Spin Relaxation Effects Research Articles

Theory of the bipolar spin switch.

We extend the Valet-Fert model of the perpendicular magnetoresistance in magnetic multilayers to describe the spin accumulation and relaxation effects in the spin switch structure introduced by Johnson and calculate the output voltage of the device. In contrast to the usual treatment by Johnson, we take into account the spin relaxation in the ferromagnetic layers, and also the influence of interface resistances and interface spin flips. We show that, for thin nonmagnetic layers, the output voltage is limited by the spin relaxation in the ferromagnetic layers. We find that the interpretation of the experimental data requires surprisingly long spin diffusion lengths in both the magnetic and nonmagnetic layers, much longer than those derived from perpendicular magnetoresistance in multilayers of similar materials. \textcopyright{} 1996 The American Physical Society.

Electron structure near abrupt edges in 2DEG under strong magnetic field

AbstractEnergies and wave functions of edge states in a two‐dimensional electron gas are evaluated for a finite step potential barrier model. The spectrum, instead of smooth bending of Landau branches in the vicinity of the barrier acquires a step‐like form; unexpected edge plateaus and significant energy gap reduction take place between the neighbouring Landau branches above the barrier tops. The origin of these phenomena is traced. Stability with respect to modifications is established. Manifestation of the qualitatively new features of electron densities of states in abrupt confinements through magnetooptical and nuclear magnetic spin relaxation effects is proposed.

Magnetoresistance of UPd 2Ge 2

Magnetoresistance Δϱ/ϱ of intermetallic UPd 2Ge 2 has been studied in the temperature range 4.2–160 K in a magnetic field up to 6 T. The field and temperature dependence of Δϱ/ϱ indicate an antiferromagnetic state of the material below 20 K, and a random spin-glass like behavior in the region 20 K < T < 80 K. The field induced metamagnetic transitions are found below 80 K, and the anisotropy energy has a very strong and unusual temperature dependence with a minimum at around 10–20 K. At T < 50 K, a gap in the magnetoresistance for increasing and decreasing field periods is found both at H = 0 and 6T, and it is explained by spin relaxation effects.

Spin relaxation effects in the perpendicular magnetoresistance of magnetic multilayers.

The spin-diffusion length is the scaling length of the perpendicular giant magnetoresistance in magnetic multilayers and, according to recent experiments, the magnetoresistance ratio can be significantly reduced by enhancing the spin relaxation and thus shortening the spin-diffusion length. We present a theoretical picture of these spin-diffusion-length effects on the magnetoresistance. We also calculate the contribution from several mechanisms (spin-orbit scattering, exchange scattering by paramagnetic impurities, electron-magnon scattering) to the spin relaxation and spin-diffusion length.

A generalized Solomon-Bloembergen-Morgan theory for arbitrary electron spin quantum numberS

The modified Solomon-Bloembergen and Morgan theory has been generalized to systems of arbitrary electron spin quantum number and non-extreme narrowing conditions. The effects of multi-exponential electron spin relaxation give rise to a correction term which can generally be derived in closed analytical form. However, only the spectral density at zero electron spin resonance frequency has a noticeable correction value due to multi-exponential electron spin relaxation. For S = 5/2, the maximum value of the correction term for the paramagnetically enhanced nuclear spin-lattice relaxation rate is 5·4% and for nuclear spin-spin relaxation 6·3%. It is therefore likely that the influence of multi-exponential electron spin relaxation on water 1H T 1 NMRD profiles is, within the accuracy of the experimental measurements, difficult to observe.

Interactions of Cu2+and VO2+Ions with Phosphatidylcholine Vesicular Membranes as Studied by the1H NMR Paramagnetic Relaxation Method

The spin-relaxation effects of the paramagnetic ions Cu2+ and VO2+ on the protons of solvent water and lipids in phosphatidylcholine unilamellar vesicles were first observed in order to elucidate the details of the interactions in the membrane surface region. VO2+ ions gave higher rates of the effective paramagnetic relaxation induced by binding to a membrane while Cu2+ ions had a larger partition coefficient from water to the membrane phase. It is concluded that VO2+ occupies a binding site closer to the N-methyl groups in the polar-head network and less accessible to a cation, which causes stronger motional restriction to a bound cation, compared with the site occupied by Cu2+.

Pulse techniques for CIDNP

AbstractThe peculiarities of measurements of chemically induced dynamic nuclear polarization (CIDNP) with pulsed and Fourier transform NMR spectrometers are discussed. If detection is accomplished in this way, new classes of experiments become possible. First, CIDNP net and multiplet effects can be separated by pulse techniques. Second, kinetic measurements (“flash” CIDNP) can be performed with a time resolution in the submicrosecond range. Third, pseudostationary experiments allow recording of time‐independent CIDNP spectra that are free from the effects of nuclear spin relaxation in the products and that possess improved signal‐to‐noise ratios while maintaining excellent suppression of unchanging background magnetization. Finally, signals of unpolarized nuclei in the products can be made observable by means of coherence transfer techniques.

Intermolecular exchange in a triarylborane-phosphine complex: a multinuclear magnetic resonance study

It has been shown using 1 H, 19 F, and 31 P NMR spectroscopy that the tris -(pentafluorophenyl) borane-phosphine adduct undergoes intermolecular exchange in toluene solution involving transfer of the phosphine group. The rate coefficient (k) for the exchange process was measured using the 31 P NMR selective inversion experiment on a series of samples containing excess phosphine. At 254 K, k is 3.60 ± 0.15 s -1 and is independent of the excess phosphine concentration. The mechanism for the exchange process is proposed to be two step, a slow initial dissociation of the adduct followed by a rapid recombination step. The rate coefficient is strongly temperature dependent in the range 243-263 K and a value of 126 kJ mol -1 is obtained for the activation energy of the dissociation step. The appearance of the 1 H two-dimensional exchange spectrum (NOESY) is described in terms of the competing effects of nuclear spin relaxation and chemical exchange.

Time-resolved and modulation methods in the study of the effects of magnetic fields on the yields of free-radical reactions

In an attempt to understand the apparently disparate results reported in magnetic field effect studies on the same reaction system from different experimental groups, two different techniques, time-resolved methods and field-modulated ones, have been applied to systems comprising pyrene and either dicyanobenzene or dimethylaniline. It is shown that these yield different results whose origin lies less in the detection method than in the light sources which produce the radicals. The results from the time-resolved experiments, in which the effects of spin relaxation in a singlet-correlated pair are apparent, have been analyzed using a novel approach applicable to more complex reaction systems than have been studied in the past. An original experiment is introduced to observe the effects of relaxation at field strengths where the hyperfine-driven spin mixing has reached its asymptotic rate. The discrepancy between the results originates in a much shortened radical pair lifetime, resulting from a very low radical concentration under the conditions used in the modulation experiment as opposed to the time-resolved one. This lifetime effect has been confirmed by using degenerate electron-exchange reactions to affect the chemistry in the period of observation in each technique. Low-field minima in the field effect curves observed using the modulation method are shown to originate in hyperfine and effects and not to be due to J-resonances.

Spin-glass-like magnetic properties for RFe10Mo2 compounds (R=Y or Lu) with the ThMn12-type structure.

dc and ac magnetic measurements combined with M\"ossbauer spectroscopy have been used to study the evolution of magnetic order below the critical temperature ${\mathit{T}}_{\mathit{c}}$ in polycrystalline powders of the intermetallic compounds ${\mathrm{YFe}}_{10}$${\mathrm{Mo}}_{2}$ and ${\mathrm{LuFe}}_{10}$${\mathrm{Mo}}_{2}$. Irreversibility of the dc thermomagnetic curves after zero-field-cooling (ZFC) and field-cooling (FC) processes has been observed. A hysteresis loop has been detected for both compounds at 4.5 K and a shifting of the loop after FC has been observed in ${\mathrm{LuFe}}_{10}$${\mathrm{Mo}}_{2}$. Strong spin-relaxation effects were recorded in $^{57}\mathrm{Fe}$ M\"ossbauer absorption spectra in temperatures well below the ${\mathit{T}}_{\mathit{c}}$=360 and 260 K for R=Y and Lu alloys, respectively. Furthermore, a number of magnetic properties common to conventional spin glasses and true ferromagnets has been revealed, which suggest the existence of a spin-glass-like magnetic phase below ${\mathit{T}}_{\mathit{c}}$.

A two-dimensional Fourier transform electron-spin resonance (ESR) study of nuclear modulation and spin relaxation in irradiated malonic acid

Nuclear modulation in electron-spin-echo spectroscopy is conventionally studied by one-dimensional electron-spin-echo envelope modulation (1D-ESEEM). Two-dimensional Fourier transform electron-spin resonance (2D-FTESR) studies of nuclear modulation have the promise of enhancing the spectral resolution and clarifying the key details of the relaxation processes. We present a 2D-FTESR study on single proton nuclear modulation from γ-irradiated malonic acid single crystals to test the validity of the Gamliel–Freed theory and to assess the value of the new methods. The two pulse spin-echo correlation spectroscopy (SECSY) spectra as a function of orientation of the single crystal show very good agreement with the Gamliel–Freed theory extended to the general case of nonaxially symmetric hyperfine interaction. It is very simply affected by spin relaxation, such that relative intensities are essentially unaffected. Thus SECSY-ESR can most reliably be utilized for studying nuclear modulation. Stimulated SECSY provides the simplest nuclear modulation patterns, which, however, do exhibit the suppression effect well known in three-pulse ESEEM studies. Two-dimensional electron–electron double resonance (2D-ELDOR) provides nuclear modulation patterns similar to that of SECSY-ESR, so the suppression effect is absent. Both three-pulse methods exhibit complex relaxation behavior which can affect relative intensities. This is a feature characteristic of three-pulse ESEEM, but is not well understood. It is shown how the 2D-FTESR methods enable one to obtain the details of the complex spin relaxation, and in the process, obtain very good agreement between experiment and theory. 2D-ELDOR exhibits exchange cross peaks as well as coherence peaks from the nuclear modulation. It is shown how experiments, as a function of mixing time, enable one to separate the effects of the two. It is pointed out that such experiments are in the spirit of 3D spectroscopy. A new observation of the broadening of the 2D-ELDOR main peaks with an increase in mixing time is ascribed to the effects of solid-state dynamical processes that are slow on the ESR time scale and may thus be studied in ‘‘real time’’ in such experiments. The analysis of spin relaxation in this study is enabled by a full Liouville space derivation of the combined effects of nuclear modulation and spin relaxation in 2D-FTESR.

Anomalous Nuclear Spin Relaxation Effects in the Presence of Paramagnetic Substances

Anomalous Nuclear Spin Relaxation Effects in the Presence of Paramagnetic Substances

A “magic-angle” sample-spinning nuclear magnetic resonance spectroscopic study of interference effects in the nuclear spin relaxation of polymers

The proton-coupled, carbon-13 “magic-angle” sample-spinning nuclear magnetic resonance spin-lattice relaxation behavior of poly ( cis-isoprene) and poly ( cis-butadiene) as a function of temperature is reported. Analysis of the relaxation behavior of the methine carbons reveals strong temporal correlations between the 13CH dipolar and 13C anisotropic chemical shielding interactions. Equations are presented which enable the interpretation of these interferences for non-axially symmetric shielding tensors, subject to cylindrically symmetric reorientation. In the context of these expressions, the results indicate that the methine moiety in each of the two polymers executes relatively rapid, isotropic motions, on the time scale of the carbon-13 Larmor frequency. It is also shown that in certain polymeric systems, the sign of the interference term may change with frequency. This intriguing property provides additional insight into the relative reorientational anisotropies of the low- and high-frequency motions, and the specific case of the spin-lattice relaxation and differential line broadening of the methine carbons in poly (cis-isoprene) and poly ( trans-isoprene) is considered in detail.

A general approach for determining scalar coupling constants in polypeptides and proteins

A general approach is described for measuring homo- and heteronuclear spin coupling constants in polypeptides and small proteins. This method uses selective magnetization transfer to generate cross peaks similar to exclusive correlated spectroscopy (E.COSY) and a large direct spin coupling (1J) in one dimension to "pull apart" cross-peak components by frequencies much larger than the resonance line width. A general description of this method is presented, along with a brief discussion of spin topology and relaxation effects that must be considered in designing multidimensional nmr pulse sequences for measuring vicinal coupling constants. The principles are demonstrated in designs of several two-dimensional nmr experiments for determining coupling constants in polypeptides and proteins. These include experiments for measuring 3J(HN-H alpha), 3J(H alpha i-1-15Ni), 3J(15N-H beta), and 3J(H alpha-H beta) coupling constants, which depend on the polypeptide dihedral angles phi, psi, and chi 1. Multidimensional nmr experiments developed with this approach will allow measurements of many vicinal coupling constants in peptides, proteins, and other molecules. Coupling constants measured in these spectra can be used to determine backbone and side-chain conformations, to obtain stereospecific resonance assignments of prochiral atoms, and to characterize conformational distributions of dihedral angles. Combined with information obtained from nuclear Overhauser effect measurements, these data will provide more precise determinations of protein solution structures by nmr spectroscopy.

A spin-product-operator analysis of the dephasing requirement in STEAM-localized NMR spectroscopy

The STEAM sequence (I) is frequently used in volume-localized NMR spectroscopy. Its application requires that the spin coherence created by the first 90” pulse be completely dephased before the application of the second pulse (2, 3). Although the fulfillment of this dephasing requirement is essential for the correct interpretation of the STEAM-localized spectra, its importance has not always been fully recognized in the analysis of the sequence. A recent calculation (4)) which ignored the dephasing requirement, yielded undesired modulations of the stimulated-echo (STE) signal by the chemical shift. The purpose of this paper is to analyze the time evolution of a spin system subjected to the STEAM sequence and to demonstrate the importance of the dephasing requirement when using the STEAM sequence for volume-localized NMR spectroscopy. The time evolution of a single spin (I = 4) system is calculated using the spin product operator formalism (5), with the effects of spin relaxation and diffusion ignored. The STEAM sequence used for the analysis is depicted in Fig. 1 with indication of the points on the time axis where the spin density operator is calculated. For simplicity, the gradients in the three time intervals are the averaged gradients. The calculation is performed under the following conditions: (a) the TE gradients are balanced, i.e., GTr( 1) = GTE( 2); (b) the TM gradient is applied to destroy the transverse magnetization after the second RF pulse. Calculation of the density operator at the successive time points yields

One-dimensional and two-dimensional 1H- and 13C-nuclear magnetic resonance (NMR) analysis of vitamin E raw materials or analytical reference standards.

Two-dimensional spectral analysis (COSY, HETCOR) was utilized to make the complete 13C- and 1H-NMR assignments for alpha-, beta-, gamma-, and delta-tocopherol as well as for the acetate and succinate esters of alpha-tocopherol. 13C-NMR was found to be especially useful in distinguishing between the various tocopherols and distinguishing between the d-isomer and the d,l-racemic mixture. HETCOR spectra were also found to be useful for the qualitative identification of mixtures of the tocopherols and sesame oil. Using a procedure designed to minimize errors arising from spin relaxation and nuclear Overhauser effects, 13C-NMR peak integrals were used to quantitate alpha-tocopherol and delta-tocopherol in the presence of sesame oil using benzoic acid as the standard for calibration of the quantitation. The NMR results were compared to a capillary column gas chromatographic analysis of the individual alpha-tocopherol and delta-tocopherol reference materials.

Hole Propagation in a 2D Quantum Spin-1/2 Antiferromagnet –Effect of Local Spin Relaxation–

We discuss density of states of a hole doped into a spin-1/2 Heisenberg antiferromagnet (AF). From the multi-spin correlation functions, moments of the density of states are estimated up to the 8-th order taking into account the effect of local spin relaxation around a hole. We show the density of states for AF shows divergent behavior near the band edge.

Mossbauer study of Fe substituted YBa 2Cu 3O 7−δ

Resistivity, D.C. susceptibility and Mo¨ssbauer spectra (MS) of YBa 2, 985Fe 0, 015O 7−δ have been obtained. The spectra in zero field show the presence of three inequivalent sites. Spectra at 4.2, 11, 25, 47 and 155 K in field of 5.5 Tesla reveal that in superconducting (SC) state the diamagnetic shielding of external field (H ext) is different for the different sites of iron. Ionic spin relaxation (ISR) effects are present at low temperature.

Microscopic interpretation of magnetic ordering and critical slowing down in potassium ferrate

Experimental M\"ossbauer results are reported for the antiferromagnet ${\mathrm{K}}_{2}$Fe${\mathrm{O}}_{4}$ from 0.16 to 4.2 K which show significant spin-relaxation effects. The data are analyzed using the Clauser-Blume theory of stochastic spin relaxation, applied to the case spin $S=1$. This analysis yields the N\'eel temperature ${T}_{N}=3.60$ K, and verifies the crystal-field parameters $D=0.11$ ${\mathrm{cm}}^{\ensuremath{-}1}$, and $E=0.02$ ${\mathrm{cm}}^{\ensuremath{-}1}$ in the presence of significant spin-fluctuation effects. The analysis also shows that, except in the critical region, the strength of the relaxation mechanism is temperature independent, indicating that the spin-spin interaction is the dominant relaxation mechanism in this temperature range. In the region of 3.60 K, the spin-relaxation rate is observed to decrease by a factor of 700, showing the critical slowing down of the iron ions' spin fluctuations in the critical region. Additionally, to fit the data between 3.50 and 3.70 K, it is necessary to include contributions from a range of values of reduced magnetization and relaxation rate. These results are discussed in terms of a proposed model including critical fluctuations.

Recombination ina-Si: H: Spin-dependent effects

Spin-dependent recombination in $a$-Si: H is studied using optically detected magnetic resonance (ODMR). Measurements are reported of the dependence of ODMR on temperature, illumination intensity, luminescence energy, microwave power, and defect density. Time-resolved ODMR using both transient illumination and transient microwave pulses are shown to be particularly informative. Samples with high-defect density exhibit a quenching spin-dependent effect which is identified with nonradiative recombination between unthermalized spins. The recombination is attributed to tunneling of band-tail electrons to dangling bonds. Low-defect-density samples have an additional quenching nonradiative mechanism, tentatively identified with Auger recombination, and a radiative enhancing process which we attribute to band-tail luminescence. The results support previous luminescence studies. We discuss the effects of microwave power, spin relaxation, and exchange on the ODMR results. The spin-lattice relaxation time ${T}_{1}$ and its temperature dependence are obtained from time-resolved ODMR and from ESR saturation measurements. Relaxation mechanisms are identified from the data.