Spin Concentration Research Articles

EPR Investigation of Radical-Production Cross Sections for Sucrose and L-alanine Irradiated with X-rays and Heavy-ions

Using electron paramagnetic resonance (EPR), we investigated stable radical-production cross sections (σ) of sucrose and L-alanine radicals produced by heavy-ion irradiations with various linear energy transfers (LET). The heavy-ion irradiation results were compared with those of X-ray irradiation at the same dose. The EPR signal areas for the two compounds showed a linear relation with the absorbed dose, as well as a logarithmic correlation with the LET. Further analysis was carried out for the radical-production cross section, which showed that stable radicals of the two compounds were produced through collisions of several particles with a single molecule. The relative σ value of sucrose for C ion irradiation was (1.29 ± 0.64) × 10−12 μm2. The σ value of alanine for C ion irradiation was (6.83 ± 0.42) × 10−13 μm2. Considering the structural molecular sizes of sucrose and alanine, the σ values are similar. In addition, a comparison of the EPR results for the C ions and X-rays at 50 Gy dose was made. Sucrose spin concentrations produced by C ions at the LET value of 13.1 keV/μm and X-rays were similar unlike alanine. Thus, the noble EPR results with X-ray and heavy-ion irradiations imply that sucrose can be useful as a radiation indicator.

Ferromagnetism in water-doped vanadium oxide nanotubes

We have investigated the magnetic properties of vanadium oxide nanotubes (VOx-NTs) of various V4+ concentrations. Ferromagnetic response was observed at ∼100 K only in the V4+-rich water-intercalated VOx-NTs, whose saturation moment is close to that of Li-doped VOx-NTs, a recently-reported room-temperature ferromagnet. The ferromagnetism may have to do with charge localization as well as the spin concentration giving rise to removal of spin frustration according to our previous study.

ChemInform Abstract: High-Spin Polyphenoxyl Based on Poly(1,4-phenyleneethynylene).

Rodlike poly(1,4-phenyleneethynylene) 2-substituted with multiple pendant phenoxyls 1 was synthesized by polymerizing 4-bromo-2-(3,5-di-tert-butyl-4-hydroxyphenyl)ethynylbenzene 10a using the catalyst of a palladium−triphenylphosphine complex and cuprous iodide and subsequent heterogeneous oxidation. The corresponding dimer 2 was also synthesized; X-ray analysis of its precursor 4 indicated a linear phenyleneethynylene backbone and twisted dihedral angles of 50 and 77° for the pendant phenol groups. ESR spectra suggested a delocalized spin distribution from the pendant phenoxyl to the backbone. The diphenoxyl 2 had a triplet (S = 2/2) ground state. The spin concentration of the polyphenoxyl 1 could not be increased beyond 0.7 spin/unit due to its low solvent solubility; 1 with a spin concentration of 0.62 had an average S of 3/2.

Structure and magnetic properties of curved graphene networks and the effects of bromine and potassium adsorption

The structure and magnetic properties of nanoporous zeolite-templated carbon (ZTC) and its guest-adsorbed systems are investigated using potassium and bromine as guest species. In ZTC, nanopores formed by a three-dimensional curved-graphene network, whose structure is transcribed from the zeolite nanopore structure, are regularly arranged (periodicity: 1.4 nm). The regularity of the nanopore structure is preserved during vacuum heat treatment $(<380\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C})$ and potassium adsorption, though it is destroyed by bromine adsorption. The pristine ZTC has localized spins, whose origin is attributed to those created in the region having a negative curvature in the graphene network, as well as to the contribution of an edge state. The spin concentration is estimated as 1 spin/nanopore and is independent of the heat-treatment temperature up to $380\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$. In potassium-adsorbed ZTC, potassium atoms forming networked clusters in the nanopores are magnetic and undergo a weak ferromagnetic transition at 24 K. The ZTC spins, which are not directly coupled with the potassium spins through exchange interaction, accurately mirror the magnetic behavior of the potassium spins.

Features of the phase transitions in the three-dimensional diluted potts model with number of spin states q = 3

The effect of frozen disorder, implemented in the form of nonmagnetic impurities, on phase transitions in the three-dimensional Potts model with number of states q = 3 has was investigated by the Monte Carlo method, using the Wolf single-cluster algorithm. Systems with linear sizes L = 20–44 were considered at spin concentrations p = 1.00–0.65. The method of fourth-order Binder cumulants was used to demonstrate that a first-order phase transition is observed for the pure model (p = 1.00) and a second-order phase transition occurs at concentrations p = 0.90, 0.80, 0.70, and 0.65.

Short-time dynamics and critical behavior of the three-dimensional site-diluted Ising model

Monte Carlo simulations of the short-time dynamic behavior are reported for three-dimensional weakly site-diluted Ising model with spin concentrations p=0.95 and 0.8 at criticality. In contrast to studies of the critical behavior of the pure systems by the short-time dynamics method, our investigations of site-diluted Ising model have revealed three stages of the dynamic evolution characterizing a crossover phenomenon from the critical behavior typical for the pure systems to behavior determined by the influence of disorder. The static and dynamic critical exponents are determined with the use of the corrections to scaling for systems starting separately from ordered and disordered initial states. The obtained values of the exponents demonstrate a universal behavior of weakly site-diluted Ising model in the critical region. The values of the exponents are compared to results of numerical simulations which have been obtained in various works and, also, with results of the renormalization-group description of this model.

Electronic structure ofGa1−xMnxAsanalyzed according to hole-concentration-dependent measurements

We study the effects of variable hole concentration on the transport, thermoelectric, and magnetic properties of ${\text{Ga}}_{1\ensuremath{-}x}{\text{Mn}}_{x}\text{As}$. The hole concentration in samples with fixed Mn content has been varied using high energy particle irradiation, which introduces donorlike defects that compensate Mn acceptors without changing the concentration of localized Mn spins. As expected, a decrease of the hole concentration results in a reduction of the Curie temperature and an increase in electrical resistivity and thermoelectric power. The mobility and thermopower data are then analyzed in terms of models based on free holes in the valence band and holes localized in a Mn impurity band. The energetic structure of the impurity band is described by the valence-band anticrossing model. We show that the electronic structure provided by the impurity band model is consistent with the experimental results.

Gd[formula omitted] substituted of Mn[formula omitted]Zn[formula omitted]Gd[formula omitted]Fe[formula omitted]O[formula omitted] ([formula omitted], 0.2, 0.3, 0.4) ferrites for ferrofluids application

Gd3+ substituted MnZn ferrites [ Mn0.5Zn0.5GdxFe2−xO4 (x=0.1, 0.2, 0.3, 0.4)] nanoparticles have been explored for ferrofluid applications. The oleic acid coated water base ferrofluids of these ferrites are prepared by chemical co-precipitation. The effect of Gd3+ ions on structural morphological and magnetic properties of these samples are characterized by X-ray diffraction, FTIR, EPR and dc magnetization techniques. The crystallite size, particle size and saturation magnetization increases with the increase of Gd3+ ions concentration while lattice parameter and lattice strain reduces. The g-value, peak-to-peak linewidth and spin concentration evaluated from EPR spectra correlated with cations occupancy.

Magnetism of nanographene network influenced by the interaction with acid species

The magnetism of nanographite (stacked nanographene sheets)-based nanoporous carbon is investigated in relation to the interaction with acid guest species. The concentration of the localized spins of non-bonding π-electron state (edge state) localized in the nanographene edges decreases upon the sulfonation of nanographene edges through charge-transfer interaction with sulfonic groups. The sulfonation of nanographene edges enhances the hydrophilic nature of the edges, resulting in the easiness in the water adsorption into the nanopores. This enhances the mechanical compression effect of water molecules condensed in the nanopores on the nanographite domains, resulting in the decrease in the spin concentration. The change in the magnetism upon water uptake reveals ferrimagnetic nature of individual nanographene sheets. The adsorption of HCl having no oxidation ability shows a mechanical effect on the edge-state spins similar to water adsorption. The spin concentration is reduced in two-step manner by the charge-transfer interaction with guest concentrated HNO 3 that is strong oxidant. In the presence of H 2O molecules in diluted HNO 3 the cooperation of mechanical and charge-transfer interactions creates also a two-step change in the magnetism.

Identifying the electron spin resonance of conduction electrons in alkali doped SWCNTs

AbstractWe study the electron spin resonance (ESR) signal of pristine and potassium doped SWCNTs. We identify signals of a super‐paramagnetic background, a low intensity impurity, and of the conduction electron spin resonance (CESR). The latter only appears upon the alkali atom doping. To identify the CESR signal, we critically assess whether it could come from residual graphitic carbon, which we clearly exclude. We give accurate values for the signal intensities and the corresponding concentration of spins and for the g‐factors. The CESR signal intensity allows to determine the density of states on the SWCNT assembly.

Compensation temperature of 3d mixed ferro-ferrimagnetic ternary alloy

In this study, we have considered the three dimensional mixed ferro-ferrimagnetic ternary alloy model of the type AB p C 1 - p where the A and X ( X = B or C ) ions are alternately connected and have different Ising spins S A = 3 / 2 , S B = 1 and S C = 5 / 2 , respectively. We have investigated the dependence of the critical and compensation temperatures of the model on concentration and interaction parameters by using MC simulation method. We have shown that the behavior of the critical temperature and the existence of compensation points strongly depend on interaction and concentration parameters. In particular, we have found that the critical temperature of the model is independent on concentration of different types of spins at a special interaction value and the model has one or two compensation temperature points in a certain range of values of the concentration of the different spins.

Long alkyl chain bearing derivatives of poly(3,4-ethylenedioxythiophene) studied by in situ EPR spectroelectrochemistry

Electrochemically synthesised polymers of long, 2-oxaalkyl chain derivatives of 3,4-ethylenedioxythiophene (EDOT) have been studied using in situ EPR spectroelectrochemistry. Originally developed as promising candidates for preparation of soluble PEDOT derivatives, their polymers themselves reveal interesting spectroelectrochemical properties, in some cases even superior to their family's parent. Our results indicate notable differences in potential dependencies of concentration of spins and Δ B pp width between the two polymers, indicating a major influence of the alkyl side chain length on specific properties of paramagnetic charge carriers appearing in these polymers in the course of their redox processes. Certain features of their spectroscopic response however, like the hysteresis of potential dependence of concentration of spins and Δ B pp widths, point to similarities of their doping - dedoping mechanism with the one of PEDOT.

Syntheses, Crystal Structures, Spectroscopic Properties, and Catalytic Aerobic Oxidations of Novel Trinuclear Non‐Heme Iron Complexes

AbstractA series of 2,6‐diacylpyridine ligand precursors 5a–d·HCl with different tether lengths between the carboxyl and pyridine moiety was prepared and converted into the correspondig trinuclear Fe3(μ3‐O) complexes 8a–d and 10. Under slow precipitation conditions a tetranuclear complex 9 was isolated instead of 8a. Single‐crystal X‐ray diffraction analyses were performed on ligands 5a–d and complexes 9 and 10. Characterization by X‐ray absorption spectroscopy (XAS) proved a trinuclear Fe3(μ3‐O) core for complexes 8a–d. When complex 8a was submitted to Gif‐type oxidations (O2, Zn, pyridine, HOAc), Mössbauer and nuclear inelastic scattering (NIS) suggested the formation of mononuclear species. The trinuclear ferric complex 10 has an isosceles molecular structure, which is manifested in the 57Fe Mössbauer spectrum by two quadrupole doublets with a 2:1 intensity ratio. The magnetic measurements reveal two moderate antiferromagnetic exchange interactions of –22.1 and –33.8 cm–1. Spin concentrations of complex 10 were determined by EPR spectroscopy, which supports the Mössbauer and magnetic studies. Complexes 8–10 were employed in catalytic aerobic oxidations of adamantane 11, cyclohexene 19, and α‐pinene 23. For adamantane 11, the oxidation of secondary C–H bonds to the corresponding ketone 14 is favored. In addition, adamantylpyridines 15–18 were isolated, thus supporting a radical pathway. A strong preference of allylic oxidation versus epoxidation was found for cyclohexene 19 and α‐pinene 23. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Green luminescence and EPR studies on Mn-activated yttrium aluminum garnet phosphor

Yttrium aluminum garnet (Y3Al5O12) and Mn activated Y3Al5O12 phosphors have been prepared by urea combustion route in less than 5 min. The phosphors are well characterized by powder X-ray diffraction, Scanning electron microscopy and Fourier-transform infrared spectroscopic techniques. Photoluminescence tests on the pure Y3Al5O12 showed a strong green emission at 525 nm (2.36 eV) attributed to the strongly allowed transition of F+ center whereas in Mn2+ activated YAG the green emission at 519 nm is due to the 4T1 (G)→6A1 (S) transition of Mn2+ ions. EPR studies have been carried out on Mn2+ activated Y3Al5O12 phosphor at 300 and 110 K. From EPR spectra the spin-Hamiltonian parameters have been evaluated. The magnitude of the hyperfine splitting (A) indicates that the Mn2+ ions are in a moderately ionic environment. The spin concentration (N) and paramagnetic susceptibility (χ) have been evaluated and discussed.

Phase transition properties of three-dimensional systems described by diluted potts model

Monte Carlo simulations are performed to analyze phase transitions in three-dimensional systems described by the 3-state Potts model with nonmagnetic impurities. Numerical results are presented for systems with spin concentrations p = 1.00, 0.95, 0.90, 0.80, 0.70, and 0.65 on lattices of size L varying between 20 and 44. Binder’s cumulant analysis shows that the introduction of quenched disorder in the form of non-magnetic impurities induces a crossover from first-order to second-order phase transition. The finite-size scaling method is used to calculate the static critical exponents α, γ, β, and ν for specific heat, susceptibility, magnetization, and correlation length, respectively.

Exchange coupled pairs of dangling bond spins as a new type of paramagnetic defects in nanodiamonds

EPR in detonation nanodiamonds (DND) reveals two different signals associated with intrinsic carbon inherited paramagnetic defects. Main carbon inherited EPR signal is narrow intensive Lorentzian-like singlet with g=2.0028 and spin concentration Ns=(6−7)×1019 spin/g that yields on average 13–15 spins per each DND particle. Additional chemical treatment of DND powder allows practically complete removal of trace amounts of transition metal impurities that reveals a new doublet EPR signal consisting of two relatively narrow lines within the half-field region (g∼4) separated by a distance of 10.4 mT. The intensity of the doublet signal is five orders of magnitude lower than that of the main singlet signal. The former signal has been observed in a wide variety of DND samples disregarding of the impurity level reached and thus may be attributed to some intrinsic defects in DND particles. Such half-field EPR signals correspond to “forbidden” ΔMs=2 transitions within thermally populated triplet (S=1) levels observed in polycrystalline samples containing exchange dimers—antiferromagnetically coupled spin pairs. Estimates suggest that the concentration of such defects is about one dimer per hundreds DND particles.

EPR and photoluminescence studies on lithium-potassium borophosphate glasses doped with Mn 2+ ions

The mixed alkali borophosphate xLi 2O–(30 − x)K 2O–35B 2O 3–34.5P 2O 5 (5 ≤ x ≤ 25) glasses doped with 0.5 mol% of manganese ions have been studied using EPR and photoluminescence techniques. The EPR spectra of all the investigated samples exhibit resonance signals which are characteristic of Mn 2+ ions. The resonance signal with effective g value at g = 2.02 exhibits a six line hyperfine structure. The zero-field splitting parameter ( D) at room temperature has been evaluated from the intensities of allowed hyperfine lines. The EPR spectra at different temperatures (123–295 K) have also been studied. It is found that the spin concentration ( N) exhibits non-linear behaviour with composition showing the mixed alkali effect (MAE). From the EPR data, the paramagnetic susceptibility ( χ) has been calculated at various temperatures. The Curie constant ( C) and paramagnetic Curie temperature ( θ p) have been evaluated from the 1/ χ versus T graph. The photoluminescence (PL) spectrum exhibits a green emission band at 582 nm and this band has been assigned to a transition from the upper 4T 1g → 6A 1g ground state of Mn 2+ ions.

Coherence of single spins coupled to a nuclear spin bath of varying density

The dynamics of single electron and nuclear spins in a diamond lattice with different $^{13}\text{C}$ nuclear spin concentration is investigated. It is shown that coherent control of up to three individual nuclei in a dense nuclear spin cluster is feasible. The free-induction decays of nuclear spin Bell states and single nuclear coherences among $^{13}\text{C}$ nuclear spins are compared and analyzed. Reduction in a free-induction-decay time ${T}_{2}^{\ensuremath{\ast}}$ and a coherence time ${T}_{2}$ upon increase in nuclear spin concentration has been found. For pure diamond, ${T}_{2}^{\ensuremath{\ast}}$ as long as $30\text{ }\ensuremath{\mu}\text{s}$ and ${T}_{2}$ of up to 0.65 ms for the electron spin has been observed. The $^{13}\text{C}$ concentration dependence of ${T}_{2}^{\ensuremath{\ast}}$ is explained by Fermi contact and dipolar interactions with nuclei in the lattice. It has been found that ${T}_{2}$ decreases approximately as $1/n$, where $n$ is $^{13}\text{C}$ concentration, which corresponds to the reported theoretical line of ${T}_{2}$ for an electron spin interacting with a nuclear spin bath.

EPR study of radiation damage to DNA irradiated with synchrotron soft X-rays around nitrogen and oxygen K-edge

In order to obtain detailed insights into the physicochemical mechanism of DNA damage induction, “ in situ” measurement of electron paramagnetic resonance (EPR) signal from DNA constituent nucleobases, guanine and adenine, has been performed in a vacuum using monochromatic synchrotron soft X-rays. We found that short-lived unpaired electron species arise only during irradiation to the evaporated thin film on a surface. The EPR spectrum of the short-lived species significantly depends on the photon energy irradiated, and the spin concentration obtained from the EPR spectra shows a similar fine structure to the X-ray photoabsorption spectra (X-ray absorption near edge structure; XANES). For the adenine sample, the spin concentration alters strikingly by water absorption on the sample surface. Trapping of photo- or Auger electrons into a newly generated potential in the nucleobases as the consequence of photoelectric effect is suggested as mechanisms of the induction of the short-lived species.

Electron spin resonance and spectral studies of bovine serum albumin-methylglyoxal complexes

Electron spin resonance and spectrophotometric measurements are reported for complexes of bovine serum albumin with methylglyoxal. The optical absorbance at 318 nm was found to be proportional to the unpaired spin concentration of the protein complexes. The constant of proportionality, however, was found to be a sensitive function of the exposure of samples to molecular oxygen, the color of the complexes remaining roughly constant while the spin concentration increased. The relevance of these observations to the previously reported electronic activity of the protein-methylglyoxal complexes is discussed, and a possible mechanism for the electronic decoupling that gives rise to the observed electron spin resonance signal is proposed.