Electron Spin Resonance Linewidth Research Articles

Paramagnetic spin correlations and spin dynamics in doped manganites as the precursors of their magnetic ordering

To confront the magnetic ordering in bulk and nanometer-sized La1−xCaxMnO3 manganites with the Ca doping level x=0.1 and 0.6, the fitting of the temperature dependences of electron paramagnetic resonance linewidth and intensity to the theoretical models was combined with the magnetic measurements. In the x=0.1 case, the magnetic ordering in nanosamples changes as compared to bulk due to transition from the confined state of charge carriers in chemically disordered bulk to a mobile one in an impuritylike band in homogeneous nanocrystals. While both bulk and nano-x=0.6 samples demonstrate stable antiferromagnetic ordering in spite of strong ferromagnetic correlations in the paramagnetic state. This may be associated with the localization of doped electrons, which (together with the elastic interaction between Jahn-Teller) ions may be a prerequisite for electron-hole doping asymmetry in the phase diagram of La1−xCaxMnO3.

Paramagnetic spin dynamics in the nonhomogeneous crystals of low-doped lanthanum manganites

The X-band electron paramagnetic resonance (EPR) technique and the model analysis of the EPR linewidth versus temperature were employed to characterize the dopant distribution in a number of La-manganite crystals, low doped with Ca, Sr, and Ba. Such distribution appears to be inhomogeneous in accordance with the results of previous study of EPR susceptibility. The technological originated competition between the quenching and annealing effects during the cooling of the crystals from the high temperatures seems to be the reason of their inherent inhomogeneity. It depends strongly on the dopant nature and, in the crystal doped with 15% of Ba, the inhomogeneity is small as compared to those in the Ca- and Sr-doped samples. It is emphasized that such chemical disorder influences strongly the crystal structure, magnetic state, and transport of these objects.

Development of polymer-coated paramagnetic Implants for biomedical oximetry Applications

There are a variety of methods available for measuring and mapping of oxygen concentration in biological tissues. The particulate-based, electron paramagnetic resonance (EPR) oximetry is a powerful technology that allows non-invasive and repetitive monitoring of oxygenation in tissues. Lithium naphthalocyanine (LiNc) forms highly stable paramagnetic crystals which can be used for high resolution oximetry in tissues. To show efficient biocompatibility, long-term in vivo stability and responsiveness of LiNc oximetry probes, as well as for making the probes surgically implantable/ retrievable, we coated LiNc microcrystals with Teflon AF 2400 (TAF) polymeric materials. EPR linewidths of polymer-coated LiNc probes under anoxic conditions as well as at varying partial pressures of oxygen (pO2) did not show appreciable change relative to uncoated LiNc particulates, yet the linewidth of coated LiNc crystals was linearly dependent on varying pO2. The coated implantable probes responded to changes in pO2 quickly and reproducibly, enabling dynamic measurements of oxygenation in real time. The implants were unaffected by biological oxidoreductants. The oxygen sensitivity and stability of the coated LiNc was demonstrated in vivo in mice for more than two months. Thus, new TAF polymer-coated LiNc crystals are potential candidates for future in vivo studies including clinical trials for oxygen measurements in pathophysiological tissues.

High-frequency electron paramagnetic resonance of the hole-trapped antisite bismuth center in photorefractive bismuth sillenite crystals

The hole-trapped antisite bismuth center has been directly observed by $W$-band (94 GHz) electron paramagnetic resonance (EPR) in the series of sillenite crystals, ${\text{Bi}}_{12}M{\text{O}}_{20}$ ($M=\text{Ge},\text{Si},\text{Ti}$, denoted as BMO), either nondoped or doped with transition ions (Cr, Cu, Ru, Ce). Blue light illumination influences the EPR intensity in most crystals, while in nondoped ${\text{Bi}}_{12}\mathrm{Ge}{\text{O}}_{20}$ and ${\text{Bi}}_{12}\mathrm{Si}{\text{O}}_{20}$ the signals only appear upon illumination. The spectra can be attributed to a single species and no anisotropy could be detected eliminating any significant deviation from tetrahedral symmetry due to a perturbing defect in the near neighborhood or to static lattice distortion. The large and isotropic hyperfine parameter, in good agreement with previous optically detected magnetic-resonance measurements [Phys. Rev. B 47, 5638 (1993)], reveals that only $\ensuremath{\sim}25%$ of the hole is in the $\text{Bi}\text{ }6{s}^{1}$ orbital, by delocalization mainly to the neighboring oxygen ions, with extremely small spin densities on the surrounding ${\text{Bi}}^{3+}$ lattice ions as derived from the EPR linewidths. The parameter variations between the three crystalline hosts are very small, showing a near-identical degree of delocalization of the trapped hole.

Field-Induced ESR Spectroscopy on Rubrene Single-Crystal Field-Effect Transistors

AbstractWe investigate the electron spin resonance (ESR) spectroscopy for the field-induced carriers in rubrene single-crystal field-effect transistors (SC-FETs), and compare the results with those on pentacene thin-film transistors (TFTs). We observe Lorentz-type ESR signal in rubrene SC-FETs whose linewidth is narrowed with increasing gate voltage and temperature. It demonstrates that the ESR linewidth is determined by motional narrowing effect as we reported on pentacene TFTs. Based on the observations, we discuss the multiple trap-and-release (MTR) processes in the two systems with and without grain boundaries.

Molecular packing and magnetic properties of lithium naphthalocyanine crystals: hollow channels enabling permeability and paramagnetic sensitivity to molecular oxygen.

The synthesis, structural framework, magnetic and oxygen-sensing properties of a lithium naphthalocyanine (LiNc) radical probe are presented. LiNc was synthesized in the form of a microcrystalline powder using a chemical method and characterized by electron paramagnetic resonance (EPR) spectroscopy, magnetic susceptibility, powder X-ray diffraction analysis, and mass spectrometry. X-Ray powder diffraction studies revealed a structural framework that possesses long, hollow channels running parallel to the packing direction. The channels measured approximately 5.0 × 5.4 Å(2) in the two-dimensional plane perpendicular to the length of the channel, enabling diffusion of oxygen molecules (2.9 × 3.9 Å(2)) through the channel. The powdered LiNc exhibited a single, sharp EPR line under anoxic conditions, with a peak-to-peak linewidth of 630 mG at room temperature. The linewidth was sensitive to surrounding molecular oxygen, showing a linear increase in pO(2) with an oxygen sensitivity of 31.2 mG per mmHg. The LiNc microcrystals can be further prepared as nano-sized crystals without the loss of its high oxygen-sensing properties. The thermal variation of the magnetic properties of LiNc, such as the EPR linewidth, EPR intensity and magnetic susceptibility revealed the existence of two different temperature regimes of magnetic coupling and hence differing columnar packing, both being one-dimensional antiferromagnetic chains but with differing magnitudes of exchange coupling constants. At a temperature of ∼50 K, LiNc crystals undergo a reversible phase transition. The high degree of oxygen-sensitivity of micro- and nano-sized crystals of LiNc, combined with excellent stability, should enable precise and accurate measurements of oxygen concentration in biological systems using EPR spectroscopy.

Electron spin resonance study of polycrystallineLa0.75(CaxSr1−x)0.25MnO3 (x = 0,0.45, 1)

Electron spin resonance (ESR) spectra of polycrystallineLa0.75(CaxSr1−x)0.25MnO3 (x = 0, 0.45, 1) were studied within the temperature range110 K≤T≤470 K. The temperature dependence of the ESR intensity for the samples is described by athermally activated model in the paramagnetic regime. It is found that the activationenergy in the orthorhombic phase is higher than that in the rhombohedral phase forLa0.75(Ca0.45Sr0.55)0.25MnO3. It is suggested that a higher energy is required to destroy the correlated polarons due tothe fact that correlated polarons only exist in the orthorhombic phase. This proposition isconfirmed by the analysis of the ESR linewidth data, which can be well fitted by the modelof adiabatic hopping motion of small polarons. In addition, it is found that, ata fixed temperature, the linewidth decreases with increasing Sr doping, whichreveals that the structural tolerance factor has a significant effect on the linewidth.

EPR Investigations of Spin‐Probe Dynamics in Aqueous Dispersions of a Nonionic Amphiphilic Compound

AbstractDynamics of various spin probes in aqueous dispersions of nonionic amphiphilic compound, [poly(oxyethylene) hydrogenated castor oil, HCO], were investigated by EPR (electron paramagnetic resonance) and saturation recovery (SR) spectroscopies. Partitioning, rotational correlation time (τR), rotational diffusion coefficient, and electron spin‐lattice relaxation time (T1e) in dispersions of the HCO membrane were obtained. The partitioning of water soluble spin probes, DTBN and TEMPO, in the aqueous and vesicle phases was determined by an EPR linewidth simulation as a function of temperature. The results suggest that DTBN and TEMPO have a similar partitioning in the vesicle phase throughout the temperatures studied. The longer τR and shorter T1e (~0.33 μs) values of DTBN in the vesicle phase were obtained, and could be attributed to the probe environment in the membrane. The simulation results for fast tumbling probes were quite different from those of conventional intensity analysis (spectral parameter, f). Thus, the simulation and T1e analyses have provided a quantitative understanding of the probe dynamics in both phases. Aliphatic spin probes, doxylstearic acids (DSAs) and 3β‐doxyl‐5α‐cholestane (CHL), were used for monitor of various membrane motions. The EPR spectra were quantitatively analyzed by a slow tumbling simulation. The rotational diffusion coefficients and order parameter were obtained by the simulation. In addition, the direct observations of the behavior of the probes were measured by SR method. The results were consistent with T1e obtained for spin probes. Thus, the quantitative results regarding EPR, SR method, various simulation analyses have provided detailed information regarding physicochemical properties of the various moieties of the probe region in the amphiphilic compound.

Electron spin resonance study of Fe doping effect in La 0.67Ca 0.33MnO 3

Electron spin resonance (ESR) study was carried out on La 0.67Ca 0.33Mn 1− x Fe x O 3 ( x=0.0, 0.04) samples. The temperature dependence of the ESR spectra indicates the presence of phase separation above and below T C in x=0.0 and 0.04 sample, respectively. The increase of the g-value in the high-temperature region indicates the existence of local spin correlations even in the paramagnetic state. The activation energy obtained from both the temperature dependence of the ESR intensity and linewidth exhibits a smaller value in the Fe-doped sample. Our study suggests that the ferromagnetic spin correlations would be significantly weakened by a slight doping of Fe ions on Mn sites.

Continuous-wave far-infrared ESR spectrometer for high-pressure measurements

We present a newly-developed microwave probe for performing sensitive high-field/multi-frequency electron spin resonance (ESR) measurements under high hydrostatic pressures. The system consists of a BeCu-made pressure-resistant vessel, which accommodates the investigated sample and a diamond microwave coupling window. The probe’s interior is completely filled with a pressure-transmitting fluid. The setup operates in reflection mode and can easily be assembled with a standard oversized microwave circuitry. The probe-head withstands hydrostatic pressures up to 1.6 GPa and interfaces with our home-built quasi-optical high-field ESR facility, operating in a millimeter/submillimeter frequency range of 105–420 GHz and in magnetic fields up to 16 T. The overall performance of the probe was tested, while studying the pressure-induced changes in the spin-relaxation mechanisms of a quasi-1D conducting polymer, KC 60. The preliminary measurements revealed that the probe yields similar signal-to-noise ratio to that of commercially available low-frequency ESR spectrometers. Moreover, by observing the conduction electron spin resonance (CESR) linewidth broadening for KC 60 in an unprecedented microwave frequency range of 210–420 GHz and in the pressure range of up to 1.6 GPa, we demonstrate that a combination of high-pressure ESR probe and high-field/multi-frequency spectrometer allows us to measure the spin relaxation rates in conducting spin systems, like the quasi-1D conductor, KC 60.

Ferromagnetic spin fluctuations in antiferromagnetic Pr 1−xCa xMnO 3: An ESR study

Electron spin resonance (ESR) measurements have been performed on polycrystalline samples of Pr 1− x Ca x MnO 3 ( x=0.4, 0.5) in the temperature range of 100–300 K. The temperature dependence of ESR intensity, g value and linewidth shows the existence of ferromagnetic spin correlations in the paramagnetic state. With decreasing temperature, the ferromagnetic spin correlations switch to antiferromagnetic spin correlations in the charge ordering state and vanish at the antiferromagnetic ordering temperature T N.

Recent Progress in the Dynamic Nuclear Polarization of Solid Deuterated Butanol Targets

Polarized solid targets have been used in nuclear and particle physics experiments since the early 1960s, and with the development of superconducting magnets and 3He/4He dilution refrigerators in the early 1970s, proton polarization values of 80–100% have been achieved routinely in various target materials at two standard magnetic field and temperature conditions (2.5 T, <0.3 K and 5 T, 1 K). Due to the much lower magnetic moment of the deuteron compared with that of the proton, deuteron polarization values have been considerably lower, typically 30–40% in chemically doped alcohols (d-butanol), 40–45% in radiation-doped ammonia (ND3), and 50–55% in irradiated lithium-deuteride. Now, however, research at the University of Bochum – including systematical electron spin resonance (ESR) investigations in order to study the properties of the paramagnetic dopants (i.e., their ESR line width) – is yielding alcohol and diol materials with deuteron polarizations as high as 80%.

Studies of Dynamic Nuclear Polarization with Nitroxides in Aqueous Solution

The dynamic nuclear polarization effect between the nitroxide radical TEMPO and the 1H protons of water solution was investigated at an electron pumping frequency of 9.7 GHz for different experimental conditions. In particular, we compared 14N-4-hydroxy-TEMPO (TEMPOL) with 4-oxo-TEMPO and evaluated the effect of 2H and 15N isotope labelling. Furthermore, we tested the effect of concomitant irradiation on both electron paramagnetic resonance (EPR) lines with the 15N-labelled compound. Our results show that the reduction in the EPR line width given by the 2H and 15N labelling and the collapse in the number of hyperfine lines due to 15N substitution leads to the highest enhancement of ε = −140 ever reported in the literature. Concomitant pumping on two hyperfine lines does not give higher enhancements at our experimental conditions.

Dzyaloshinsky-Moriya Anisotropy in the Spin-1/2 Kagome CompoundZnCu3(OH)6Cl2

We report the determination of the Dzyaloshinsky-Moriya interaction, the dominant magnetic anisotropy term in the kagome spin-1/2 compound ZnCu3(OH)6Cl2. Based on the analysis of the high-temperature electron spin resonance (ESR) spectra, we find its main component |Dz|=15(1) K to be perpendicular to the kagome planes. Through the temperature dependent ESR linewidth, we observe a building up of nearest-neighbor spin-spin correlations below approximately 150 K.

Polaron Motional Narrowing of Electron Spin Resonance in Organic Field-Effect Transistors

Polaron states in organic thin-film transistors (TFTs) were investigated by the electron spin resonance (ESR) technique. Gate-field-dependent and temperature-dependent single-Lorentzian ESR spectra were observed for field-induced polarons in pentacene TFTs, demonstrating the effect of motional narrowing due to polaron diffusion. Analyses of the ESR linewidth revealed a considerably long trapping time (tau_(C) approximately 0.7 ns), the variation of which is discussed in terms of the multiple trap-and-release model.

Effects of quenched disorder on Griffiths phase and EPR line width in La0.67–2xNd2xCa0.33–xSrx MnO3 manganites

AbstractIt is widely accepted that there is Griffiths phase (GP) present in the paramagnetic phase in doped manganites. We studied the effect of quenched disorder on the region of GP in connection with the temperature dependence of the line width (ΔH) of the electron paramagnetic resonance (EPR) in La0.67–2x Nd2x Ca0.33–x Srx MnO3 (x = 0, 0.1, 0.15, 0.2 and 0.25). With this approach it is found that ΔH (T), the minimum in the line width (ΔHmin) and Tmin are reproduced. With increasing values of the quenched disorder the values of ΔHmin are found to increase. A numerical relation that correlates the temperature range in which GP exists and the cation size disorder is obtained and found to be in agreement with some reported data. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Nanostructure and paramagnetic centres in diamond-like carbon: Effect of Ar dilution in PECVD process

Diamond-like carbon (DLC) films were deposited utilising plasma enhanced chemical vapour deposition (PECVD) with acetylene precursor, diluted with 0–45% argon. Electron paramagnetic resonance (EPR) measurements show the presence of one paramagnetic centre with no change in spin population over the range of film deposition conditions. However, the EPR linewidth decreases with increasing argon content of the precursor mix, suggesting an enhancement of motional narrowing due to an increase in electron delocalization, related to an increase in the sp 2 cluster size. Atomic force microscopy (AFM) measurements indicate that the surface of the DLC is formed of nanoscale asperities of material. With radii of tens of nanometres for films deposited with zero argon, the size of the features increases with the argon dilution of the acetylene. Energy dispersive X-ray analysis and electrical measurements further elucidate the changes in film structure.

EPR Evidence for Premonitory Charge-Ordering Fluctuations in Nanomanganite Pr0.57Ca0.41Ba0.02MnO3

Electron paramagnetic resonance (EPR) and magnetic properties of nanocrystalline $Pr_{0.57}Ca_{0.41}Ba_{0.02}MnO_3$ are studied and compared with those of the bulk material. The nanoscale samples prepared by the sol–gel method were annealed at different temperatures to obtain particles of different sizes. The crystallite and particle sizes were estimated by X-ray diffraction and transmission electron microscopy and found to be approximately equal to 30, 60 and 100 nm. Magnetization studies on the 60 and 100 nm particles show charge-ordering at 230 K similar to the bulk sample prepared by solid-state reaction, while in the 30 nm particles the charge-ordering signature is absent. At low temperatures all the three nanosamples show ferromagnetic transitions. This result is confirmed by the EPR intensity behavior as well. However, the EPR line width, which is reflective of the spin dynamics, shows a shallow minimum for the 30 nm particles at the temperature of 230 K, corresponding to the charge-ordering transition. We interpret this result as an indication of the presence of charge-ordering fluctuations in the nanoparticles of 30 nm size even though the static charge order is absent, thus heralding the occurrence of charge order in the larger particles.

Electron spin resonance of antiferromagnetism in Nd0.43Sr0.57MnO3 and Nd0.35Sr0.65MnO3

The manganites Nd0.43Sr0.57MnO3 and Nd0.35Sr0.65MnO3, with A-type and C-type antiferromagnetic orders, respectively, are prepared and their physical properties are studied by electrical resistivity, ac∕dc susceptibility, and electron spin resonance (ESR) measurements. Results of the temperature dependent ESR linewidth suggest that the spin-spin interaction is the dominant relaxation mechanism in these layered manganites. The g factors are nearly temperature independent in the paramagnetic state, but it varies dramatically near TN, indicating a strong spin-orbit coupling near the transition. Different temperature-dependencies of g factor in these two samples reflect the distinct features in the layer∕chain antiferromagnetism.

Anisotropic exchange interactions in CuTe2O5

Electron paramagnetic resonance (EPR) was used to study CuTe2O5 single crystals at frequencies of 9.4 and 160 GHz. Analytic expressions for the second and fourth moments of the EPR line are deduced with inclusion of the difference between the exchange couplings of the copper spin with its different neighbors. From comparing the calculated and measured EPR linewidths, the positions of copper ions with the strongest exchange interactions are identified. The parameters of the anisotropic exchange interaction between copper ions in a pair are found. The parameter of the exchange interaction between magnetically nonequivalent copper centers is determined from the frequency dependence of the EPR linewidth. The directions of the principal axes of the g tensors are established. The data obtained count in favor of a quasi-one-dimensional model of magnetism in CuTe2O5.