Spin-lattice Relaxation Mechanism Research Articles

Low-frequency spin dynamics and NMR spin-lattice relaxation in antiferromagnetic rings

We develop a general theory of the spin dynamics of Heisenberg antiferromagnetic rings (HAFRs) that explains the mechanism of NMR spin-lattice relaxation at low temperatures. In HAFRs, the imaginary parts of the q-summed dynamic spin susceptibilities parallel and perpendicular to an applied static field, χ ′′ sum‖(ω) and χ ′′ sum⊥(ω), are composed of the sum of many slightly broadened δ-functional modes at many frequencies. The NMR relaxation is caused by the quasielastic mode in χ ′′ sum‖(ω) at around zero frequency. This quasielastic mode is characterized by two physical quantities, intensity P0‖ and frequency width 0‖. Although P0‖ has to date been assumed to be identical to the uniform static susceptibility, we point out that the two quantities are not identical. Without making this unreliable assumption for P0‖, we demonstrate experimentally how P0‖ and 0‖ behave, by analyzing the NMR relaxation rates of two different nuclei, 1H and 13C, in a real HAFR. This analysis is more rigorous and thus can be used to estimate 0‖ and P0‖ more precisely than previously possible. We find that the temperature dependence of P0‖ exhibits activation-type behavior reflecting the first excitation gap. We also find that 0‖ decreases monotonically on cooling but saturates to a nonzero value at zero temperature. This strongly suggests that 0‖ is dominated not only by the electron-phonon interactions but also by internanomagnet dipole interactions, which have been neglected to date.

Spin-lattice relaxation via quantum tunneling in anEr3+-polyoxometalate molecular magnet

We investigate the mechanism of spin-lattice relaxation of Er ions encapsulated in polyoxometalate clusters, which below 4 K can only reverse its spin via quantum tunneling processes. The temperature-independent rate ${\ensuremath{\tau}}^{\ensuremath{-}1}$ is, at zero field, ten orders of magnitude larger than the rates predicted for direct phonon-induced processes. In addition, we observe that ${\ensuremath{\tau}}^{\ensuremath{-}1}$ is suppressed by external magnetic bias and hyperfine interactions but enhanced by increasing the concentration of Er ions. The observed relaxation agrees with predictions for pure quantum tunneling, showing that this phenomenon drives the thermalization of electronic spins. A possible link between these two phenomena is discussed, involving the collective emission of phonons from particular spin configurations attained via quantum tunneling.

Reverse phenomena of magnetic field effects and time-resolved EPR spectra in the photogenerated biradical from intramolecular electron-transfer in a phenothiazine–C60 linked compound with a semi-rigid spacer

Photoinduced electron-transfer reactions and magnetic field effects (MFEs) on the decay rates of the photogenerated biradical in a phenothiazine (Ph)–C60 linked compound with a biphenyl group (Ph(BP)C60) were examined in benzonitrile and benzene. Fluorescence and transient absorption spectra indicate that the intramolecular electron-transfer for Ph(BP)C60 from the Ph to the singlet or triplet excited state of C60 was suppressed by the biphenyl group. The decay rates of the photogenerated biradical decreased in the 0–0.2 T magnetic field range and increased in the 0.2–1 T magnetic field range. The reverse phenomena of the MFEs in Ph(BP)C60 were strongly enhanced with increasing temperature and similar to those in Ph(n)C60 (n = 6−12). The MFEs in Ph(BP)C60 can be governed by spin-lattice relaxation and/or spin-spin relaxation mechanisms as observed in Ph(n)C60 (n = 6−12). Time-resolved EPR spectra of Ph(BP)C60 showed absorption, emission, absorption and emission patterns, and are quite different from those in Ph(n)C60 (n = 4−12). The result indicates that the magnitude and distribution of the exchange interaction |2J| in Ph(BP)C60 are smaller than those in Ph(n)C60 (n = 4−12) and charge recombination occurs in the inverted region because the sign of the J is positive.

Spin-lattice relaxation of 113Cd and 19F nuclear spins in the crystal lattice of CdF2 semiconductor crystals with DX centers

Temperature dependences of spin-lattice relaxation rates of the 113Cd and 19F lattice nuclei in the CdF2 semiconductor crystals containing bistable In and Ga impurity centers show that the relaxation mechanisms in the CdF2:In and CdF2:Ga crystals are different. The basic mechanism of spin-lattice relaxation of the 113Cd nuclei in the CdF2:In crystal is the scalar contact’s interaction of nuclear spins with spins of mobile charge carriers of the conduction band. In the CdF2:Ga crystal, relaxation of the 113Cd nuclei is controlled by the contact interaction with electrons moving within a narrow band of impurity states. The same mechanism is apparently responsible for relaxation of the 19F nuclei in this crystal. In the CdF2:In crystal, the 19F nuclei relax by the dipole-dipole interaction with electron spins localized at the hydrogen-like orbits of shallow donors.

1H NMR Study Of Hydrogenated Nanocrystalline Silicon Thin Films

AbstractHydrogenated nano-crystalline silicon (nc-Si:H) is a promising material for multi-junction solar cells. We investigated the local hydrogen environments in nano-crystalline silicon thin films by nuclear-magnetic-resonance (NMR). At room temperature, 1H NMR spectra have broader components than those observed in standard device grade hydrogenated amorphous silicon (a-Si:H). As the temperature decreases, the 1H NMR exhibits a broadening of the line shape attributed to hydrogen atoms at the interfaces between the amorphous silicon (a-Si) and the crystalline silicon (c-Si) regions. These results suggest that the local hydrogen structure in nc-Si:H is very different from that in a-Si:H. In particular, the hydrogen clusters contributing to broadened spectra may exist on the surfaces of the a-Si/c-Si interfaces which do not exist in the more dense matrix of a-Si:H and may contribute to certain unique optoelectronic properties of these nc-Si:H thin films. The dependence of the spin-lattice relaxation time (T1) on temperature, however, is very similar to that in a-Si:H, which indicates the spin-lattice relaxation mechanism, i.e via spin diffusion through molecular hydrogen, is common to both systems.

Feasibility of 19F-NMR for Assessing the Molecular Mobility of Flufenamic Acid in Solid Dispersions

The purpose of the present study was to clarify the feasibility of 19F-NMR for assessing the molecular mobility of flufenamic acid (FLF) in solid dispersions. Amorphous solid dispersions of FLF containing poly(vinylpyrrolidone) (PVP) or hydroxypropylmethylcellulose (HPMC) were prepared by melting and rapid cooling. Spin-lattice relaxation times (T1 and T(1rho)) of FLF fluorine atoms in the solid dispersions were determined at various temperatures (-20 to 150 degrees C). Correlation time (tauc), which is a measure of rotational molecular mobility, was calculated from the observed T1 or T1rho value and that of the T1 or T1rho minimum, assuming that the relaxation mechanism of spin-lattice relaxation of FLF fluorine atoms does not change with temperature. The tauc value for solid dispersions containing 20% PVP was 2-3 times longer than that for solid dispersions containing 20% HPMC at 50 degrees C, indicating that the molecular mobility of FLF in solid dispersions containing 20% PVP was lower than that in solid dispersions containing 20% HPMC. The amount of amorphous FLF remaining in the solid dispersions stored at 60 degrees C was successfully estimated by analyzing the solid echo signals of FLF fluorine atoms, and it was possible to follow the overall crystallization of amorphous FLF in the solid dispersions. The solid dispersion containing 20% PVP was more stable than that containing 20% HPMC. The difference in stability between solid dispersions containing PVP and HPMC is considered due to the difference in molecular mobility as determined by tauc. The molecular mobility determined by 19F-NMR seems to be a useful measure for assessing the stability of drugs containing fluorine atoms in amorphous solid dispersions.

Mechanism of nuclear spin-lattice relaxation and its field dependence for ultraslow atomic motion

The contribution of ultraslow self-diffusion of polycrystalline benzene molecules to the spin-lattice relaxation of protons is studied as a function of effective magnetic field H2 in a doubly rotating frame (DRF). Proton relaxation time T1ρρ is measured by direct recording of NMR in a rotating frame (RF). The effective fields have a “magic” orientation corresponding to angles arccos(1/√3) in the RF and π/2 in the DRF so that the secular part of the dipole-dipole interactions of protons is suppressed in two orders of perturbation theory, while the nonsecular part becomes predominant. It is found that the diffusion contribution of benzene molecules to proton relaxation time T1ρρ is a linear function of the square of field H2 and exhibits all peculiarities typical of the model of strong collisions generalized to only fluctuating nonsecular dipole interactions in fields exceeding the local field. This means that the model can also be employed in the given conditions. It is shown that perfect agreement with such a dependence can also be obtained in the model of weak collisions if we take into account the concept of the locally effective quantization field, whose magnitude and direction are controlled by the vector sum of field H2, and the nonsecular local field perpendicular to it.

Electron spin relaxation via flexural phonon modes in semiconducting carbon nanotubes

This work considers the $g$-tensor anisotropy induced by the flexural thermal vibrations in one-dimensional structures and its role on electron spin relaxation. In particular, the mechanism of spin-lattice relaxation via flexural modes is theoretically studied for localized and delocalized electronic states in semiconducting carbon nanotubes in the presence of a magnetic field. The calculation of a one-phonon spin-flip process predicts distinctive dependencies of the relaxation rate on temperature, magnetic field, and nanotube diameter. A comparison to the spin relaxation caused by the hyperfine interaction clearly suggests the relative efficiency of the proposed mechanism at sufficiently high temperatures. Specifically, the longitudinal spin relaxation time in the semiconducting carbon nanotubes is estimated to be as short as $30\text{ }\ensuremath{\mu}\text{s}$ at room temperature.

Electron spin relaxation and quantum localization in carbon nanoparticle: Electron spin echo studies

We present pulsed electron spin resonance measurements of activated carbon fibers (ACFs) for pristine samples and after adsorption of nitrobenzene in the fibers' porous structure. Two-level tunneling state (TLS) model is discussed in order to define the nature of paramagnetic centers and to explain the electron spin-lattice relaxation mechanism in the studied system. Electron spin echo decay is dominated by spin diffusion at low temperatures (below $7\phantom{\rule{0.3em}{0ex}}\mathrm{K}$) and for higher temperatures the echo dephasing rate is governed by thermal excitations of the TLS model. The model enables us to treat the nanographitic units being the basic structural units of ACF, as quantum wells (dots) separated by potential barriers of the order of $25(2)\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ determined from temperature dependence of the spin-lattice relaxation rate. Stimulation of these barriers leads to spin localization within nanographitic units. A possible control of the potential barriers is the crucial point for the problem of nanographitic unit system treated as a quantum dot matrix.

Spin Resonance Absorption in a 2D Electron Gas

We analyze the power absorption at electron spin resonance excited by the Bychkov{Rashba spin{orbit fleld in a 2D electron gas. We show that, as long as the absorbed power is dissipated by the usual spin lattice relaxation mechanisms, the resonance line shape is expected to be of pure absorption type, i.e., it can be described by the imaginary part of the dynamic magnetic susceptibility. Therefore, the Dysonian line shape observed in ESR of 2D electron gas cannot result from this type of the resonance signal.

Magnetic field effects and time-resolved EPR studies on the photogenerated biradicals in phenothiazine–C 60 linked systems: Clarification of the mechanism of novel magnetic field effects by dependences of methylene chain length and temperature

Magnetic field effects (MFEs) on photogenerated biradical in phenothiazine (Ph)–C 60 linked compound with four methylene group (Ph(4)C 60 ) have been investigated. Transient absorption spectra showed photoinduced intramolecular electron-transfer reactions from the Ph to triplet excited state of C 60 ( 3 C 60 ∗ ) . With increasing magnetic field, the decay rate constant of the photogenerated biradical decreased steeply at lower magnetic fields (<0.2 T), and then recovered in the 0.2 T < H < 1.0 T region. Temperature dependence on the reverse phenomenon of the MFEs in Ph(4)C 60 was clearly different from those in other linked compounds ( Ph( n)C 60 ( n = 6, 8, 10, 12)). The present MFEs can be explained by the contribution of not only spin–lattice relaxation mechanism but also spin–spin relaxation mechanism related to ∣2 J∣. In time-resolved EPR measurements, the spectra in Ph( n)C 60 ( n = 4–12) are assigned to spin-correlated radical pairs. The methylene chain dependence of the time-resolved EPR spectra also supports the mechanism suggested in MFEs.

Resonant susceptibility of dielectric glasses in magnetic field

The anomalous magnetic field dependence of dielectric properties of insulating glasses in the temperature interval $10mK<T<50mK$ is considered. In this temperature range, the dielectric permittivity is defined by the resonant contribution of tunneling systems. The external magnetic field regulates nuclear spins of tunneling atoms. This regulation suppresses a nuclear quadrupole interaction of these spins with lattice and, thus, affects the dielectric response of tunneling systems. It is demonstrated that in the absence of an external magnetic field the nuclear quadrupole interaction $b$ results in the correction to the permittivity $\delta\chi\sim| b| /T$ in the temperature range of interest. An application of a magnetic field results in a sharp increase of this correction approximately by a factor of two when the Zeeman splitting $m$ approaches the order of $| b| $. Further increase of the magnetic field results in a relatively smooth decrease in the correction until the Zeeman splitting approaches the temperature. This smooth dependence results from tunneling accompanied by a change of the nuclear spin projection. As the magnetic field surpasses the temperature, the correction vanishes. The results obtained in this paper are compared with experiment. A new mechanism of the low temperature nuclear spin-lattice relaxation in glasses is considered.

Spin-Wave Contribution to the Nuclear Spin-Lattice Relaxation in Triplet Superconductors

We discuss collective spin-wave excitations in triplet superconductors with an easy axis anisotropy for the order parameter. Using a microscopic model for interacting electrons, we estimate the frequency of such excitations in Bechgaard salts and ruthenate superconductors to be 1 and 20 GHz, respectively. We introduce an effective bosonic model to describe spin-wave excitations and calculate their contribution to the nuclear spin-lattice relaxation rate. We find that, in the experimentally relevant regime of temperatures, this mechanism leads to the power law scaling of 1/T1 with temperature. For two- and three-dimensional systems, the scaling exponents are 3 and 5, respectively. We discuss experimental manifestations of the spin-wave mechanism of the nuclear spin-lattice relaxation.

Evidence for disorder in l-alanine lattice detected by Pulsed-EPR spectroscopy at cryogenic temperatures

The unusual behavior of lattice dynamics of l-alanine has been assigned to intermolecular dynamics and localization of vibrational energy. Recent heat capacity and Pulsed-EPR measurements support presence of thermally activated dynamic orientational disorder in the l-alanine lattice below 20 K. In the present study, the additional evidence for possible thermally activated disordered behavior of l-alanine lattice have been obtained by investigating dependences of longitudinal relaxation time of first stable l-alanine radical, SAR1, on sample cooling rates for the same low temperature interval. The obtained relaxation time by Pulsed-EPR shows clear dependence on cooling rates and this behavior can be explained within two types of suggested spin-lattice relaxation mechanisms for the paramagnetic centers in the hydrogen-bonded organic crystal.

Magnetic field effects on photoelectrochemical reactions of modified electrodes with C60-phenothiazine nanoclusters

Magnetic field effects on the photoelectrochemical reactions of photosensitive electrodes modified with C60 nanoclusters were examined, intended for the development of nanodevices where photofunctions were controllable with magnetic field. Mixed clusters of a C60 derivative (C60N+) containing a positive charge and methylphenothiazine (MePH) were prepared by dissolving them in the THF–H2O mixed solvent. Diameters of the clusters were estimated to be ca. 100nm from dynamic light scattering and AFM measurements. Photofunctional electrodes with the nanoclusters of C60N+ and MePH were fabricated on the electrodes modified with a self-assembled monolayer of on a gold substrate. The photocurrent action spectrum of the cluster-modified electrode strongly indicates that the photocurrents are ascribed to the photoexcitation of the nanoclusters of C60N+. Furthermore, the photocurrents increased with increasing an external magnetic field, typically 3% at 0.5 T. The profile of magnetic field dependence on the photocurrents is in good agreement with the results on the dynamics of radical pair in the nanoclusters of C60N+ and MePH in the THF–H2O mixed solvent. This is suggested that the magnetic field effects are ascribable to the contribution of triplet radical pair that are generated by photoinduced intermolecular electron-transfer from MePH to triplet excited state of nanocluster of C60N+, and are explained in terms of spin-lattice relaxation mechanism in radical pair mechanism.

Photoinduced electron-transfer and magnetic-field effects on the decay rates of photogenerated biradicals in a phenothiazine-C60 linked compound with six methylene groups: temperature dependence

Abstract Photoinduced electron-transfer reactions and magnetic-field effects (MFEs) on the decay of photogenerated biradical in a phenothiazine-C60 linked compound with six methylene groups were examined in benzonitrile at various temperatures (283–343 K). The decay rate constants of the biradicals were dramatically decreased in the lower magnetic fields (0–0.2 T) and increased in the higher magnetic fields (0.2–1.0 T). These reverse phenomena in the MFEs were more clearly observed with the increase of temperature and are explained by the contribution of spin-lattice relaxation mechanism due to anisotropic Zeeman interaction. The temperature dependence on the MFEs supports the above mechanism. To cite this article: S. Moribe et al., C. R. Chimie 9 (2006).

Study of NMR spin-lattice relaxation mechanism and mutual viscosity in some substituted alcohols

The present communication reports the experimental values of NMR spin-lattice relaxation time ( T 1) and dielectric relaxation time ( τ) of α-naphthol, β-naphthol, o-aminophenol, benzyl alcohol, phenol, pyrogallol, catechol, and the experimental values of mutual viscosity η 12 of o-aminophenol, m-aminophenol, and p-aminophenol. The correlation of mutual viscosity with dielectric relaxation time ( τ) of the compounds investigated leads to the conclusion that mutual viscosity is a better representation of the resistance to the rotation of the individual solute molecule. The experimental values of T 1 have been correlated with calculated values of T 1 obtained using various equations of dielectric relaxation time. It has been concluded that Murty's equation is a better substitute of dielectric relaxation phenomenon.

Spin relaxation processes in the defect hole centres of Be doped MgO single crystals

The dependence of the width and intensity of the EPR spectrum lines of various paramagnetic hole centers in MgO:Be on the measurement temperature and applied microwave power has been measured. Using these data, the longitudinal and transverse relaxation times are estimated. With the aid of the found relaxation times, the temperature dependences of the EPR spectra of these centres are calculated. The validity of the obtained results is estimated by comparing the calculated temperature dependences of the EPR spectra with the measured ones. The mechanism of spin-lattice relaxation is analyzed using the models of the centres. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Temperature dependence on magnetic field effects on the decay rates of triplet biradical photogenerated from intramolecular electron-transfer in a phenothiazine-C 60 linked compound

The decay rate constants of the photogenerated biradical of a phenothiazine-C 60 linked compound dramatically decreased in lower magnetic fields (0–0.2 T) and then gradually increased in higher magnetic fields (0.2–1 T) in benzonitrile at various temperatures (283–343 K). As temperature increased, the magnitudes of magnetic field effects (MFEs) in the lower magnetic fields were reduced, while those in the higher magnetic fields were enhanced. These reverse phenomena in the MFEs are explained by the contribution of spin-lattice relaxation mechanism due to anisotropic Zeeman interaction. The temperature effect supports the above mechanism and is probably ascribed to the molecular motion in the biradical.

An NMR study of the origin of dioxygen-induced spin-lattice relaxation enhancement and chemical shift perturbation

Due to its depth-dependent solubility, oxygen exerts paramagnetic effects which become progressively greater toward the hydrophobic interior of micelles, and lipid bilayer membranes. This paramagnetic gradient, which is manifested as contact shift perturbations ( 19F and 13C NMR) and spin-lattice relaxation enhancement ( 19F and 1H NMR), has been shown to be useful for precisely determining immersion depth, membrane protein secondary structure, and overall topology of membrane proteins. We have investigated the influence of oxygen on 19F and 13C NMR spectra and spin-lattice relaxation rates of a semiperfluorinated detergent, (8,8,8)-trifluoro (3,3,4,4,5,5,6,6,7,7)-difluoro octylmaltoside (TFOM) in a model membrane system, to determine the dominant paramagnetic spin-lattice relaxation and shift-perturbation mechanism. Based on the ratio of paramagnetic spin-lattice relaxation rates of 19F and directly bonded 13C nuclei, we conclude that the dominant relaxation mechanism must be dipolar. Furthermore, the temperature dependence of oxygen-induced chemical shift perturbations in 19F NMR spectra suggests a contact interaction is the dominant shift mechanism. The respective hyperfine coupling constants for 19F and 13C nuclei can then be estimated from the contact shifts 〈 ( Δ ν / ν 0 ) 19 F 〉 and 〈 ( Δ ν / ν 0 ) 13 C 〉 , allowing us to estimate the relative contribution of scalar and dipolar relaxation to 19F and 13C nuclei. We conclude that the contribution to spin-lattice relaxation from the oxygen induced paramagnetic scalar mechanism is negligible.