 ms/wt% (R² = 0.887). Furthermore, the slope of the T₁-concentration curve using linear fitting showed the values for trehalose to have almost no difference when comparing the internal and the external areas; internal, − 2.79 ms/wt% (R² = 0.759); external, − 3.02 ms/wt% (R² = 0.795). However, the slope of the T₂-concentration curve using linear fitting for PEG showed that there was only a slight change between the internal and the external areas; internal, 0.26 ms/wt% (R² = 0.642); external, 0.18 ms/wt% (R² = 0.920). The slope of the T₂-concentration curve did not show a change in linear relationship between the internal and the external areas; internal, 0.06 ms/wt% (R² = 0.175); external, − 0.14 ms/wt% (R² = 0.043). In conclusion, using visualization of relaxation time T₁, it is possible to obtain more detail information noninvasively concerning the state of impregnation treatment of internal waterlogged wood.