Electron Spin Resonance Linewidth Research Articles

Electron paramagnetic resonance investigation of the electron-doped manganite La1−xTexMnO3 (0.1 ≤x≤0.2)

The electron paramagnetic resonance (EPR) properties of the electron-doped manganite La(1-x)Te(x)MnO(3) (0.1 ≤ x ≤ 0.2) are investigated based on the data of EPR spectra, resistivity, and magnetic susceptibility. With decreasing temperature from 400 K, the EPR linewidth ΔH(PP) decreases and passes through a minimum at T(min), then substantially increases with further decreasing temperature. The broadening of the EPR linewidth above T(min) can be understood in terms of the increase in the relaxation rate of spin of e(g) polarons to the lattice with increasing temperature due to the similarity between the temperature dependence of the linewidth ΔH(pp)(T) and the conductivity σ(T). For the samples with x = 0.1 and 0.15, the conductivity activation energy E(σ) is comparable with the activation energy E(a) deduced from the linewidth. Whereas for the x = 0.2 sample, there is a large difference between E(σ) (0.2206 eV) and E(a) (0.0874 eV).

Intrinsic magnetism in Fe doped SnO 2 nanoparticles

Iron doped semiconducting nanoparticles Sn 1− x Fe x O 2 with x=0, 0.001, 0.002, 0.003, 0.004, 0.01 and 0.03 were prepared by a sol–gel method. The X-ray diffraction, Transmission Electron Microscopy measurements confirm the rutile structure with no impurity phase. The three characteristic lines of electron spin resonance (ESR) are observed in the doped samples for all compositions, which is a clear evidence for rhombic Fe 3+ in rutile phase. The line width of ESR increases with increase in Fe concentration due to induced disorder. The spin-pumping effect is observed at temperatures below 250 K for the samples with x=0.01 and 0.03. However, based on the Curie–Weiss susceptibility, iron is in paramagnetic state and is subject to weak antiferromagnetic interaction. Blue shift in the optical band gap is observed with increase in the Fe content.

Reduced dimensionality effects on phonon transport

Electron paramagnetic resonance (EPR) spectroscopy has been used to study energy transport properties of both bulk and nanophosphor (nominally 30 nm) oxyorthosilicate samples, and we were able to separate the effects of crystal disorder and relaxation lifetime broadening on the EPR linewidths. The low temperature linewidths (T<10 K) were inhomogeneously broadened and dominated by crystal disorder effects and the nanophosphors showed an order of magnitude increase in crystal disorder. Both bulk and nanophosphor samples showed significant lifetime broadening involving direct relaxation via phonons and the Orbach relaxation process. At low temperatures, the lifetimes of the bulk samples displayed the influence of the lattice-bath relaxation time as well as the spin-lattice relaxation time while the lifetimes of the nanophosphor samples were not influenced by the lattice-bath relaxation time. The results imply that reduced dimensionality in insulators does reduce the lattice-bath relaxation time, although the exact relationship cannot be confirmed by this study largely due to the nonideal nature of the materials studied.

Mechanical compression induced short-range ordering of nanographene spins

Nanographene has electronic structure which depends crucially on its edge shape; that is, the zigzag edge has an edge state having a localized spin in spite of the absence of such a state in the armchair edge. Nanoporous activated carbon fibers, which consist of a three-dimensional random network of nanographite domains (stacked nanographene sheets), are investigated to clarify the magnetism of the edge-state spins in relation to Ar physisorption. In addition to the deviation of the susceptibility from the Curie-Weiss behavior, the electron spin resonance linewidth is found to increase abruptly below the boiling point (87 K) of Ar condensed in the nanopores. This is the evidence of mechanically induced magnetic short-range ordering of edge-state spins in the nanographite upon the condensation of Ar molecules.

Magnetic phase separation in Nd0.5(Ca,Sr)0.5MnO3

The phase separation of Nd0.5Ca0.5−ySryMnO3, for 0≤y≤0.5, is studied via electron spin resonance (ESR) in the temperature range 80–400 K. Two types of magnetic phases are found to coexist in the samples with y>0 below a phase separation temperature T∗. The ESR line width exhibits a minimum near the charge order transition temperature TCO for samples with y<0.25, and near T∗ for y≥0.25. A systematic decrease in line width is observed as y increases from 0 to 0.5. The g factors increase slightly and the ESR intensities increase exponentially when the temperature decreases in the paramagnetic region. Our results demonstrate presence of two local magnetic structures due to the intrinsic phase separation.

Radical Formation in Lithium Formate EPR Dosimeters after Irradiation with Protons and Nitrogen Ions

Radical formation in polycrystalline lithium formate monohydrate after irradiation with gamma rays, protons and nitrogen ions at room temperature was studied by continuous-wave electron paramagnetic resonance (EPR) spectroscopy. The linear energy transfer (LET) of the various radiation beams was 0.2, 0.7-3.9 and 110-164 keV/microm for gamma rays, protons and nitrogen ions, respectively. Doses between 5 and 20 Gy were given. The EPR reading (the area under the EPR absorption resonance) increased linearly with dose for all types of radiation. As the LET increased, the relative effectiveness (the EPR reading per dose relative to that for gamma rays) decreased, while the EPR line width increased. Track structure theory and modeling of detector effectiveness predicted the dosimeter response observed after proton and nitrogen-ion irradiation. A semi-empirical line broadening model including dipolar spin-spin interactions was developed that explained the dependence of the line width on LET. The findings indicate that the local radical density in lithium formate is increased after high-LET irradiation.

Electron Spin Resonance Spectroscopic Study of Size-Controlled Ink Particles Isolated from Sepia officinalis

The paramagnetic properties of size-controlled ink particles isolated from the ink sacs of Sepia officinalis were studied by electron spin resonance (ESR) spectroscopy. Both the size-controlled ink particles and synthetic melanins seemingly yielded similar ESR spectra consisting of a singlet with a slightly asymmetrical signal. However, the progressive microwave power saturation revealed a clear difference between their paramagnetic behaviors. In comparison with synthetic melanins, the ESR spectra of the ink particles readily reached saturation, indicating a long spin–lattice relaxation time. On the other hand, the ESR linewidth depended on particle size. This implies that the particle size is related to the distance between paramagnetic species in the particles. Hence, it is reasonable that the large ink particle has the weakest spin–spin interaction among these samples. The employment of the size-controlled ink particles enabled us to determine precisely the paramagnetic parameters of Sepia inks.

Novel Type of Career Generated System: Magnetic Investigations of TTF-Based Self-Doped Hydrogen-Bonding Conductor

Magnetic investigations, including static magnetic susceptibility and high-field electron spin resonance (ESR) measurements, were carried out for an organic conductor, (TTFCOO)[(NH 4 +1 ) 1- x (NH 3 ) x ]. Anisotropic ESR parameters were determined for powder samples using a high-frequency W-band (93.9817 GHz) ESR spectrometer. The observed principal values of the g -tensor and quantum chemical calculation results indicate that the observed spin is distributed on quasi-hole-like TTF skeletons, and that the TTFCOO mainframe partially becomes a neutral radical. The temperature dependence of the spin susceptibility is well fitted with a Curie–Weiss term and an activation-type term, with the activation-type term dominant at high temperatures. The high absolute value of the spin susceptibility and the extremely small activation energy, Δ, indicate that a quasi-degenerate (metallic) state is stabilized in (TTFCOO)[(NH 4 +1 ) 1- x (NH 3 ) x ], although the weakly temperature-dependent ESR linewidth indicates a l...

A variable temperature EPR study of the manganites (La 1/3Sm 2/3) 2/3Sr xBa 0.33− xMnO 3 ( x=0.0, 0.1, 0.2, 0.33): Small polaron hopping conductivity and Griffiths phase

Four manganite samples of the series, (La 1/3Sm 2/3) 2/3Sr x Ba 0.33− x MnO 3, with x=0.0, 0.1, 0.2 and 0.33, were investigated by X-band (∼9.5 GHz) electron paramagnetic resonance (EPR) in the temperature range 4–300 K. The temperature dependences of EPR lines and linewidths of the samples with x=0.0, 0.1 and 0.2, containing Ba 2+ ions, exhibit similar behavior, all characterized by the transition temperatures ( T C) to ferromagnetic states in the 110–150 K range. However, the sample with x=0.33 (containing no Ba 2+ ions) is characterized by a much higher T C=205 K. This is due to significant structural changes effected by the substitution of Ba 2+ ions by Sr 2+ ions. There is an evidence of exchange narrowing of EPR lines near T min, where the linewidth exhibits the minimum. Further, a correlation between the temperature dependence of the EPR linewidth and conductivity is observed in all samples, ascribed to the influence of small-polaron hopping conductivity in the paramagnetic state. The peak-to-peak EPR linewidth was fitted to Δ B pp( T)=Δ B pp,min+ A/ Texp(− E a/ k B T), with E a=0.09 eV for x=0.0, 0.1 and 0.2 and E a=0.25 eV for x=0.33. From the published resistivity data, fitted here to σ( T)∝1/ T exp(− E σ/ k B T), the value of E σ, the activation energy, was found to be E σ=0.18 eV for samples with x=0.0, 0.1 and 0.2 and E σ=0.25 eV for the sample with x=0.33. The differences in the values of E a and E σ in the samples with x= 0.0, 0.1and 0.2 and x=0.33 has been ascribed to the differences in the flip-flop and spin-hopping rates. The presence of Griffiths phase for the samples with x=0.1 and 0.2 is indicated; it is characterized by coexistence of ferromagnetic nanostructures (ferrons) and paramagnetic phase, attributed to electronic phase separation.

Pyrophosphate-Bridged CuII Chain Magnet: {[Na3Cu(P2O7)(NO3)]·3H2O}n

A Cu(II)...Cu(II) pyrophosphate-bridged compound of formula {[Na(3)Cu(P(2)O(7))(NO(3))].3H(2)O}(n) (1) has been characterized. X-ray diffraction measurements show that it crystallizes in the monoclinic space group P2(1)/m, with unit cell dimensions a = 7.2492(5) A, b = 8.2446(6) A, c = 9.9050(7) A, beta = 107.123(1) degrees, and Z = 2. The structure consists of chains of Cu(II) cations at inversion symmetry sites bound to four equatorial oxygen atoms provided by two pyrophosphate anions halved by a symmetry plane and two axial oxygen atoms of nitrate anions. The molar magnetic susceptibility chi(0) of a powdered sample was measured in the temperature range 2 K < T < 273 K, and an isothermal magnetization curve, M(B(0),T), was obtained at T = 30 K, with the magnetic field B(0) between 0 and 5 T. Fitting a spin-chain model to the susceptibility data, we evaluate an antiferromagnetic exchange coupling 2J = -24.3(1) cm(-1) (defined as H(ex) = -2JS(i)S(j)) between Cu(II) neighbors. For any orientation of B(0), single-crystal electron paramagnetic resonance (EPR) spectra obtained at 9.8 and 33.9 GHz at 300 K display a single signal having a g matrix with orthorhombic symmetry, arising from the merger produced by the exchange interaction of the resonances corresponding to the two rotated Cu(II) sites. The g matrices of the individual molecules calculated assuming axial symmetry yielded principal values g(parallel) = 2.367(1) and g(perpendicular) = 2.074(1) at both frequencies, indicating a d(x(2)-y(2)) ground-state orbital for the Cu(II) ions. The angular variation of the EPR line width suggests exchange narrowing in a system with one-dimensional spin dynamics, as expected from the structure and susceptibility data. The results, discussed in terms of the crystal and electronic structures and of the spin dynamics of the compound, are compared with those obtained in other materials.

Electron paramagnetic resonance of boron acceptors in isotopically purified silicon

The electron paramagnetic resonance (EPR) linewidths of B acceptors in Si are found to reduce dramatically in isotopically purified 28Si single crystals. Moreover, extremely narrow substructures in the EPR spectra are visible corresponding to either an enhancement or a reduction of the absorbed microwave on resonance. The origin of the substructures is attributed to a combination of simultaneous double excitation and spin relaxation in the four level spin system of the acceptors. A spin population model is developed which qualitatively describes the experimental results.

Correlation between the negative magnetoresistance effect and magnon excitations in single-crystalline CuCr1.6V0.4Se4

Structural, electrical and magnetic measurements, as well as electron spin resonance (ESR) spectra, were used to characterise the single-crystalline CuCr1.6V0.4Se4 spinel and study the correlation between the negative magnetoresistance effect and magnon excitations. We established the ferromagnetic order below the Curie temperature T C ≈ 193 K, a p-type semiconducting behaviour, the ESR change from paramagnetic to ferromagnetic resonance at T C, a large ESR linewidth value and its temperature dependence in the paramagnetic region. Electrical studies revealed negative magnetoresistance, which can be enhanced with increasing magnetic field and decreasing temperature, while a detailed thermopower analysis showed magnon excitations at low temperatures. Spin–phonon coupling is explained within the framework of a complex model of paramagnetic relaxation processes as a several-stage relaxation process in which the V3+ ions, the exchange subsystem and conduction electron subsystem act as the intermediate reservoirs.

Electron paramagnetic resonance (EPR) of antiferromagnetic nanoparticles of La 1− xSr xCrO 3 (0.000 ≤ x ≤ 0.020) synthesized by combustion reaction

Nanocrystalline particles of La 1− x Sr x CrO 3 (0.000 ≤ x ≤ 0.020) compounds were synthesized in order to investigate the antiferromagnetic (AFM) to paramagnetic (PM) phase transition temperature, g-factor, line width and intensity by electron paramagnetic resonance (EPR). All samples were synthesized by combustion reaction method using strontium nitrate, lanthanum nitrate, chromium nitrate and urea as fuel without subsequent heat treatment. X-ray diffraction patterns of all systems showed broad peaks consistent with orthorhombic structure of LaCrO 3. The absence of extra reflections in the diffraction patterns of as-prepared materials ensures the phase purity. The average crystallite sizes determined from the prominent (1 1 2) peak of the diffraction using Scherrer's equation was independent of the addition of Sr 2+ ions; being ca. 31–29 nm for x = 0.000 and 0.020, respectively. The EPR line width and intensity were found to be dependent on Sr 2+ addition and temperature. However, the AFM–PM transition temperature was found to be independent of strontium concentration, being ca. 296 K. In the PM phase, g-factor was nearly temperature independent with increasing of x. The EPR results indicated that the addition of Sr 2+ ions may induce creation of Cr 3+–Cr 4+ clusters.

Why could electron spin resonance be observed in a heavy fermion Kondo lattice?

We develop a theoretical basis for understanding the spin relaxation processes in Kondo lattice systems with heavy fermions as experimentally observed by electron spin resonance (ESR). The Kondo effect leads to a common energy scale that regulates a logarithmic divergence of different spin kinetic coefficients and supports a collective spin motion of the Kondo ions with conduction electrons. We find that the relaxation rate of a collective spin mode is greatly reduced due to a mutual cancelation of all the divergent contributions even in the case of the strongly anisotropic Kondo interaction. The contribution to the ESR linewidth caused by the local magnetic field distribution is subject to motional narrowing supported by ferromagnetic correlations. The developed theoretical model successfully explains the ESR data of YbRh2Si2 in terms of their dependence on temperature and magnetic field.

Detection of vortex excitations in a Bi2Sr2Ca2Cu3O10 crystal at temperatures above the critical temperature by EPR of the surface layer

Vortex excitations have been detected at temperatures both below and above the critical temperature when investigating local magnetic fields on the surface of a Bi2Sr2Ca2Cu3O10 single crystal by means of an electron paramagnetic resonance (EPR) probe. A thin layer of a diphenyl picrylhydrazyl organic radical deposited on the crystal surface is used as the EPR probe. A narrow EPR signal makes it possible to detect weak distortions of the magnetic field appearing at T ≿ Tc. The analysis of the temperature dependences of the resonance field and the EPR linewidth is thebasis of the assumption of the vortex nature of magnetic excitations in this temperature range.

Electron paramagnetic resonance linewidth narrowing of Gd3+ ions in Y-doped ceria nanocrystals with decreasing crystallite size

Electron paramagnetic resonance (EPR) spectra of Gd(3+) ions in crystalline Ce(1-x-y)Gd(x)Y(y)O(2-[0.5*(x+y)]) (x=0.0025, y=0.10, and 0.25) with crystallite sizes ranging from 600 nm down to 5 nm have been measured at X-band and at Q-band near liquid He and room temperatures. The EPR line shape is controlled by the low-symmetry surrounding of Gd(3+) ions in a coordination environment with one oxygen-vacancy and seven oxygen nearest-neighbors forming GdO(7) polyhedra. These coordination polyhedra are characterized by a wide distribution of crystal field parameters that primarily controls the EPR linewidth. The EPR linewidth of the central (-1/2 <--> +1/2) transition is observed to decrease systematically with decreasing crystallite size. This observation implies that the size of the crystallites in the nanoregime may have important influence on the energetics of vacancy distribution in crystalline materials.

Magnetocrystalline interactions inMnCr2O4spinel

In this work we present a magnetic, structural, and electron spin resonance (ESR) study of the geometrically frustrated ${\text{MnCr}}_{2}{\text{O}}_{4}$ spinel in an extended range of $T$ (2--1000 K). At the lowest temperature $(Tl18\text{ }\text{K})$ the ESR lineshape is compatible with the coexistence of spiral and ferrimagnetic spin ordering. Above ${T}_{C}$ $(\ensuremath{\approx}41\text{ }\text{K})$, magnetic susceptibility $(\ensuremath{\chi})$ and ESR intensity coincide with each other showing the typical behavior of a ferrimagnet. From the $\ensuremath{\chi}(T)$ vs $T$ dependence, absolute values for the three exchange constants ${J}_{\text{MnCr}}$, ${J}_{\text{CrCr}}$, and ${J}_{\text{MnMn}}$ were determined. Our results indicate that (i) these values are approximately independent of $T$, (ii) the antiferromagnetic direct Cr-Cr exchange is the main interaction, and (iii) ${J}_{\text{MnMn}}$ is indeed not negligible as compared with ${J}_{\text{MnCr}}$ and ${J}_{\text{CrCr}}$. One noticeable anomaly in the temperature dependence of the ESR linewidth is observed at $T\ensuremath{\approx}450\text{ }\text{K}$. This behavior is accounted by a 30% variation in the extrapolated high-temperature linewidth and it is attributed to a crystalline distortion not reported previously. Experiments of high-temperature x-ray diffraction allowed us to associate this distortion to a deformation of the oxygen sublattice. ESR and x-ray results are both compatible with a more symmetric high-temperature structure.

Evidence for Conformational Movement and Radical Mechanism in the Reaction of 4-Thia-l-lysine with Lysine 5,6-Aminomutase

We demonstrate that the steady state reaction of lysine 5,6-aminomutase with substrate analogue 4-thia-l-lysine generates a radical intermediate, which accumulates in the enzyme to an electron paramagnetic resonance (EPR) detectable level. EPR line width narrowing of approximately 1 mT due to [4'-(2)H] labeling of the pyridoxal-5'-phosphate (PLP), an isotropic hyperfine coupling of 40 MHz for the proton at C4' of PLP derived from (2)H electron nuclear double resonance (ENDOR) measurement, and spin density delocalization onto the (31)P of PLP realized from observations of the (31)P ENDOR signal provide unequivocal identification of the radical as a substrate-PLP-based species. X- and Q-band EPR spectra fittings demonstrate that this radical is spin coupled with the low spin Co(2+) in cob (II) alamin and the distance between the two species is about 10 A. These results provide direct evidence for the active site motion upon substrate binding, bringing the adenosylcobalamin to the proximity of substrate-PLP for subsequent H-atom abstraction and for the notion that lysine 5,6-aminomutase functions by a radical mechanism. Observation of (2)H-ENDOR signal also provides a reliable hyperfine coupling constant for future comparison with quantum-mechanical-based calculations to gain further insight into the molecular structure of this steady state radical intermediate.

Dynamic nuclear polarization of high- and low-crystallinity polyethylenes

We carried out a dynamic nuclear polarization (DNP) study of high- and low-crystallinity polyethylene films defined as HPE and LPE, respectively, which were doped with TEMPO (2,2,6,6-tetra-methyl-piperidine-1-oxyl) by the same amount (1×1019spins/cm3). The proton polarization of 32±3% was obtained for LPE and 23±4% for HPE at 3.35T and 1.4K. With increase in the concentration of TEMPO, the electron spin resonance (ESR) linewidth of TEMPO in HPE increased more steeply than that in LPE, indicating that TEMPO did not incorporated into the crystalline part, and was localized only at the amorphous part, hence the local concentration of TEMPO in the amorphous regions of HPE varied more steeply than that of LPE with the net amount of TEMPO fed into the film. We suggest that the smaller nuclear polarization for HPE is related to the localization of TEMPO in the smaller amorphous regions.

Thermomagnetic properties of the Finland trityl radical

The Finland trityl paramagnet is characterized by magnetic susceptibility and a new form of quantitative electron paramagnetic resonance (EPR) that utilizes a superconducting quantum interference device (SQUID) as a detection method. This radical is of interest due to its use as a dynamic nuclear polarization agent as well as a potential magnetic refrigerant and quantum computing bit. The SQUID-EPR data show that the EPR linewidth of a concentrated trityl powder decreases dramatically from 4.4 to 1.4 mT as the temperature is increased from 1.8 to 10 K. The quantitative nature of SQUID-EPR is used to thermodynamically quantify the EPR energy transfer times and saturated fractions. At 95 GHz and 1.8 K, only 40% of the spins are in resonance at the onset of saturation. Conventional dc magnetic susceptibility over 1.8-150 K indicates an S=12 Curie-Weiss relationship with little long range interaction. Magnetization versus applied field at 1.8 and 4 K fits a Brillouin function with >80% electronic polarization at a normalized field of gmu(B)mu(0)HkT approximately 3. These results provide information required for theoretical modeling and engineering of the trityl radical for a wide range of applications.