High Electron Spin Research Articles

Spin dynamics in perovskites, pyrochlores, and layered manganites

High temperature electron spin resonance (ESR) and magnetic susceptibility (χ) are analyzed for manganites related with colossal magnetoresistance (CMR). The properties of compounds with different crystalline structures: three-dimensional (3D) perovskites, pyrochlore, and La1.2Sr1.8Mn2O7, a two-dimensional layer, are compared. In the paramagnetic regime, and outside the critical regions associated with phase transitions, the temperature dependence of the ESR linewidth presents a universal behavior dominated by the variations of χ(T), ΔHpp(T)=[C/Tχ(T)]ΔHpp(∞). The high temperature limit of the linewidth, ΔHpp(∞), is related to the parameters of the Hamiltonian describing the interactions of the spin system. The role played by magnetic anisotropy, isotropic superexchange, and double exchange is revealed and discussed in the analysis of the experimental data. In CMR and non-CMR pyrochlores, ΔHpp(∞)∝ωp2/J where J is proportional to the Curie–Weiss temperature, including the hybridization mechanism producing CMR. Instead, ΔHpp(∞) of CMR perovskites seems not to be affected by the double-exchange interaction. In contrast with the 3D perovskites, the ESR linewidth and resonance field of La1.2Sr1.8Mn2O7, a bilayer compound, although isotropic at high temperatures, becomes anisotropic for Tc=125 K<T<Tp≈450 K.

Development of frequency tunable, medium power gyrotrons (Gyrotron FU series) as submillimeter wave radiation sources

High frequency, medium power gyrotrons (Gyrotron FU series) have been developed at Fukui University as radiation sources covering a broad band from millimeter to submillimeter wave region. They have already been applied to high frequency electron spin resonance and to submillimeter wave scattering in plasma. Many parameters of the gyrotron series could be useful for applications in several new areas. The development and some applications of the Gyrotron FU series are summarized in this paper.

Electron spin resonance and thermoluminescence in powder form of clear fused quartz: effects of grinding

Clear fused quartz (CFQ) tubes were powdered either manually by using a mortar and pestle (for coarse production) or mechanically, using a micronising mill (for fine production). A high and multisignal electron spin resonance (ESR) background was found in the fine powder even after annealing it at 900°C for 20 min. In the case of the coarse powder, the signal (ESR background) varied inversely with particle size and was quite high for particle sizes lower than 38 μm. In a subsidiary experiment, using fine SiO 2 powder (99.8% pure, with the particle size of ∼0.007 μm), manufactured by using flame hydrolysis, only a weak background signal was found. The 60Co gamma-ray irradiated powders (∼22 Gy) were subjected to ESR analysis or thermoluminescence (TL) readout. The ESR intensity of the coarse powder varied directly with particle size. Thus, the intensity for a particle size of 20–38 μm was very low and almost the same as the unirradiated intensity. In TL readout the results were the opposite: the TL intensity of the coarse powder varied inversely with the particle size down to 38 μm, after which it decreased with decreasing particle size of the material. The fine powder, produced by grinding the CFQ tubes, was insensitive to gamma-rays (at least at doses of up to 50 Gy); but for the flame hydrolysis SiO 2 the situation was the opposite. The minimum detectable dose (MDD) for the CFQ in powder form using ESR was ∼2 Gy, which is ∼2 times higher than that for the bulk form, while the MDD for the powder using TL was ∼20 μGy, which is ∼2 times lower than that for the bulk form of the material.

EPR Observation of the dimensional crossover in (TMTSF) 2ClO 4

High temperature electron spin dynamics in the organic superconductor (TMTSF) 2ClO 4 (tetramethyltetraselenafulvalenium-perchlorate) was studied by means of EPR measurements. Distinct temperature dependencies were observed from the line components attributed to the highly conductive stacking direction and the transverse plane, respectively. In particular, while the stacking direction component shows a metallic behavior in the whole temperature range investigated, the other component shows an apparent dimensional crossover at around 160 K.

Steric Influence on the Structure, Magnetic Properties, and Reactivity of Hexa- and Octaisopropylmanganocene

The reaction of KCpni (Cpni = (C3H7)nC5H5-n; n = 3, 4) with MnCl2 in THF produces the metallocenes (Cp3i)2Mn or (Cp4i)2Mn in good yield. Solid-state magnetic susceptibility measurements indicate that orange-red (Cp3i)2Mn is in high-spin/low-spin equilibrium at room temperature. In the solid-state, it has a centrosymmetric sandwich structure with an average Mn−C bond length (2.130(6) Å) typical of low-spin manganocenes. (Cp3i)2Mn reduces tetracyanoethylene (TCNE) to generate the TCNE radical anion. Despite the presence of additional electron-donating isopropyl groups, pale yellow (Cp4i)2Mn is entirely high-spin from room temperature to 10 K. The crystal structure of (Cp4i)2Mn reveals a bent metallocene structure, with a centroid−Mn−centroid angle of 167.4° and an average Mn−C distance of 2.42(2) Å. This structure is consistent with its high-spin electron configuration; the longer Mn−C bonds provide relief from the steric strain that would be present in a low-spin complex. The reaction of (Cp4i)2Mn with TCNE produces a tricyanovinylation product, C5(i-Pr)4HC(CN)C(CN)2, which was characterized by X-ray diffraction.

Evidence for an Antiferromagnetic Transition in a Zeolite-Supported Cubic Lattice ofFCenters

Evidence for a bcc lattice of $F$ centers in sodium-electro-sodalite (SES), synthesized by exposing dehydrated sodalite to sodium vapor, is presented. A high electron spin density in SES produces isotropic contact shifts in nuclear magnetic resonance (NMR) spectra of the framework nuclei whose magnitude is a discrete function of local electron density. Strong exchange coupling between unpaired electrons gives rise to an antiferromagnetic phase transition in SES at ${T}_{N}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}48\ifmmode\pm\else\textpm\fi{}2\mathrm{K}$, providing the first example of an $s\ensuremath{-}\mathrm{electron}$ antiferromagnet.

Structure, Stability, and Redox Properties of MnII, FeII, CoII, and CoIII Complexes with 1,3,5-Triamino-1,3,5-trideoxy-cis-inositol1

The interaction of MnII, FeII, and CoII with the ligand taci (=1,3,5-triamino-1,3,5-trideoxy-cis-inositol) was studied in the solid state and in aqueous solution. Magnetic susceptibility measurements revealed a high-spin electron configuration for the Mn and the Fe complexes. Crystal data: [Mn(taci)2](NO3)2·2H2O (1), C12H34MnN8O14, monoclinic, space group P21/c, a = 7.107(5) Å, b = 8.742(2) Å, c = 18.527(6) Å, β = 100.20(4)°, Z = 2; [Fe(taci)2]Cl2·6H2O (2), C12Cl2FeH42N6O12, monoclinic, space group C2/m, a = 11.420(2) Å, b = 9.300(2) Å, c = 13.330(3) Å, β = 111.97(3)°, Z = 2; [Co(taci)2](NO3)2·1.5H2O (3), C12CoH33N8O13.5, monoclinic, space group P21/c, a = 7.081(1) Å, b = 8.743(1) Å, c = 18.181(2) Å, β = 99.49(1)°, Z = 2. All three structures exhibited distorted octahedral MN6 coordination. The stability constants for the complexes [M(taci)]2+ (M = Mn, Fe, Co) and [M(taci)2]2+ (M = Fe, Co) in aqueous media have been evaluated by potentiometric titration. Comparison with other divalent cations showed that the order of stability increases MnII < CdII ≈ FeII < ZnII < CoII < CuII < NiII. In alkaline aqueous media, the complexes 1−3 are readily oxidized by dioxygen. The redox properties were analyzed by a series of polarographic and cyclic voltammetric measurements. A new isomeric form of a bis-taci CoIII complex was formed, where one of the taci ligands coordinates the metal cation by one nitrogen and two oxygen atoms. The complex was isolated in crystalline form as [Co(taci)(Htaci)](NO3)4·0.5H2O (4), C12CoH32N10O18.5, monoclinic, space group C2/c, a = 30.541(3) Å, b = 9.015(1) Å, c = 17.852(2) Å, β = 95.63(1)°, Z = 8. [Co(tach)(H2O)x(NO3)3-x]x+ (tach = all-cis-1,3,5-triaminocyclohexane, 0 ≤ x ≤ 3) was used as a model to demonstrate that, for CoIII, the substitution of the three monodentate coligands by a taci molecule resulted generally in an asymmetric binding. An N,O,O and an N,N,O coordination mode were observed for [Co(taci)(tach)](NO3)3·3H2O (5), and [Co(H-1taci)(tach)](NO3)2·(3 − x)H2O·xMeOH, 0.2 ≤ x ≤ 0.5 (6), respectively. Crystal data: 5 (C12CoH36N9O15), monoclinic, P21/n, a = 8.942(5) Å, b = 30.07(3) Å, c = 9.354(5) Å, β = 109.25(4)°, Z = 4; 6 (x ≈ 0.2, C12.2CoH35.4N8O12), monoclinic, P21/n, a = 7.337(2) Å, b = 19.847(4) Å, c = 15.226(3) Å, β = 93.59(3)°, Z = 4.

Influence of radiation treatment on theodrenaline: ESR and HPLC study

The use of ionizing radiation for sterilization of pharmaceuticals is now a well established technology. The purpose of the present work was to apply High performance liquid chromatography (HPLC) and Electron spin resonance (ESR) spectroscopy to study the degradation of theodrenaline after gamma radiation treatment. Numerical simulations of the free radicals dependence on dose at ambient temperature were performed using linear regression, quadratic fit and power function. Tests were carried out to investigate the effects of storage on the free radicals concentration. The decay was fit using bi-exponential regression. Degradation of theodrenaline was studied by ion pair chromatography (IPC) and reversed-phase chromatography (RPLC). The pre-existent impurities did not show a significant increase with dose but the initial amount of impurities (nearly 2%) caused the radiosterilization of this sample not to be technically feasible.

Substrate bias effects on the structural and electronic properties of tetrahedral amorphous carbon.

Tetrahedral amorphous carbon deposited by a filtered cathodic arc has a very high unpaired electron spin density of around ${10}^{20}$--${10}^{21}$ spin/g (c.f. ${10}^{18}$--${10}^{20}$ spin/g for a-Si and a-Ge). Trap states associated with such unpaired spins have a detrimental effect on the electronic properties of both amorphous silicon and germanium and it is therefore desirable to reduce them in tetrahedral amorphous carbon (ta-C). In this paper, we report on the effect of a negative substrate bias, applied during deposition, on the electron spin resonance (ESR) of ta-C films and on their structure, bonding, conductivity, and Raman spectra. The results show a slow decrease in the unpaired electron spin density with increasing bias up to -1350 V followed by an abrupt decrease at -1750 V. Electron energy loss and electrical conductivity measurements indicate a steady increase in the ${\mathit{sp}}^{2}$ hydridized bonding in these films with bias, and it is understood that it is the coupling between electrons at these ${\mathit{sp}}^{2}$ sites that leads to a reduction in the ESR signal. Investigations using electron diffraction and Raman spectroscopy show the formation of ${\mathit{sp}}^{2}$-bonded graphitelike planes in the sample deposited at -1750 V, which provides a mechanism for the absence of unpaired electron spins at this deposition bias. \textcopyright{} 1996 The American Physical Society.

Pressure and light effects on the phase transition of deuterated Cu(2,5-dimethyl-dicyanoquinonediimine)2

The deuterated dicyanoquinonediimine salt Cu(2-CD3,5-CD3-DCNQI)2, (d6), a highly conductive organic metal, exhibits a phase transition from the metallic to the insulating state at Tc≊60 K. In a wide temperature range we observe an anticoincidence of high conductivity and electron spin resonance (ESR). These experiments were performed simultaneously in our ESR apparatus equipped for application of He pressure up to 200 bar. Near Tc there is a coexistence of conducting (ESR-silent) and insulating (ESR-detectable) domains. This correlation of ESR and conductivity σ points to a percolation limited conductivity. Upper and lower limits of the size of the domains are given. Light shifts Tc to lower temperatures, pressure to higher ones. This could be due to shrinking of the lattice which induces the phase transition when critical structural parameters are approached. This is most probably explained by a simultaneous development of a charge density wave (CDW) and a spin pairing within the DCNQI stack together with the trimerization of the lattice. Systematic measurements by variation of temperature, pressure, and light led to phase diagrams of (d6) and to the understanding of the mechanism of the phase transitions.

Production of a spinpolarized keV Sr + ion beam by laser-optical pumping

Using a well collimated Sr + ion beam with very small energy spread in combination with collinear laser-optical pumping, we have obtained a beam with high electron spin polarization. The Sr + ions were produced with μA intensity by surface ionisation in a point-like hot iridium-plated Ta tube and extraction into a beam guiding system. The 5 2S 1 2 5 2P 1 2 transition was pumped by circularly polarized laser light (421.6 nm) propagating antiparallel to the ion beam. The laser is actively stabilized to the SP transition by means of a newly developed method. The final electron spin polarisation of the ground state for the most abundant 88Sr + isotope was measured to be greater than 90%.

17O hyperfine and quadrupole interactions for water ligands in frozen solutions of high spin Mn2+

The magnetic couplings of 17O in H217O coordinated to high spin Mn2+ in a frozen aqueous solution were determined using the complementary magnetic resonance techniques of pulsed and continuous wave (cw) ENDOR (electron nuclear double resonance), ESEEM (electron spin echo envelope modulation), and PFSEPR [pulse field sweep electron paramagnetic resonance (EPR)]. Several complications arise from the high electron spin multiplicity of the d5, Mn2+ ion and the high nuclear spin multiplicity, I=5/2, of the 17O nucleus. At the applied magnetic field strengths in 9 GHz EPR studies, the zero-field splitting of the S=5/2 Mn2+ ion in aqueous frozen solution is small relative to the electron spin Zeeman interaction so that the MS=±1/2,±3/2,±5/2 electron spin states all contribute to the ENDOR spectrum. This results in a complex spectrum in which the 17O ENDOR powder pattern arising from the MS=±1/2 manifolds are separately resolved but the powder patterns from the MS=±3/2,±5/2 manifolds overlap the multiple 1H ENDOR lines arising from all six MS manifolds [X. Tan, M. Bernardo, H. Thomann, and C. P. Scholes, J. Chem. Phys. 98, 5147 (1993)]. Given this complexity, a combination of complementary spectroscopic techniques and numerical simulations are used to deconvolute the overlapping spectra and to assign the spectral lines. The ENDOR spectra provided an experimental description of H217O hyperfine couplings to high spin Mn2+ in a frozen solution. The ESEEM results are consistent with the first-order assignments of the ENDOR lines and demonstrate the feasibility of ESEEM measurements of 17O ligand hyperfine couplings to Mn2+. Simulations of the 17O ENDOR hyperfine patterns of aqueous frozen solutions of Mn2+, especially those near 20 MHz, indicated an A-tensor anisotropy of A⊥=−6.5±0.5 MHz and A∥=−9.5±0.5 MHz, consistent with couplings observed by single crystal ENDOR of H217O ligated to Mn2+ doped in [LaMg(NO2)12⋅24(H2O)]. More detailed simulations of the ENDOR pattern below 10 MHz indicated the need for quadrupole couplings consistent with those measured by single crystal ENDOR and with those determined by gas phase measurements on H17OD. Simulations of the ENDOR spectra recorded by the cw and pulsed techniques have delineated important features of the techniques which must be taken into account for a quantitative analysis of the ENDOR amplitudes. It is expected that the general ENDOR conditions employed and the theory developed will be useful in frozen solution studies of 17O involved as a ligand to Mn2+ in enzymes.

ESR spectrometer with a wide frequency range using a gyrotron as a radiation power source

The development of a high frequency electron spin resonance (ESR) spectrometer with a wide frequency range using a gyrotron as the radiation power source is described. GYROTRON FU-E, optimized for use in an ESR spectrometer in the millimeter wave range, was developed in Fukui University. In order to test the normal operation of the spectrometer, the ESR of two standard samples, single crystal and polycrystalline DPPH, has been measured, in the pulsed mode over the frequency range from 65 GHz to 135 GHz.

Laser optical pumping of potassium in a high magnetic field using linearly polarized light

It is shown experimentally that in a high magnetic field a potassium vapor can be optically pumped to a high electron spin polarization by light polarized parallel to the magnetic field and incident normal to the magnetic field. The polarization of the K vapor is measured both by observing the fluorescence and by the Faraday effect. This method of optical pumping may be useful for spin polarized targets.

Synthesis and structural investigation of some mixed-ligand selenito complexes of cobalt(II)

Two complexes, [Co(An)2(O2SeO)] and [Co(Iso)2(H2O)2(O2SeO)], have been isolated and characterised by elemental analyses, IR spectra, magnetic measurements and thermogravimetric analysis. Their lattice constants have been determined with the help of X-ray powder diffraction data. Kinetic parameters like energy and entropy of activation for the thermal degradation process have been arrived at using Piloyan-Novikova, Coats-Redfern and Horowitz-Metzger equations. The mechanism of non-isothermal decomposition of complexes is by phase boundary nucleation. The probable structure for Co(II) complexes is six-coordinated octahedral in a high-spin electron configuration. The order of the thermal stability of complexes is [Co(An)2(O2SeO)]>[Co(Iso)2(H2O)2(O2SeO)].

Nuclear modulation effects in high-spin electron systems with small zero-field splittings

An analytic theory has been developed to simulate the nuclear modulations arising from the application of the two-pulse electron-spin-echo envelope modulation (ESEEM) technique to a high-spin-electron system with S=5/2, coupled both to its own nuclear spin, I′=5/2, and a distant nuclear spin, I=1/2. This theory differs from that previously used by Mims [Electron Paramagnetic Resonance, edited by S. Geschwind (Plenum, New York, 1972), pp. 263–351] to describe the ESEEM results of a simpler system (S=1/2, I=1/2) by the inclusion of a zero-field splitting (ZFS) term in the spin Hamiltonian. The major assumption made in the derivation is that only transitions that do not share a common electron energy level are excited. This and other assumptions are discussed. The resulting analytic form suggests that each of the five possible single-quantum electron transitions in the S=5/2 system make a contribution to the ESEEM spectrum. The contribution from each electron transition is similar to that described by Mims for the simpler system, i.e., two nuclear frequencies, a sum frequency and a difference frequency. The frequencies and intensities, however, differ from those predicted for the simpler spin system. The nuclear frequencies are shown to be a function of the particular electron transition excited by the microwave field. Whether or not a specific electron transition makes a contribution to the ESEEM spectrum is shown to be a function of the magnitudes and relative orientations of the ZFS and superhyperfine tensors. This results in an orientation selective property of the ESEEM experiment imparted by the anisotropy of the ZFS term. This is shown to affect the predicted powder-pattern-averaged spectra. Although there is a specific application to the case of Mn(II), the result is general and is applicable to all-electron high-spin systems with S≥1/2.

Interrupted σ-Bonds in Organic Materials with Colligative Magnetic Properties

Abstract Solutions of tris(2, 6-dimethoxyphenyl)methyl radical (1) and lithium salts form solids showing unusual magnetic behavior. The propeller-shaped radicals, with their tripods of nucleophilic oxygens, are preorganized polydentate ligands. By coordination with small alkali metal cations the radicals may stack, enforcing spin-spin interactions in extended chains of paramagnets. Ab initio calculations on a linear H3C-Li+-CH3 model shows that high-spin (ferromagnetic) electron coupling is favored for a wide range of radical-Li+distances.

Use of high performance electron spin resonance materials for the design of scalar and vectorial magnetometers

Extensive work on radical ESR (electron spin resonance) materials has led to the discovery of a number of products exhibiting intense and narrow ESR lines, suitable for magnetometry. These materials can be classified using a convenient figure of merit. Two of them were used in two specific architectures for a scalar magnetometer-(FA/sub )/2P((fluoranthenyl)/sub 2/PF/sub 6/)-/sub 6/ with a resolution of 1 nT-Hz/sup -1/2/ (white noise) and a vectorial magnetometer (AcTCNQ/sub 2/) with a resolution of 0.2 nT-Hz/sup -1/2/ (white noise). These magnetometers can be used in high gradients (usual indoor environments) and have bandwidths which can be extended from DC to several kHz.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The correlation of various properties of thin films of MoO3 and of the mixed oxide systems MoO3-In2O3 and MoO3-SiO

A discussion of the general properties of three systems of dielectric films i.e. MoO3 and the mixed oxide systems MoO3-In2O3 and MoO3-SiO is presented. Composition, film thickness, substrate deposition temperature and annealing all have a substantial effect on the structure and various properties of the films. The general properties of these three systems of dielectric films include analysis by X-ray photoelectron spectroscopy, UV-visible and infrared spectroscopy including the Fourier transform technique, electrical properties both d.c. and a.c. at both low and high fields and electron spin resonance. An attempt is made to show how the variations of properties depend on many disposable parameters. In particular the use of X-ray photoelectron spectroscopy shows how the local atomic and molecular bonding changes as a result of varying preparation parameters and this feature is a recent facility in the field of thin dielectric-semiconducting films. It is expected that the general discussion in this paper may help in the interpretation of results on other thin dielectric and semiconducting films.

Avian kidney mitochondrial hemeprotein P-450 1 α: isolation, characterization and NADPH-ferredoxin reductase-dependent activity

We describe the isolation of cytochrome P-450 1 α from chick-kidney mitochondria. Although, gel permeation HPLC yielded 41% of the total amount of P-450 present in cholate-solubilized hemeproteins, it produced a highly purified mixture from which the P-450 1 α could be purified to homogeneity in a final detergent-free state by a single-step application of hydrophobic interaction HPLC using hydroxypropyl silica. The purified P-450 1 α traveled as a single band in SDS gel electrophoresis with an apparent M r = 57 000. The absolute spectrum of the P-450 1 α (Fe 3+) form gave a λ max at 403 nm. This characteristic lends support to the anomalous high-spin heme electron paramagnetic resonance spectrum and the heme structure of P-450 1 α which we have previously reported (Ghazarian et al. (1980) J. Biol. Chem. 255, 8275–8281; Pedersen et al. (1976) J. Biol. Chem. 251, 3933–3941). In reconstitution experiments with ferredoxin-dependent NADPH-cytochrome c ( P-450) reductase complexes, P-450 1 α catalyzed the hydroxylation of 25-hydroxy-9,10-secocholesta-5,7,10(19)-trien-3β-ol at the C-1 position exclusively with a turnover number of 0.03 min −1. This number is identical to that obtained from measurements of the catalytic activity in intact mitochondria, indicating that only one major species of cytochrome P-450 occurs in chick-kidney mitochondria. The complete responsiveness of cytochrome P-450 concentrations in intact mitochondria to the vitamin D status of chicks provided additional evidence that the major cytochrome P-450 species present in renal mitochondria is uniquely associated with vitamin D metabolism.