Showing papers on "Relaxation (NMR) published in 2010"

Slow Magnetic Relaxation in a High-Spin Iron(II) Complex

Abstract: Slow magnetic relaxation is observed for [(tpaMes)Fe]−, a trigonal pyramidal complex of high-spin iron(II), providing the first example of a mononuclear transition metal complex that behaves as a single-molecule magnet. Dc magnetic susceptibility and magnetization measurements reveal a strong uniaxial magnetic anisotropy (D = −39.6 cm−1) acting on the S = 2 ground state of the molecule. Ac magnetic susceptibility measurements indicate the absence of slow relaxation under zero applied dc field as a result of quantum tunneling of the magnetization. Application of a 1500 Oe dc field initiates slow magnetic relaxation, which follows a thermally activated tunneling mechanism at high temperature to give an effective spin-reversal barrier of Ueff = 42 cm−1 and follows a temperature-independent tunneling mechanism at low temperature. In addition, the magnetic relaxation time shows a pronounced dc-field dependence, with a maximum occurring at ∼1500 Oe.

Intrinsic phonon relaxation times from first-principles studies of the thermal conductivities of Si and Ge

Abstract: Using a first-principles approach, we present forms for the intrinsic phonon relaxation times in semiconductors, which properly reflect the physically distinct behaviors of the normal and umklapp scattering processes. We find that accurate representation of the phonon-phonon scattering strength and inclusion of scattering of acoustic phonons by optic phonons are essential ingredients, which are missing from the decades old derivations of commonly used intrinsic relaxation times. We also assess the validity of the relaxation time approximation itself for silicon and germanium.

Slow magnetic relaxation in a family of trigonal pyramidal iron(II) pyrrolide complexes.

Abstract: We present a family of trigonal pyramidal iron(II) complexes supported by tris(pyrrolyl-α-methyl)amine ligands of the general formula [M(solv)(n)][(tpa(R))Fe] (M = Na, R = tert-butyl (1), phenyl (4); M = K, R = mesityl (2), 2,4,6-triisopropylphenyl (3), 2,6-difluorophenyl (5)) and their characterization by X-ray crystallography, Mossbauer spectroscopy, and high-field EPR spectroscopy. Expanding on the discovery of slow magnetic relaxation in the recently reported mesityl derivative 2, this homologous series of high-spin iron(II) complexes enables an initial probe of how the ligand field influences the static and dynamic magnetic behavior. Magnetization experiments reveal large, uniaxial zero-field splitting parameters of D = -48, -44, -30, -26, and -6.2 cm(-1) for 1-5, respectively, demonstrating that the strength of axial magnetic anisotropy scales with increasing ligand field strength at the iron(II) center. In the case of 2,6-difluorophenyl substituted 5, high-field EPR experiments provide an independent determination of the zero-field splitting parameter (D = -4.397(9) cm(-1)) that is in reasonable agreement with that obtained from fits to magnetization data. Ac magnetic susceptibility measurements indicate field-dependent, thermally activated spin reversal barriers in complexes 1, 2, and 4 of U(eff) = 65, 42, and 25 cm(-1), respectively, with the barrier of 1 constituting the highest relaxation barrier yet observed for a mononuclear transition metal complex. In addition, in the case of 1, the large range of temperatures in which slow relaxation is observed has enabled us to fit the entire Arrhenius curve simultaneously to three distinct relaxation processes. Finally, zero-field Mossbauer spectra collected for 1 and 4 also reveal the presence of slow magnetic relaxation, with two independent relaxation barriers in 4 corresponding to the barrier obtained from ac susceptibility data and to the 3D energy gap between the M(S) = ±2 and ±1 levels, respectively.

Magnetic interactions between nanoparticles.

Abstract: We present a short overview of the influence of inter-particle interactions on the properties of magnetic nanoparticles. Strong magnetic dipole interactions between ferromagnetic or ferrimagnetic particles, that would be superparamagnetic if isolated, can result in a collective state of nanoparticles. This collective state has many similarities to spin-glasses. In samples of aggregated magnetic nanoparticles, exchange interactions are often important and this can also lead to a strong suppression of superparamagnetic relaxation. The temperature dependence of the order parameter in samples of strongly interacting hematite nanoparticles or goethite grains is well described by a simple mean field model. Exchange interactions between nanoparticles with different orientations of the easy axes can also result in a rotation of the sub-lattice magnetization directions.

Measurement of Fast Electron Spin Relaxation Times with Atomic Resolution

Abstract: Single spins in solid-state systems are often considered prime candidates for the storage of quantum information, and their interaction with the environment the main limiting factor for the realization of such schemes. The lifetime of an excited spin state is a sensitive measure of this interaction, but extending the spatial resolution of spin relaxation measurements to the atomic scale has been a challenge. We show how a scanning tunneling microscope can measure electron spin relaxation times of individual atoms adsorbed on a surface using an all-electronic pump-probe measurement scheme. The spin relaxation times of individual Fe-Cu dimers were found to vary between 50 and 250 nanoseconds. Our method can in principle be generalized to monitor the temporal evolution of other dynamical systems.

Observation of a secondary slow relaxation process for the field-induced single-molecule magnet U(H2BPz2)3.

Abstract: The trigonal prismatic complex U(H2BPz2)3 is characterized by single crystal X-ray diffraction and ac magnetic susceptibility measurements. The ac susceptibility data demonstrate the presence of multiple processes responsible for slow magnetic relaxation. Out-of-phase signals observed at ac switching frequencies between 1 and 1500 Hz in dc fields of 500−5000 Oe indicate a thermal relaxation barrier of ca. 8 cm−1 for the molecule, with a temperature-independent process taking over at the lowest temperatures probed. Significantly, an unprecedented, slower relaxation process becomes apparent for ac switching frequencies between 0.06 and 1 Hz, for which a monotonic increase of the relaxation time with an applied dc field suggests a direct relaxation pathway.

Slow relaxation processes and single-ion magnetic behaviors in dysprosium-containing complexes.

Abstract: A series of one-dimensional complexes [Ln(L1)3(HOCH2CH2OH)]n (L1 = 2-furoate anion; Ln = Nd (1), Sm (2), Gd (3), Tb (4), Dy (5), Er (6)) have been synthesized. The complexes were crystallized in the monoclinic space group P2(1)/c and show a chain-like structure determined by single-crystal X-ray diffraction. Magnetic properties indicate that carboxyl group of 2-furoate mediates different magnetic couplings in light and heavy rare earth complexes, namely, antiferromagnetic interaction between light rare earth ions and ferromagnetic interaction between heavy ones. Noticeably, complex 5 displays a strong frequency dependence of alternating current (AC) magnetic properties. Further magnetic studies show a distribution of a single relaxation process in 5. While 1,10-phenanthroline and phthalate anion (L2) were employed, [Dy2(L2)6(H2O)]n (7) was isolated by hydrothermal reactions and characterized magnetically. Research results also show the frequency dependence of AC magnetic susceptibilities, although the pht...

[ReCl4(CN)2]2-: a high magnetic anisotropy building unit giving rise to the single-chain magnets (DMF)4MReCl4(CN)2 (M = Mn, Fe, Co, Ni).

Abstract: An S = 3/2, high-anisotropy building unit, trans-[ReCl4(CN)2]2−, representing the first paramagnetic complex with a mixture of just cyanide and halide ligands, has been synthesized through the reaction of (Bu4N)CN with ReCl4(THF)2. This species is characterized in detail and employed in directing the formation of a series of one-dimensional coordination solids of formula (DMF)4MReCl4(CN)2 (M = Mn (2), Fe (3), Co (4), Ni (5)). Variable-temperature dc magnetic susceptibility measurements demonstrate the presence of intrachain antiferromagnetic (2) and ferromagnetic (3−5) exchange coupling within these solids. In addition, probing the ac magnetic susceptibility as a function of both temperature and frequency reveals that all of the chain compounds exhibit slow relaxation of the magnetization. The relaxation time is shown to be thermally activated, with energy barriers to relaxation of Δτ = 31, 56, 17, and 20 cm−1 for 2−5, respectively. Notably, the field-dependent magnetization of the iron congener exhibits ...

Ultrafast transient absorption microscopy studies of carrier dynamics in epitaxial graphene.

Abstract: Transient absorption microscopy was employed to image charge carrier dynamics in epitaxial multilayer graphene. The carrier cooling exhibited a biexponential decay that showed a significant dependence on carrier density. The fast and slow relaxation times were assigned to coupling between electrons and optical phonon modes and the hot phonon effect, respectively. The limiting value of the slow relaxation time at high pump intensity reflects the lifetime of the optical phonons. Significant spatial heterogeneity in the dynamics was observed due to differences in coupling between graphene layers and the substrate.

Analysis of Amorphous Solid Dispersions Using 2D Solid-State NMR and 1H T1 Relaxation Measurements

Abstract: Solid-state NMR (SSNMR) can provide detailed structural information about amorphous solid dispersions of pharmaceutical small molecules. In this study, the ability of SSNMR experiments based on dipolar correlation, spin diffusion, and relaxation measurements to characterize the structure of solid dispersions is explored. Observation of spin diffusion effects using the 2D 1H−13C cross-polarization heteronuclear correlation (CP-HETCOR) experiment is shown to be a useful probe of association between the amorphous drug and polymer that is capable of directly proving glass solution formation. Dispersions of acetaminophen and indomethacin in different polymers are examined using this approach, as well as 1H double-quantum correlation experiments to probe additional structural features. 1H−19F CP-HETCOR serves a similar role for fluorinated drug molecules such as diflunisal in dispersions, providing a rapid means to prove the formation of a glass solution. Phase separation is detected using 13C, 19F, and 23Na-de...

Influence of silver ion reduction on electrical modulus parameters of solid polymer electrolyte based on chitosan- silver triflate electrolyte membrane

Abstract: The electric modulus properties of solid polymer electrolyte based on chitosan: AgCF3SO3 from 303 to 393 K have been investigated by using impedance spectroscopy. The shift of the M" peak spectra with frequeny depends on the dissociation and association of ions. The lowest conductivity relaxation time τσ, was found for the sample with the highest conductivity. The real part of electrical modulus shows that the material is highly capacitive. The asymmetric peak of the imaginary part of electric modulus M", predicts a non Debye type relaxation. The distribution of relaxation times was indi- cated by a deformed arc form of Argand plot. The increase of Mand M" values above 358 K can be attributed to the trans- formation of silver ions to silver nanoparticles. The complex impedance plots and ultraviolet-visible (UV-vis) absorption spectroscopy indicate the temperature dependent of silver nanoparticles in chitosan-silver triflate solid electrolyte. The for- mation of silver nanoparticles was confirmed by transmission electron microscopy (TEM). The scaling behavior of M" spectra shows that the dynamical relaxation processes is temperature independent for aparticular composition. The β expo- nent value indicate that the conductivity relaxation is highly non exponential.

Investigation of conduction and relaxation phenomena in LaFe 0.9 Ni 0.1 O 3 by impedance spectroscopy

Abstract: Polycrystalline LaFe0.9Ni0.1O3 is prepared by the solid state reactions route. X-ray diffraction is used to analyse the phase purity of the compound. Impedance spectra of LaFe0.9Ni0.1O3 over the frequency range of 1?Hz to 10?MHz are investigated at different temperatures from 100 to 373?K. Two relaxation processes with different relaxation times are observed at each temperature. An equivalent circuit model (R1Q1) (R2Q2) is applied to explore the physical parameters associated with grains and grain boundaries. Frequency and temperature dependence of the relaxation processes and extracted parameters are discussed in terms of hopping between Fe+4 and Fe+3 and trap state scattering at grain boundaries. A change in conduction mechanism from variable range hopping to adiabatic small polaron hopping at 296?K is evident from these results. Traps are active below 296?K in capturing the charge.

Nuclear-Magnetic-Resonance Measurements Reveal the Origin of the Debye Process in Monohydroxy Alcohols

Abstract: Monohydroxy alcohols show a structural relaxation and at longer time scales a Debye-type dielectric peak. From spin-lattice relaxation experiments using different nuclear probes, an intermediate, slower-than-structural dynamics is identified for n-butanol. Based on these findings and on translational diffusion measurements, a model of self-restructuring, transient chains is proposed. The model is demonstrated to explain consistently the so-far puzzling observations made for this class of hydrogen-bonded glass forming liquids.

Relaxation Phenomena in Vitrifying Polymers and Molecular Liquids

Abstract: Recent experimental results on the dynamics of glass-forming materials, particularly polymers, are surveyed. The focus is on aspects of the behavior that are connected to or correlated with structural relaxation. These results include the invariance to thermodynamic conditions (temperature, pressure, volume) of a number of properties—breadth of the relaxation dispersion, number of dynamically correlating molecules, Johari−Goldstein secondary relaxation time, onset of the dynamic crossover, and the product of temperature and specific volume with the latter raised to a material constant—provided the structural relaxation time is maintained constant. Additional salient experimental findings include the correlation of various high-frequency processes, usually measured in the glassy state, with properties of the equilibrium material above Tg. These correlations indicate that the glass transition, although conventionally defined by the relaxation time becoming larger than experimental time scales (>100 s), has ...

In-plane phonon transport in thin films

Abstract: The in-plane phonon thermal conductivities of argon and silicon thin films are predicted from the Boltzmann transport equation under the relaxation time approximation. We model the thin films using bulk phonon properties obtained from harmonic and anharmonic lattice dynamics calculations. The input required for the lattice dynamics calculations is obtained from interatomic potentials: Lennard-Jones for argon and Stillinger–Weber for silicon. The effect of the boundaries is included by considering only phonons with wavelengths that fit within the film and adjusting the relaxation times to account for mode-dependent, diffuse boundary scattering. Our model does not rely on the isotropic approximation or any fitting parameters. For argon films thicker than 4.3 nm and silicon films thicker than 17.4 nm, the use of bulk phonon properties is found to be appropriate and the predicted reduction in the in-plane thermal conductivity is in good agreement with results obtained from molecular dynamics simulation and ex...

Evidence for thermal mechanisms in laser-induced femtosecond spin dynamics

Abstract: Recent pump-probe experiments using powerful femtosecond lasers and x-ray magnetic circular dichroism have opened a debate on the origin of the magnetization modification on the femtosecond time scale. We show a quantitative agreement between femtosecond optical pump-probe experiments and thermal micromagnetic modeling in nickel, which reveals a predominant thermal demagnetization mechanism. Magnetic fluctuations are introduced in the system as spin-flip processes due to scattering mechanisms in the electron system. In our model the Landau-Lifshitz-Bloch equation for a macrospin (containing the statistically averaged magnetic fluctuations) is coupled to the electronic temperature of the two-temperature model whose parameters are extracted from the measured reflectivity. We show that the demagnetization and the magnetization recovery time slow down as the laser pump fluence is increased and identify the longitudinal relaxation as a key factor for the observed behavior.

Magnetic quantum tunneling: insights from simple molecule-based magnets

Abstract: This perspectives article takes a broad view of the current understanding of magnetic bistability and magnetic quantum tunneling in single-molecule magnets (SMMs), focusing on three families of relatively simple, low-nuclearity transition metal clusters: spin S = 4 NiII4, MnIII3 (S = 2 and 6) and MnIII6 (S = 4 and 12). The MnIII complexes are related by the fact that they contain triangular MnIII3 units in which the exchange may be switched from antiferromagnetic to ferromagnetic without significantly altering the coordination around the MnIII centers, thereby leaving the single-ion physics more-or-less unaltered. This allows for a detailed and systematic study of the way in which the individual-ion anisotropies project onto the molecular spin ground state in otherwise identical low- and high-spin molecules, thus providing unique insights into the key factors that control the quantum dynamics of SMMs, namely: (i) the height of the kinetic barrier to magnetization relaxation; and (ii) the transverse interactions that cause tunneling through this barrier. Numerical calculations are supported by an unprecedented experimental data set (17 different compounds), including very detailed spectroscopic information obtained from high-frequency electron paramagnetic resonance and low-temperature hysteresis measurements. Comparisons are made between the giant spin and multi-spin phenomenologies. The giant spin approach assumes the ground state spin, S, to be exact, enabling implementation of simple anisotropy projection techniques. This methodology provides a basic understanding of the concept of anisotropy dilution whereby the cluster anisotropy decreases as the total spin increases, resulting in a barrier that depends weakly on S. This partly explains why the record barrier for a SMM (86 K for Mn6) has barely increased in the 15 years since the first studies of Mn12–acetate, and why the tiny Mn3 molecule can have a barrier approaching 60% of this record. Ultimately, the giant spin approach fails to capture all of the key physics, although it works remarkably well for the purely ferromagnetic cases. Nevertheless, diagonalization of the multi-spin Hamiltonian matrix is necessary in order to fully capture the interplay between exchange and local anisotropy, and the resultant spin-state mixing which ultimately gives rise to the tunneling matrix elements in the high symmetry SMMs (ferromagnetic Mn3 and Ni4). The simplicity (low-nuclearity, high-symmetry, weak disorder, etc.) of the molecules highlighted in this study proves to be of crucial importance. Not only that, these simple molecules may be considered among the best SMMs: Mn6 possesses the record anisotropy barrier, and Mn3 is the first SMM to exhibit quantum tunneling selection rules that reflect the intrinsic symmetry of the molecule.

Non-adiabatic dynamics of pyrrole: Dependence of deactivation mechanisms on the excitation energy

Abstract: Non-adiabatic dynamics simulations were performed for pyrrole at time-dependent density functional theory level using the trajectory surface hopping approach. Initial conditions were prepared based on the UV-absorption spectrum so as to simulate monochromatic absorption in three distinct spectral regions. The results showed predominance of the NH-stretch mechanism for excited-state relaxation. With increasing initial energy, however, other mechanisms are activated as well, even though they still occurred for a minor fraction of the trajectories. Dynamics starting at the origin of the absorption spectrum exhibited internal conversion to the ground state with a time constant of 20 fs. In contrast, dynamics starting at higher energies gave rise to much longer time constants for internal conversion near 200 fs.

Ultrafast Relaxation Dynamics of [Au25(SR)18]q Nanoclusters: Effects of Charge State

Abstract: The ultrafast electron relaxation dynamics of anionic and neutral Au25(SR)18 nanoclusters are investigated using broad-band time-resolved optical spectroscopy. From an analysis of the wavelength-dependent transient absorption kinetics, we have obtained valuable information on the spectral features that originate from excitation of “core” and “core−shell” states. In both clusters, photoexcitation occurs into two nondegenerate states near the HOMO−LUMO gap that are derived from the core orbitals. A large difference in the lifetime of the core excitations is observed, with [Au25(SR)18]− exhibiting a decay rate more than 1000 times slower than the neutral cluster. Both clusters show strong coupling to two different coherent phonon modes, which are observed at 2.4 and 1.2 THz. The electron−phonon coupling is analyzed in terms of the spectral distribution and damping of the coherent modes.

Viscosity effect on the ultrafast bond twisting dynamics in an amyloid fibril sensor: thioflavin-T.

Abstract: The femtosecond fluorescence upconversion technique is used to study the effect of viscosity on the excited state relaxation dynamics of an amyloid fibril sensor, thioflavin-T, in different solvent media. The excited state decay in all of the solvents is seen to be dependent on the emission wavelength. From the constructed time-resolved emission spectra, it is seen that the present system shows dynamic Stokes’ shift as well as an appreciable increase in the spectral width with time. These temporal spectral characteristics of time-resolved emission spectra have been assigned to the formation of a new emissive species from the locally excited state of the thioflavin-T molecule. The formation of the new emissive state from the locally excited state is also supported by the fact that an iso-emissive point appears in the time-resolved area normalized emission spectra. From the detailed study on the excited state dynamics of thioflavin-T as a function of solvent viscosity, it is concluded that the new emissive ...

Systematic dielectric and NMR study of the ionic liquid 1-alkyl-3-methyl imidazolium.

Abstract: The dynamic behaviors of ionic liquid samples consisting of a series of 1-alkyl-3-methylimidazolium cations and various counteranionic species are investigated systematically over a wide frequency range from 1 MHz to 20 GHz at room temperature using dielectric relaxation (DR) and nuclear magnetic resonance (NMR) spectroscopies. DR spectra for the ionic liquids are reasonably deconvoluted into two or three relaxation modes. The slowest relaxation times are strongly dependent upon sample viscosity and cation size, whereas the relaxation times of other modes are almost independent of these factors. We attribute the two slower relaxation modes to the rotational relaxation modes of the dipolar cations because the correlation times of the cations evaluated using longitudinal relaxation time (T(1) (13)C NMR) measurements corresponded to the dielectric relaxation times. On the other hand, the fastest relaxation mode is presumably related to the inter-ion motions of ion-pairs formed between cationic and anionic species. In the case of the ionic liquid bis(trifluoromethanesulfonyl)imide, the system shows marked dielectric relaxation behavior due to rotational motion of dipolar anionic species in addition to the relaxation modes attributed to the dipolar cations.

Long-range magnetic order in CeRu2Al10 studied via muon spin relaxation and neutron diffraction

Abstract: The low temperature state of CeRu2Al10 has been studied by neutron powder diffraction and muon spin relaxation (muSR). By combining both techniques, we prove that the transition occurring below T*~27K, which has been the subject of considerable debate, is unambiguously magnetic due to the ordering of the Ce sublattice. The magnetic structure with propagation vector k=(1,0,0) involves collinear antiferromagnetic alignment of the Ce moments along the c-axis of the Cmcm space group with a reduced moment of 0.34(2)mu_B. No structural changes within the resolution limit have been detected below the transition temperature. However, the temperature dependence of the magnetic Bragg peaks and the muon precession frequency show an anomaly around T2~12 K indicating a possible second transition.

Mn2+ Complexes with Pyridine-Containing 15-Membered Macrocycles: Thermodynamic, Kinetic, Crystallographic, and 1H/17O Relaxation Studies

Abstract: Given its five unpaired d-electrons, long electronic relaxation time, and fast water exchange, Mn2+ is a potential candidate for contrast agent application in medical magnetic resonance imaging. Nevertheless, the design of chelators that ensure stable Mn2+ complexation and optimal relaxation properties remains a coordination chemistry challenge. Here, we report the synthesis of two pyridine-containing ligands L1 and L2, with 15-membered triaza-dioxa-crown and pentaaza-crown ether macrocycles, respectively, and the characterization of their Mn2+ complexes. Protonation constants of the ligands and stability constants of various metal complexes were determined by potentiometry. The presence of the pyridine in the macrocyclic ring induces rigidity of the complexes which results in a greater thermodynamic stability with respect to the nonpyridine analogues. Solid-state structures of MnL1 and MnL2 confirmed seven-coordination of Mn2+ with Cl− and H2O in axial positions. The dissociation kinetics of MnL2 in the ...

Ultrafast magnon generation in an Fe film on Cu(100).

Abstract: We report on a combined experimental and theoretical study of the spin-dependent relaxation processes in the electron system of an iron film on Cu(100). Spin-, time-, energy- and angle-resolved two-photon photoemission shows a strong characteristic dependence of the lifetime of photoexcited electrons on their spin and energy. Ab initio calculations as well as a many-body treatment corroborate that the observed properties are determined by relaxation processes involving magnon emission. Thereby we demonstrate that magnon emission by hot electrons occurs on the femtosecond time scale and thus provides a significant source of ultrafast spin-flip processes. Furthermore, engineering of the magnon spectrum paves the way for tuning the dynamic properties of magnetic materials.