Spin Relaxation Effects Research Articles

Spin-relaxation and magnetoresistance in FM/SC/FM tunnel junctions

The effect of spin relaxation on tunnel magnetoresistance (TMR) in a ferromagnet/superconductor/ferromagnet (FM/SC/FM) double tunnel junction is theoretically studied. The spin accumulation in SC is determined by balancing of the spin-injection rate and the spin-relaxation rate. In the superconducting state, the spin-relaxation time τ s becomes longer with decreasing temperature, resulting in a rapid increase of TMR. The TMR of FM/SC/FM junctions provides a useful probe to extract information about spin-relaxation in superconductors.

Effects of spin relaxation on electron tunneling through single discrete levels in magnetic tunnel junctions

Electron tunneling through a single discrete level of a quantum dot, coupled to two ferromagnetic leads, is studied theoretically in the sequential tunneling regime. Electron correlations and spin relaxation processes on the dot are taken into account. It is shown that strong Coulomb correlations can enhance tunnel magnetoresistance in a certain bias range. The effect, however, is suppressed by spin-flip processes.

Magnetic properties of syntheticP21/c(Mg-Fe)SiO3clinopyroxenes as observed from their low-temperature Mössbauer spectra and from SQUID magnetization measurements

Magnetic properties of six Ca-free P2 1 /c clinopyroxenes along the clinoenstatite-clinoferrosilite join were studied by transmission 57 Fe Mössbauer spectroscopy (0.3-4.2 K) and SQUID magnetization measurements (2-300 K). The Fe x Mg 1-x SiO 3 members were synthesized in a multi-anvil press, and samples with x = 1.00, 0.91, 0.87, 0.78, 0.61, and 0.09 have been considered in this study. The magnetic order-disorder transition temperatures TN were determined by Mössbauer thermoscanning with zero source velocity. The magnetic Mössbauer spectra were refined using a full hyperfineinteraction Hamiltonian approach assuming different Fe 2+ -Mg 2+ next-nearest-neighbor configurations of the probe nuclei to give a distinct spectral component. The different strengths of the M1 and M2 hyperfine fields can be related to the different 5 D electronic level schemes as earlier determined from the paramagnetic Mössbauer spectra. The magnetic susceptibility measurements indicate a positive paramagnetic Curie temperature θ p for FeSiO 3 (x = 1) above 50 K and a negative θ p for x ≤ 0.91 above 200 K. The Néel temperatures as determined from the susceptibility curves are equal to those obtained from the thermoscanning, indicating that spin relaxation effects can be ruled out. The macroscopic magnetic results further suggest ferromagnetic ordering of Fe 2+ ions within ribbons consisting of two linear bands of M2 sites that enclose chains of M1 sites, and antiferromagnetic coupling between neighboring ribbons. Generally spoken the magnetic behavior of Ca-free P2 1 /c clinopyroxenes are similar to that of orthopyroxenes with similar (Mg,Fe) compositions.

C 13 – C 13 dipolar recoupling under very fast magic angle spinning in solid-state nuclear magnetic resonance: Applications to distance measurements, spectral assignments, and high-throughput secondary-structure determination

A technique is presented to recouple homonuclear dipolar couplings between dilute spin pairs such as C13–C13 systems under very fast magic angle spinning (MAS) in solid-state nuclear magnetic resonance (NMR) spectroscopy. The presented technique, finite pulse rf driven recoupling (fpRFDR), restores homonuclear dipolar interactions based on constructive usage of finite pulse-width effects in a phase- and symmetry-cycled π-pulse train in which a rotor-synchronous π pulse is applied every rotation period. The restored effective dipolar interaction has the form of a zero-quantum dipolar Hamiltonian for static solids, whose symmetry in spin space is different from that obtained by conventional rf driven recoupling (RFDR) techniques. It is demonstrated that the efficiency of recoupling by fpRFDR is not strongly dependent on chemical shift differences or resonance offsets in contrast to previous recoupling methods under very fast MAS. To realize distance measurements without effects of spin relaxation, a constant-time version of fpRFDR (CT-fpRFDR) is introduced, in which the effective evolution period is varied by refocusing dipolar evolution with a rotor-synchronized solid echo while the total recoupling period is kept constant. From CT-fpRFDR experiments at a spinning speed of 30.3 kHz in a field of 17.6 T, the C13–C13 distance of [1-13C]Ala–[1-13C]Gly–Gly was determined to be 3.27 Å, which agrees well with the value of 3.20 Å obtained by x-ray diffraction. Also, two-dimensional (2D) C13/C13 chemical-shift correlation NMR spectrum in a field of 9.4 T was obtained with fpRFDR for fibrils of the segmentally C13- and N15-labeled Alzheimer’s β-Amyloid fragments, Aβ16–22 (residues 16–22 taken from the 40-residue Aβ peptide) in which Leu-17 through Ala-21 are uniformly C13- and N15-labeled. Most C13 resonances for the main chain as well as for the side chains are assigned based on 2D C13/C13 chemical-shift correlation patterns specific to amino-acid types. Examination of the obtained C13 chemical shifts revealed the formation of β-strand across the entire molecule of Aβ16–22. Possibility of high throughput determination of global main-chain structures based on C13 shifts obtained from 2D C13/C13 chemical-shift correlation under very fast MAS is also discussed for uniformly/segmentally C13-labeled protein/peptide samples.

Probing slow time scale dynamics at methyl-containing side chains in proteins by relaxation dispersion NMR measurements: application to methionine residues in a cavity mutant of T4 lysozyme.

A relaxation dispersion-based NMR experiment is presented for the measurement and quantitation of micros-ms dynamic processes at methyl side-chain positions in proteins. The experiment measures the exchange contribution to the 13C line widths of methyl groups using a constant-time CPMG scheme. The effects of cross-correlated spin relaxation between dipole-dipole and dipole-CSA interactions as well as the effects of scalar coupling responsible for mixing of magnetization modes during the course of the experiment have been investigated in detail both theoretically and through simulations. It is shown that the complex relaxation properties of the methyl spin system do not complicate extraction of accurate exchange parameters as long as care is taken to ensure that appropriate magnetization modes are interchanged in the middle of the constant-time CPMG period. An application involving the measurement of relaxation dispersion profiles of methionine residues in a Leu99Ala substitution of T4 lysozyme is presented. All of the methionine residues are sensitive to an exchange event with a rate on the order of 1200 s(-1) at 20 degrees C that may be linked to a process in which hydrophobic ligands are able to rapidly bind to the cavity that is present in this mutant.

Electronic states of polyanionic high-spin C 60 fullerene and their clusters: anomalous spin relaxation and Jahn-Teller effects

Tri- and dianionic C 60 fullerenes and polyanionic C 60 clusters were prepared by the successive chemical reduction of C 60 with potassium metal in solution. Trianionic spin quartet C 60 and polyanionic high-spin C 60 clusters were detected by cw-ESR and pulsed ESR-based 2D Electron Spin Transient Nutation (2D-ESTN) spectroscopy. The high spin states of annealed polyanionic C 60 show a spin relaxation anomaly. A phonon bottleneck phenomenon is responsible for the anomaly and the phenomenon is attributable to the occurrence of Jahn-Teller hot phonon molecular states of polyanionic C 60 fullerenes. Wiggling behaviours of the T 1 relaxation curves were observed for both the annealed tri- and dianionic C 60 . The peculiar time profiles were identified to arise from a quantum spin tunnelling between the Jahn-Teller assisted vibronic states. Polyanionic C 60 -based molecular clusters have high-spin ground states (S=1, 3/2, 2, 3). Their clustering structures were proposed in terms of tensorial analyses of the fine structure parameters.

The effect of spin relaxation on ENDOR spectra recorded at high magnetic fields and low temperatures.

A simple theoretical model that describes the pulsed Davies electron-nuclear double resonance (ENDOR) experiment for an electron spin S = (1/2) coupled to a nuclear spin I = (1/2) was developed to account for unusual W-band (95 GHz) ENDOR effects observed at low temperatures. This model takes into account the thermal polarization along with all internal relaxation processes in a four-level system represented by the electron- and nuclear-spin relaxation times T(1e) and T(1n), respectively, and the cross-relaxation time, T(1x). It is shown that under conditions of sufficiently high thermal spin polarization, nuclei can exhibit asymmetric ENDOR spectra in two cases: the first when t(mix) >> T(1e) and T(1n), T(1x) >> T(1e), where ENDOR signals from the alpha manifold are negative and those of the beta manifold positive, and the second when the cross- and/or nuclear-relaxation times are longer than the repetition time (t(mix) << T(1e) << t(R) and T(1n), T(1x) > t(R)). In that case the polarization of the ENDOR signals becomes opposite to the previous case, the lines in the alpha manifolds are positive, and those of the beta manifold are negative. This case is more likely to be encountered experimentally because it does not require a very long mixing time and is a consequence of the saturation of the nuclear transitions. Using this model the experimental t(mix) and t(R) dependencies of the W-band (1)H ENDOR amplitudes of [Cu(imidazole)(4)]Cl(2) were reproduced and the values of T(1e) and T(1x) >> T(1e) were determined. The presence of asymmetry in the ENDOR spectrum is useful as it directly provides the sign of the hyperfine coupling. The presented model allows the experimentalist to adjust experimental parameters, such as t(mix) and t(R), in order to optimize the desired appearance of the spectrum.

Application of integral encounter theory to account for the spin effects in radical reactions: I. Δg and spin relaxation effects on recombination kinetics of free radicals

Integral encounter theory is applied to investigate spin effects in radical recombination. Effects of electron spin relaxation and of S/T 0 mixing by an external magnetic field are considered. An analytical expression for the stationary rate constant is obtained from which appropriate conditions for experimental investigations are predicted.

Spin injection and relaxation properties of anisotropic superconductors

We report the spin injection and relaxation properties of the anisotropic YBa 2Cu 3O y(YBCO) superconductor by injection of spin-polarized quasiparticles using a cobalt ferromagnetic injector. The observed current gain depends on the thickness of the Au interlayer and is directly related to the nonequilibrium magnetization due to spin relaxation effects.

Nuclear spin relaxation in paramagnetic complexes of S=1: Electron spin relaxation effects

Electron spin relaxation for an S=1 system and its field dependence in the presence of static zero-field splitting (ZFS) has been described and incorporated in a model for nuclear spin-lattice relaxation in paramagnetic complexes in solution, proposed earlier by the group in Florence. Slow reorientation is assumed and the electron spin energy level structure (at any orientation of the molecule with respect to the laboratory frame) is described in terms of the Zeeman interaction and of the static ZFS. The electron spin relaxation is assumed to be caused by a transient ZFS modulated by the deformation of the complex described as a distortional (or pseudorotational) motion and the Redfield theory is used to derive the electron spin relaxation matrices. In the description of the electron spin relaxation we neglect any contribution from mechanisms involving modulation by reorientation, such as those of the static ZFS and the less important Zeeman interaction, as we limit ourselves to the slow-rotation limit (i.e., τR≫τS). This in general covers the behavior of proteins and macromolecules. The decomposition (DC) approximation is used, which means that the reorientational motion and electron spin dynamics are assumed to be uncorrelated. This is not a serious problem, due to the slow-rotation condition, since reorientational and distortional motions are time-scale separated. The resulting nuclear magnetic relaxation dispersion (NMRD) profiles obtained using the Florence model are calculated and compared with the calculations of the Swedish approach, which can be considered essentially exact within the given set of assumed interactions and dynamic processes. That theory is not restricted by the Redfield limit and can thus handle electron spin relaxation in the slow-motion regime, which is a consequence of not explicitly defining any electron spin relaxation times. Furthermore, the DC approximation is not invoked, and in addition, the electron spin relaxation is described by reorientationally modulated static ZFS and Zeeman interaction besides the distortionally modulated transient ZFS. The curves computed with the Florence model show a satisfactory agreement with these more accurate calculations of the Swedish approach, in particular for the axially symmetric static ZFS tensor, providing confidence in the adequacy of the electron spin relaxation model under the condition of slow rotation. The comparison is also quite instructive as far as the physical meaning of the electron spin relaxation and of its interplay with the nuclear spin system are concerned.

Spin-polarized quasiparticle tunnel injection in a YBa2Cu3Oy/Au/Co junction

We report the strong suppression of YBa2Cu3Oy (YBCO) supercurrent by injection of spin-polarized quasiparticles (QP) using a cobalt ferromagnetic injector. The injection of spin-polarized QP generates a substantially larger nonequilibrium population as compared with that of an unpolarized injection current. The observed current gain depends on the thickness of Au interlayer (dAu) and is directly related to the nonequilibrium magnetization due to spin relaxation effects. For dAu=15 nm, the tunnel characteristic a YBCO/Au/Co junction exhibited a zero bias conductance peak, which may be interpreted by spin scattering processes at a ferromagnetic/d-wave superconductor junction.

Localization effects in nuclear spin relaxation of a charge-transfer complex

The measurement of nuclear spin lattice relaxation rate has been performed on charge-transfer (CT) complexes based on the donor molecule of bisethylenedioxytetrathiafulvalene (BO). The relaxation rate of a CT complex of BO and decyltetra cyanoquinodimethane (C 10TCNQ) exhibits a noticeable peak at low temperatures. The behaviors of the peak are characterized by Anderson localization in weakly disordered systems. Through the quantitative analyses of the peak, the detailed dynamics of the localized electrons of the system has been provided successfully.

Ultrafast Spin Dynamics of Ferromagnetic Thin Films Observed by fs Spin-Resolved Two-Photon Photoemission

Using a ``pump and probe'' technique, the time scale of spin relaxation effects in ferromagnetic thin films is investigated. Samples are excited by a 3 eV ``pump'' laser pulse $(\ensuremath{\tau}\ensuremath{\approx}170\mathrm{fs})$, and electrons are photoemitted by a 6 eV ``probe'' pulse, after delays ranging up to 1 ns. The spin polarization of the photoelectrons is measured using a Mott detector. The time dependence of the spin polarization of thin Ni films on Ag is explained in a local magnetic moment picture with two excitation processes. Stoner excitations are responsible for the reduction of the observed spin polarization on a time scale of 1 ps and phonon-magnon scattering leads to the loss of long range magnetic order on a time scale of 500 ps.

X-ray crystal structures and Mössbauer studies of some trismalonatoferrate(III) compounds

Three trismalonatoferrate compounds (20% enriched in 57Fe) NaBa (FeMa 3)·3H 2O ( 1), [Na 3 (FeMa 3] 2·8H 2O ( 2), and Na 3)O·1/2 Sol ( 3) (Ma is the malonato anion and Sol is a solvate molecule) have been prepared and their X-ray crystal structures determined. The Mössbauer spectra exhibit a broadened line at 293 and 4.2 K indicating the presence of spin relaxation effects. The spectra data, fitted following Afanas'ev and Wickman models, point to the presence of spin-spin isotropic relaxation mechanism.

The effect of spin relaxation on ESR spectra of spin correlated radical pairs

The effect of spin relaxation on time resolved ESR spectra of spin-correlated radicals pairs is analyzed within the exponential stochastic Liouville equation approach. The shapes and amplitudes of spectra are shown to considerably change as a spin dephasing rate increases to values of order of the difference of Zeeman frequencies of radicals. In particular, antiphase structures of different lines merge in one of the opposite line located at the center of spectrum.

Effect of spin relaxation on the polarization of excitonic luminescence in disordered systems

The kinetics of localized excitons in systems with disorder is studied with allowance for the fine structure of the excitonic state and for spin relaxation processes. The exciton distribution function, formed as a result of the competition between radiative and nonradiative recombination, spin relaxation, and intercenter transitions with an exponentially wide scatter in transition times, is calculated.

Dynamic nuclear polarization of nitrogen-15 in benzamide

A 15N dynamic nuclear polarization (DNP) experiment is reported in which a 15N DNP enhancement factor of approximately 2.6 × 10 2 is obtained on free radical doped samples of 99% 15N labeled benzamide. The free radicals BDPA (1:1 complex of α, γ-bisdiphenylene-β-phenylallyl with benzene) and DPPH (2,2-Di(4-tert-octylphenyl)-1-picrylhydrazyl) are used as dopants and the spin relaxation effects of adding these dopants are studied by means of changes in proton and nitrogen T 1, values of the samples. The combination in solids of a very low natural abundance, 0.37%, a small gyromagnetic ratio, and a long spin-lattice relaxation time for 15N nuclei create severe sensitivity problems that, in large part, are ameliorated by the signal enhancement observed in the 15N DNP experiment on samples containing free electrons.

Size Dependence in the Disordered Kondo Problem

We study here the role randomly placed nonmagnetic scatterers play on the Kondo effect. We show that spin-relaxation effects (with time ${\ensuremath{\tau}}_{s}^{0}$) in the vertex corrections to the Kondo self-energy lead to an exact cancellation of the singular temperature dependence arising from the diffusion poles. For a thin film of thickness $L$ and a mean-free path $\ensuremath{\ell}$, disorder provides a correction to the Kondo resistivity of the form ${\ensuremath{\tau}}_{s}^{0}/({k}_{F}L{\ensuremath{\ell}}^{2})\mathrm{ln}T$ that explains the disorder-induced depression of the Kondo effect observed by Blachly and Giordano [Phys. Rev. B 51, 12 537 (1995)].

Ion-recombination luminescence in squalane solutions: Spin relaxation effects

The luminescence of solutions of p-terphenyl in squalane excited by fast β-particles and its enhancement by magnetic fields has been studied. Applied fields were varied between 1 and 200 mT. Time-resolved single-photon counting was used to measure the luminescence pulse-shapes over a range of 1 µs with a resolution of 200 ns gives a direct measure of the spin–lattice relaxation rate of radical ion pairs. Analysis of the data at shorter times is more difficult but it is shown that relaxation occurs in three stages involving different ion pairs. The reaction mechanism and the nature of the relaxation processes are discussed.

Spin-diffusion lengths of Cu1-xNix using current perpendicular to plane magnetoresistance measurements of magnetic multilayers.

Spin-relaxation effects are generally believed to have a strong influence on giant magnetoresistance of ferromagnetic/noble-metal multilayers. We have measured current perpendicular to plane magnetoresistances (CPP MR's) of sputtered Co/${\mathrm{Cu}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ni}}_{\mathit{x}}$ multilayers with x=0.06, 0.10, 0.14, and 0.23. Our results indicate that the spin-diffusion length of CuNi is reduced as the concentration of Ni increases. Spin-diffusion lengths obtained from fitting the CPP MR to Valet-Fert theory are in a good agreement with theoretical calculations from spin-orbit scattering. Assuming that the permalloy(Py)-Cu interface in Py/Cu multilayers resembles a CuNi alloy, we can conclude that there will be spin-flip scattering at a Py-Cu interface, but we expect its effect on the MR to be small. \textcopyright{} 1996 The American Physical Society.