EPR study of spin and charge dynamics in slightly doped poly(3-octylthiophene)

An initial virgin plasticine-like poly(3-octylthiophene) (P3OT) and this sample modified by annealing and recrystallization were investigated at X-band (10 GHz) and mainly at D-band (140 GHz) electron paramagnetic resonance (EPR) in a wide temperature region. Paramagnetic centers with anisotropic magnetic-resonance parameters were proved to exist in all polymers, namely, mobile polarons whose concentration and susceptibility depend on the temperature and the polymer treatment. Superslow torsional motion of the polymer chains and layers was studied by the saturation transfer method at D-band EPR. Spin-spin and spin-lattice relaxation times were measured separately by the steady-state saturation method at the same waveband EPR. Intrachain and interchain spin diffusion coefficients and conductivity due to polaron dynamics were calculated. It was shown that the charge transport in P3OT is determined by the strong spin-spin interaction and is stimulated by torsion motion of the polymer chains. The total conductivity of P3OT is determined mainly by dynamics of paramagnetic charge carriers. Magnetic, relaxation and dynamics parameters of P3OT were also shown to change during the polymer treatment.     

Spin charge carrier dynamics in poly(bis-alkylthioacetylene)

Initial and laser-irradiated poly(bis-alkylthioacetylene) (PATAC) samples were investigated by electron paramagnetic resonance (EPR) at X-band (9.6 GHz), Q-band (37 GHz), and D-band (140 GHz) in a wide temperature range. Two types of paramagnetic centers were proved to exist in laser-modified polymer, namely, localized and mobile polarons with the concentration ratio and susceptibility depending on the irradiation dose and temperature. Superslow torsion motion of the polymer chains was studied by the saturation transfer method at D-band EPR. Additional information on the polymer chain segment dynamics was obtained by the spin probe method at X-band EPR. Spin-spin and spin-lattice relaxation times were measured separately by the steady-state saturation method at D-band EPR. Intrachain and interchain spin diffusion coefficients and conductivity arising from the polaron dynamics were calculated. It was shown that the polaron dynamics in laser-modified polymer is affected by the spin-spin interaction. The interchain charge transfer is stimulated by torsion motion of the polymer chains, whereas the total conductivity of irradiated PATAC is determined mainly by the dynamic of diamagnetic charge carriers. Magnetic, relaxation and dynamics parameters of PATAC were also shown to change during polymer storage.     

Spin dynamics investigated by EPR in the paramagnetic regime of La2/3Ca1/3Mn1−x Me x O3 (Me = Al, In) manganites

We report an electron paramagnetic resonance (EPR) investigation of the spin dynamics in the paramagnetic regime of the colossal magnetoresistive manganites La_2/3Ca_1/3Mn_1?x Me _x O₃ (Me=Al, In;x≤0.05). The temperature dependences of the EPR linewidth and integral intensity have been analyzed in terms of the bottleneck spin relaxation and small-polaron hopping models. The exchange coupling integral between Mn³⁺ and Mn⁴⁺ ions and the polaron activation energy decrease with increasing doping level. A discussion is given concerning the factors which could explain the observed changes.     

EPR investigation of nanosized La0.67Ca0.33MnO3−δ manganites

We report an electron paramagnetic resonance (EPR) investigation of the spin dynamics in the paramagnetic regime of nanosized La_0.67Ca_0.33MnO_3?δ manganites. The temperature dependences of the EPR line width and integral intensity have been analyzed in terms of the bottlenecked spin relaxation and small polaron hopping scenarios. The exchange coupling integral between Mn³⁺ and Mn⁴⁺ ions and the polaron activation energy decrease with the reduction of grain size. A discussion is given concerning the factors which could explain the observed changes.     

The ground state and EPR spectrum in monoclinic KY(WO4)2:Nd single crystal

The electron paramagnetic resonance (EPR) of Nd³⁺ ion in KY(WO₄)₂ single crystal was investigated at T=4.2 K using an X-band spectrometer. The observed resonance absorption represents the complex superposition of three spectra corresponding to neodymium isotopes with different nuclear momenta. The EPR spectrum is characterized by a strong g-factor anisotropy. The temperature dependences of the g-factor were caused by strong spin-orbit and orbit-lattice coupling. The resonance lines become broader as temperature increases due to the short spin-lattice relaxation time.     

Paramagnetic centers in nonmetallic amorphous polyphthalocyanines

Electron paramagnetic resonance (EPR) spectra of nonmetallic amorphous polyphthalocyanines are investigated in the temperature range 295–500 K. The EPR spectrum of nonmetallic amorphous polyphth-alocyanine samples at room temperature prior to heating is a narrow singlet of approximately Lorentzian shape with a linewidth ΔH_pp ≈ 1.7 Oe, a splitting factor g=2.00, and an intensity I_EPR ≈ 10¹⁷ spins/g. It is found that the intensity and linewidth of the EPR spectrum increase with increasing temperature. Beginning with a characteristic temperature T₁, both parameters, ΔH_pp and I_EPR, become dependent on time (under isothermal conditions). Computer calculations of the spectra demonstrate that the EPR spectrum can be represented as a superposition of two lines with substantially differing parameters whose dependences on the temperature and micro-wave power also differ significantly. The possible reasons for the existence of electron paramagnetic resonance centers of two types with different degrees of delocalization of a charge carrier with a magnetic moment in nonmetallic amorphous polyphthalocyanines are discussed.     

Effect of a structural disorder on the magnetic properties of the sodium–cobalt tellurate Na<Subscript>3.70</Subscript>Co<Subscript>1.15</Subscript>TeO<Subscript>6</Subscript>

A new layered oxide, sodium–cobalt tellurate Na_3.70Co_1.15TeO₆, is synthesized and structurally characterized, and its static and dynamic magnetic properties are studied. This compound has a new monoclinic structure type with quasi-one-dimensional cation ordering in magnetically active layers. This compound is antiferromagnetically ordered at a Néel temperature T _N ~ 3.3 K, and the temperature and field dependences of magnetization suggest competing antiferromagnetic and ferromagnetic interactions. EPR spectroscopy reveals complex spin dynamics when temperature changes and the presence of two different paramagnetic centers, which is attributed to the existence of two structurally nonequivalent (regular and antisite) positions for magnetic Co²⁺ ions.     

A new method for determining the relaxation times,T 1 andT 2, from progressive saturation of inhomogeneously broadened lines

An analytical method to determine the spin-lattice (T ₁), and spin-spin (T ₂), relaxation times for inhomogeneously broadened lines obtained from electron paramagnetic resonance (EPR) experiments is presented in this work. To apply this method, the knowledge of the lineshape of the saturation curve is not necesssary, only the Lorentzian and Gaussian widths are required and these are obtained from a non-saturated line. The relaxation times are calculated by using continuous saturation under slow passage conditions. An explicit algebraic equation for the direct calculation ofT ₁, and the general form of the saturation curve for a single line in an EPR signal are given. The equation given to calculateT ₁ can also be used for substances in which the full saturation can not be obtained experimentally. A comparison of the results obtained for some substances by using the present method with respect to other existent ones, is carried out to show its reliability.     

Effect of electron irradiation on the paramagnetic state of La0.67Ca0.33MnO3

The effect of point defects on the magnetic properties of La_0.67Ca_0.33MnO₃ polycrystals and single crystals has been studied. The magnetic susceptibility χ _dc of the initial samples and samples irradiated by electrons to the maximum dose F = 9 × 10¹⁸ cm^?2 has been measured in the temperature region 80 K < T < 650 K. Local variations of Mn-O-Mn bond angles and lengths result in a nonmonotonic dose dependence of the Curie temperature T _C. At high doses of electron irradiation, F ≥ 5 × 10¹⁸ cm^?2, the temperature of the transition from the ferromagnetic to polaron state in a single crystal is found to increase. In the paramagnetic region close to T _C, ferromagnetically ordered polarons are observed to exist, while at T > 1.2T _C, localization of e _g electrons initiates formation of paramagnetic polarons with a higher magnetic moment. Electron irradiation stimulates persistence of magnetic polarons up to higher temperatures T > 2T _C.     

EPR Spectroscopy of Impurity Thulium Ions in Yttrium Orthosilicate Single Crystals

The frequency-field and orientation dependences of the electron paramagnetic resonance (EPR) spectra are measured for impurity Tm³⁺ ions in yttrium orthosilicate (Y₂SiO₅) single crystals by stationary EPR spectroscopy in the frequency range of 50–100 GHz at 4.2 K. The position of the impurity ion in the crystal lattice and its magnetic characteristics are determined. The temperature dependences of the spin–lattice and phase relaxation times are measured by pulse EPR methods in the temperature range of 5–15 K and the high efficiency of the direct single-phonon mechanism of spin–lattice relaxation is established. This greatly shortens the spin–lattice relaxation time at low temperatures and makes impurity Tm³⁺ ions in Y₂SiO₅ a promising basis for the implementation of high-speed quantum memory based on rare-earth ions in dielectric crystals.     

EPR study of copper ions in Li<Subscript>2</Subscript>Ge<Subscript>7</Subscript>O<Subscript>15</Subscript> crystals

The EPR spectra of Cu²⁺ ions (² D _5/2) located at two structurally nonequivalent positions Cu1 and Cu2 in crystals of lithium heptagermanate Li₂Ge₇O₁₅ are recorded. The angular dependences of the EPR spectrum are measured in the paraelectric phase of the Li₂Ge₇O₁₅ compound (T = 300 K). The components of the g factor and the hyperfine interaction tensor A are determined, and the orientation of the magnetic axes with respect to the crystallographic basis is established. The EPR spectra are recorded in the temperature range in the vicinity of the temperature T _C = 283 K of the transition from the paraelectric phase to the ferroelectric phase. The position symmetry of the Cu1 and Cu2 centers is determined at temperatures above and below the phase transition temperature T _C . The localization of paramagnetic centers in the structure is discussed, An analysis of the results obtained demonstrates that the Cu1 and Cu2 centers in the Li₂Ge₇O₁₅ crystal lattice replace lithium ions located at two structurally nonequivalent positions with the symmetries described at temperatures above T _C by the triclinic C _i and monoclinic C ₂ point groups, respectively.     

Dielectric relaxation and magnetic characteristics of the Bi0.5La0.5MnO3 ceramics

The complex permittivity ?* of ceramics of bismuth-lanthanum manganite Bi_0.5La_0.5MnO₃ has been measured in ranges of temperatures T = 10–200 K and frequencies f = 10²–10⁶ Hz. Clearly pronounced regions of the non-Debye dielectric relaxation have been revealed at low temperatures (T < 90 K). To describe them, the possible mechanisms have been proposed and discussed. The temperature dependences of magnetization, the anomalous behavior of which can be associated with the phase transition from the paramagnetic phase into the ferromagnetic phase occurring at T ～ 40–80 K, have been measured in the temperature range T = 10–120 K.     

Spin-Label EPR for Determining Polarity and Proticity in Biomolecular Assemblies: Transmembrane Profiles

Hyperfine couplings and g-values of nitroxyl spin labels are sensitive to polarity and hydrogen bonding in the environment probed. The dependences of these electronic paramagnetic resonance (EPR) properties on environmental dielectric permittivity and proticity are reviewed. Calibrations are given, in terms of the Block–Walker reaction field and local proton donor concentration, for the nitroxides that are commonly used in spin labeling of lipids and proteins. Applications to studies of the transverse polarity profiles in lipid bilayers, which constitute the permeability barrier of biological membranes, are reviewed. Emphasis is given to parallels with the permeation profiles of oxygen and nitric oxide that are determined from spin-label relaxation enhancements by using nonlinear continuous-wave EPR and saturation recovery EPR, and with permeation profiles of D₂O that are determined by using ²H electron spin echo envelope modulation spectroscopy.     

Low-frequency detected EPR: Principles and applications

The experimental works described are performed by the authors over last two decades by means of the LFD EPR technique. The essence of this method is low-frequency detection of the longitudinal spin magnetization while the magnetic resonance is excited by a strong microwave field. The first kind of LFD EPR is the enchanced longitudinal susceptibility effect (ELSE) which has been elaborated and applied to study spin thermodynamics in solids since 1972. Various applications of ELSE are described such as direct measuring of the spin-spin interaction temperatureT _ss in the course of resonance saturation, spin-lattice and cross relaxation, dynamic nuclear polarization etc. Another version of LFD EPR was employed to study electron spin-lattice relaxation of paramagnetic centers in high-temperature superconductors (HTSC). Recent experimental data are presented on the electron spin-lattice relaxation of Cu²⁺ ions in YBa₂Cu₃O_6+x at various temperatures andx values.     

Study of Stable L-Alanine Radicals by W-Band EPR Spectroscopy

Stable L-alanine radicals, SAR1 and SAR2, induced by γ-irradiation of the L-alanine crystal have been investigated by electron paramagnetic resonance (EPR) technique at W-band (94 GHz) frequency. The study provides assignment of radical centers detected by continuous-wave EPR, saturation transfer mode and echo-detected field-swept EPR at W-band frequencies. The phase memory time, T _m, which was measured simultaneously at X-band (9.5 GHz) and W-band frequencies for different spectral components has been employed to estimate rotation correlation times of CH₃ protons and an effective correlation time related to the local dynamics of the entire SAR1 center at room temperature.     

Influence of zeolite water on paramagnetic and ferromagnetic resonances in the Co2[Nb(CN)8] · 8H2O molecular magnet

The contributions of Co²⁺ and Nb⁴⁺ ions to the high-frequency dynamic magnetic susceptibility of the Co₂[Nb(CN)₈] · 8H₂O molecular magnet in the paramagnetic state at T > 12 K are separated. It is found that the ferromagnetic ordering, which leads to the reconstruction of the electron paramagnetic resonance spectrum into the ferromagnetic resonance spectrum, occurs at T < 12 K. The influence of zeolite water on the spectra of the paramagnetic and ferromagnetic resonances is found. Dehydration leads to a decrease in the time of the spin relaxation of the ferromagnetic system from 50 ps to 17 ps at T = 4 K and to the variation in the temperature dependences of the widths of the lines and g factors in the electron spin resonance spectra.     

Manifestation of liquid-state and solid-state properties in electron relaxation of paramagnetic ions in solutions: Non-Markovian processes

The linewidth δH and the spin-spin relaxation time T ₂ for Gd³⁺, Mn²⁺, and Cr³⁺ ions in aqueous, water-glycerol, and water-poly(ethylene glycol) solutions at paramagnetic ion concentrations providing the dipole-dipole mechanism of spin relaxation are measured using two independent methods, namely, electron paramagnetic resonance (EPR) and nonresonance paramagnetic absorption in parallel fields. Analysis of the experimental results indicates a gradual crossover from pure liquid-state (diffusion) to quasi-solid-state (rigid lattice) spin relaxation. It is demonstrated that the limiting cases are adequately described by standard, universally accepted formulas for dipole-dipole interactions in the liquid-state (the correlation time of translational motion satisfies the condition τ _c ?τ₂) and solid-state (τ _c ?τ₂) approximations. A complete theoretical treatment of the experimental dependences (including the observed gradual crossover of spin relaxation) is performed in the framework of the non-Markovian theory of spin relaxation in disordered media, which is proposed by one of the authors. Within this approach, the collective memory effects for spin and molecular (lattice) variables are taken into account using the first-order and second-order memory functions for spin-spin and spin-lattice interactions. A correlation between the spin magnitude and the temperature-viscosity conditions corresponding to the crossover to non-Markovian relaxation is revealed, and the situations in which structural transformations occurring in the solutions favor the crossover to solid-state spin relaxation are analyzed.     

Electron spin-lattice relaxation anomaly and the local pseudo freeze-out of impurity dynamics in H-bonded ferroelectrics

The strong decrease in the electron spin-lattice relaxation rate at the ferroelectric transition temperature T_c and the simultaneous increase in the transverse spin-spin relaxation rate can be both understood in terms of the local “spontaneous freeze-out” model of impurity dynamics recently proposed to explain the spontaneous dynamic symmetry breaking observed far above T_c in the EPR spectra of H-bonded ferroelectrics doped with paramagnetic impurities.     

Magnetism and the magnetocaloric effect in PrMn1.6Fe0.4Ge2

The magnetic and magnetocaloric properties of PrMn_1.6Fe_0.4Ge₂around the ferromagnetic transitions T _C ^inter ~ 230 K and T _C ^Pr ~ 30 K have been investigated by magnetisation, ⁵⁷Fe Mössbauer spectroscopy and electron paramagnetic resonance (EPR) measurements over the temperature range 5–300 K. The broad peaks in magnetic entropy around T_C ^inter (intralayer antiferromagnetism of the Mn sublattice to canted ferromagnetism) and T_C ^Pr (onset of ferromagnetic order of Pr sublattice in addition to ferromagnetically ordered Mn sublattice) are typical of second order transitions with maximum entropy values of -ΔS _M ~ 2.0 J/kg K and -ΔS _M ~ 2.2 J/kg K respectively for ΔB = 0–6 T. The EPR signal around T = 48 K of g value g ~ 0.8 is consistent with paramagnetic free ion Pr^3?+?. Below T_C ^Pr ~ 30 K the g value increases steadily to g ~ 2.5 at 8 K as saturation of the Pr^3?+? ion is approached. The EPR measurements indicate additional effects in this system below T ~ 20 K with the appearance of EPR signals of low g value g ~ 0.6.     

The reverse shift of the EPR line of paramagnetic centers coupled to species with a fast paramagnetic relaxation

The EPR spectrum of the spin 1/2 paramagnetic centers with a relatively slow relaxation is considered in the case when they are coupled via the Heisenberg exchange interaction to partners which have short times of the longitudinal and transverse paramagnetic relaxation. Under these conditions only the EPR line of paramagnetic centers with a relatively slow relaxation is detectable in experiment. The shape of this line is analyzed by solving numerically kinetic equations for the spin density matrix for simple model systems. Depending on a ratio between the exchange integral and the paramagnetic relaxation rates of partner spins, the EPR line shifts in opposite directions. For moderate relaxation rates, as the relaxation rates decrease, the EPR line shifts toward the gravity center of the total EPR spectrum. In the case of extremely fast relaxation, as the relaxation rates decrease, the reverse shift of the EPR line is expected, the line shifts away from the gravity center of the total EPR spectrum. This type of the non-monotonous line shift was experimentally observed for the monocrystal of [CuNd₂(C₄O₄)₄(H₂O)₁₆] · 2H₂O when relaxation rates were changed by temperature variation.