Electron Paramagnetic Resonance Susceptibility Research Articles

Continuous-wave and pulsed EPR studies of glass-like carbon with high spin concentration: Evidence for triplet states

Glass-like carbons (GLCs) are a large group of disordered carbon materials which have applications in various fields, but not all their properties are well characterized. In particular, they have high concentration of paramagnetic centers (PCs) which are easily observed by electron paramagnetic resonance (EPR). However, an apparently simple EPR spectrum, together with its peculiarities caused by high electrical conductivity, make it difficult to identify the PCs and their origin remains unknown. To resolve these unsettled questions, in this work we use the vacuum Rabi splitting and Rabi oscillations which so far have not been applied to study these materials by the continuous wave (CW) and pulsed EPR. GLC produced by pyrolysis of phenol-formaldehyde resin at 1120 K was studied in the temperature range of 4.3–300 K. Using the Rabi splitting we found conditions for undistorted registration of CW EPR spectra and determined spin concentration (1×1020 spin/g). The temperature dependence of the EPR susceptibility shows the coexistence of Pauli paramagnetism due to conduction electrons and the Curie-type paramagnetism of thermally activated PCs with the activation energy of 5.7 cm-1. These PCs may be excited triplet states, the presence of which is revealed by Rabi oscillations in the pulsed EPR.

Temperature dependences of paramagnetic response in oxidized spark plasma sintered carbon nanotubes: The way of understanding the local electronic structure

Electron paramagnetic resonance (EPR) spectroscopy was used to study an electronic structure of carbon nanotubes (CNTs) consolidated by spark plasma sintering and additionally oxidized in vapor of boiling nitric acid. Two components were distinguished in the EPR absorption spectra: narrow and broad lines with different g-factors. This testified to the different surrounding of paramagnetic centers associated with the lines. Temperature dependence of spin numbers, associated with the narrow line, transfers from Curie-Weiss law to Curie law with increasing temperature. EPR susceptibility, associated with the broad line, can be attributed to a paramagnetic matrix with superparamagnetic inclusions.

Thermal stability and the magnetic properties of hybrid vanadium oxide-tetradecylamine nanotubes

Vanadium oxide nanotubes (NTs) were synthesized by the sol-gel method followed by a long-term hydrothermal treatment. The obtained nanotubes have a multiwall structure, and 70% of vanadium ions are in the V4+ state. This percentage was derived by evaluating three components of the magnetic susceptibility; namely, (i) the paramagnetic Curie-Weiss behavior, (ii) antiferromagnetic dimers, and (iii) magnetic trimers. The as-made NTs were annealed in situ in the cavity of the electron paramagnetic resonance (EPR) spectrometer. The line shape changes irreversibly at 390 K, and the EPR susceptibility presents an anomaly at 425 K. These changes are interpreted as a partial oxidation of the V4+ ions and consequently a decrease in the concentration of the magnetic species. The quantification of the V4+ ions of the annealed NTs reveals a diminution to 39% of V4+, a weakening of the Curie-Weiss and antiferromagnetic dimers contributions, and the suppression of magnetic trimers. Vibrational studies confirm the decrease of V4+ amount.

Thermal, structural and magnetic properties of some zinc phosphate glasses doped with manganese ions

(MnO) x ·(P 2O 5) 40·(ZnO) 60− x glasses containing different concentrations of MnO ranging from 0 to 20 mol% were prepared by the melt-quenching technique. The samples had a fixed P 2O 5 content of 40 mol% and the MnO:ZnO ratio was varied. The thermal, structural and magnetic properties of these glasses were investigated by means of differential thermal analysis (DTA), electron paramagnetic resonance (EPR) and magnetic susceptibility measurements. Compositional dependence of the glass transition ( T g), crystallization ( T p) and melting temperatures were determined by DTA investigations. From the dependence of the T g on the heating rate ( a), the activation energy of the glass transition ( E g) was calculated. The fragility index ( F) for the studied glasses was determined to see whether these materials are obtained from kinetically strong-glass-forming (KS) or kinetically fragile-glass forming (KF) liquids. The EPR spectra of the studied glasses revealed absorptions centered at g ≈ 2.0, 3.3 and 4.3. The compositional variations of the intensity and line width of these absorption lines was interpreted in terms of the variation in the concentration of the Mn 2+ and Mn 3+ ions in the glass and the interaction between the manganese ions. EPR and magnetic susceptibility data reveal that both Mn 2+ and Mn 3+ ions are present in the studied glasses, their relative concentration being dependent on the glass composition. Magnetic susceptibility data reveal an antiferromagnetic interaction between the manganese ions for the glasses containing 20 mol% MnO.

Influence of iron ions on the structural and magnetic properties of some zinc-phosphate glasses

Glasses in the (Fe 2O 3) x ·(P 2O 5) 40·(ZnO) 60− x (0 ≤ x ≤ 30 mol%) system have been prepared by the melt-quenching technique. The structural and magnetic properties of these glasses were investigated by means of X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, electron paramagnetic resonance (EPR) and magnetic susceptibility measurements. The XRD pattern for the prepared samples shows their vitreous state only for x ≤ 20 mol%. For the samples containing 30 mol% Fe 2O 3 the presence of a unique crystalline phase, FePO 4, embedded in an amorphous matrix was evidenced. FTIR spectroscopy data suggest that the iron ions play the network modifier role in the studied glasses. The addition of iron oxide leads to the breakage of P O bonds and the formation of the P–O–Fe bonds. The presence of the Fe–O–P bonds for the samples with higher iron oxide content in the studied system improves their chemical durability. The increases of iron ions content of the samples determine a gradual decrease in the number of bridging oxygen ions respectively the increase of the number of non-bridging oxygen ions. EPR and magnetic susceptibility measurements show the presence of the Fe 3+ ions in sites of distorted octahedral symmetry and as well as in clustered formations containing both Fe 3+ and Fe 2+ ionic species. Dipolar and superexchange interactions involving iron ions were revealed depending on the iron content of the sample.

Mössbauer, electron paramagnetic resonance, and magnetic susceptibility studies on members of a new family of cyano-bridged 3d-4f complexes. Demonstration of anisotropic exchange in a Fe-Gd complex.

The synthesis and crystallographic characterization of a new family of M(mu-CN)Ln complexes are reported. Two structural series have been prepared by reacting in water rare earth nitrates (Ln(III) = La, Pr, Nd, Sm, Eu, Gd, Dy, Ho) with K(3)[M(CN)(6)] (M(III) = Fe, Co) in the presence of hexamethylenetetramine (hmt). The first series consists of six isomorphous heterobinuclear complexes, [(CN)(5)M-CN-Ln(H(2)O)(8)].2hmt ([FeLa] 1, [FePr] 2, [FeNd] 3, [FeSm] 4, [FeEu] 5, [FeGd] 6), while the second series consists of four isostructural ionic complexes, [M(CN)(6)][Ln(H(2)O)(8)].hmt ([FeDy] 7, [FeHo] 8, [CoEu] 9, [CoGd] 10). The hexamethylenetetramine molecules contribute to the stabilization of the crystals by participating in an extended network of hydrogen bond interactions. In both series the aqua ligands are hydrogen bonded to the nitrogen atoms from both the terminal CN(-) groups and the hmt molecules. The [FeGd] complex has been analyzed with (57)Fe Mossbauer spectroscopy and magnetic susceptibility measurements. We have also analyzed the [FeLa] complex, in which the paramagnetic Gd(III) is replaced by diamagnetic La(III), with (57)Fe Mossbauer spectroscopy, electron paramagnetic resonance (EPR), and magnetic susceptibility measurements, to obtain information about the low-spin Fe(III) site that is not accessible in the presence of a paramagnetic ion at the complementary site. For the same reason, the [CoGd] complex, containing diamagnetic Co(III), was studied with EPR and magnetic susceptibility measurements, which confirmed the S = 7/2 spin of Gd(III). Prior knowledge about the paramagnetic sites in [FeGd] allows a detailed analysis of the exchange interactions between them. In particular, the question of whether the exchange interaction in [FeGd] is isotropic or anisotropic has been addressed. Standard variable-temperature magnetic susceptibility measurements provide only the value for a linear combination of J(x), J(y), and J(z) but contain no information about the values of the individual exchange parameters J(x), J(y), and J(z). In contrast, the spin-Hamiltonian analysis of the variable-field, variable-temperature Mossbauer spectra reveals an exquisite sensitivity on the anisotropic exchange parameters. Analysis of these dependencies in conjunction with adopting the g-values obtained for [FeLa], yielded the values J(x) = +0.11 cm(-1), J(y) = +0.33 cm(-1), and J(z) = +1.20 cm(-1) (S(1).J.S(2) convention). The consistency of these results with magnetic susceptibility data is analyzed. The exchange anisotropy is rooted in the spatial anisotropy of the low-spin Fe(III) ion. The condition for anisotropic exchange is the presence of low-lying orbital excited states at the ferric site that (i) effectively interact through spin-orbit coupling with the orbital ground state and (ii) have an exchange parameter with the Gd site with a value different from that for the ground state. Density functional theory (DFT) calculations, without spin-orbit coupling, reveal that the unpaired electron of the t(2g)(5) ground configuration of the Fe(III) ion occupies the xy orbital, that is, the orbital along the plane perpendicular to the Fe...Gd vector. The exchange-coupling constants for this orbital, j(xy), and for the other t(2g) orbitals, j(yz) and j(xz), have been determined using a theoretical model that relates them to the anisotropic exchange parameters and the g-values of Fe(III). The resulting values, j(yz) = -5.7 cm(-1), j(xz) = -4.9 cm(-1), and j(xy) = +0.3 cm(-1) are quite different. The origin of the difference is briefly discussed.

Effect of Gadolinium on the Structure and Magnetic Properties of Glass and Glass–Ceramic Sillenites

Glass samples belonging to xGd2O3·(1−x)[0.857Bi2O3·0.143GeO2] system, with 0.005≤x≤0.1, have been prepared by the rapid quenching method. Glass–ceramics were obtained after heat treatment of the as‐prepared glasses at 600°C for 24 h. All samples have been structurally characterized by X‐ray diffraction and electron paramagnetic resonance (EPR) spectroscopy. All as‐prepared samples are noncrystalline. The glass–ceramics contain different bismuthate crystalline phases, Bi12GeO20, BiLa type, and Bi24GeO38, depending on the gadolinium content in the glassy sample. The magnetic properties have been investigated by EPR and magnetic susceptibility measurements. The EPR and magnetic susceptibility data suggest that besides homogeneously distributed Gd3+ ions, both in the glass and glass–ceramic samples, there are clustered Gd3+ ions experiencing antiferromagnetic interactions.

EPR and magnetic susceptibility studies of iron ions in ZnO–Fe 2O 3–SiO 2–CaO–P 2O 5–Na 2O glasses

Room temperature electron paramagnetic resonance (EPR) spectra and temperature dependent magnetic susceptibility data have been obtained on bulk x(ZnO,Fe 2O 3)(65− x)SiO 220(CaO, P 2O 5)15Na 2O (6≤ x≤21 mole%) glasses prepared by melt quenching method. EPR spectra of the glasses revealed absorptions centered at g≈2.1 and 4.3. The variations of the intensity and line width of these absorption lines with composition have been interpreted in terms of the variation in the concentration of the Fe 2+ and Fe 3+ ions in the glass and the interaction between the iron ions. EPR and magnetic susceptibility data of the glasses reveal that both Fe 2+ and Fe 3+ ions are present in the glasses, with their relative concentration being dependent on the glass composition. The studies reveal superexchange type interactions in these glasses, which are strongly dependent on their iron content.

Multi-Frequency EPR and DC Conductivity of Itinerant Spins in Single-Wall Carbon Nanotubes

Sealed, deoxygenated single-wall carbon nanotubes show two characteristic electron paramagnetic resonance (EPR) signals at g = 2.07 and g = 2.00 in the temperature range from 300 to 50 K. Reversible interconversion between both components was observed. The large g-shift and the temperature dependence of the EPR susceptibility of the g = 2.07 signal indicate that this signal can be attributed to itinerant spins. At low temperatures only the g = 2.00 signal remained, which could be further characterized using microwave frequencies up to 320 GHz. The direct current conductivity of a partially aligned sample was also measured. The room temperature value was estimated as 0.7 (Ωcm)−1. The observed temperature dependence can be described by assuming temperature-activated hopping in a small-gap semiconductor with an activation energy of 3.5 meV, similar to the characteristics of the previously measured 9.4 GHz microwave conductivity.

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.

Inherent inhomogeneity in the crystals of low-doped lanthanum manganites

The X-band electron paramagnetic resonance (EPR) technique and the model analysis of the EPR susceptibility versus temperature were employed to characterize the dopant distribution in a number of La-manganite crystals, low doped with Ca, Sr, and Ba in a La site. Such distribution appears to be inhomogeneous as a result of technological-driven effect. It is emphasized that the above chemical disorder influences strongly both magnetic state and transport of the low-doped manganite crystals.

EPR AND MAGNETIC SUSCEPTIBILITY STUDIES OF Gd3+ IONS-DOPED BISMUTH–GERMANATE GLASS MATRIX

Glasses of the x Gd 2 O 3 · (100-x)[4 Bi 2 O 3 · GeO 2] system, with 1 ≤ x ≤ 30 mol% were obtained and investigated by electron paramagnetic resonance (EPR) and magnetic susceptibility measurements. The EPR spectra of the studied glasses reveal resonance sites with an unexpected high crystalline field in addition to the "U" spectrum, typical for Gd 3+ ions in glasses. This absorption line is due to Gd 3+ ions that replace Bi 3+ ions from the host glass matrix and can play former role of the unconventional network in the studied glasses. EPR and magnetic susceptibility data show that for low Gd 2 O 3 content, x ≤ 10 mol%, the Gd 3+ ions are randomly distributed in the host glass matrix and are present only as isolated species. For higher Gd 2 O 3 content, x > 10 mol%, the Gd 3+ ions appear as both isolated and antiferromagnetically coupled species.

Influence of iron ions on the magnetic properties of CaO–SiO 2–P 2O 5–Na 2O–Fe 2O 3 glass–ceramics

Room temperature Electron Paramagnetic Resonance (EPR) spectra and temperature dependent magnetic susceptibility measurements have been performed on 41CaO.(52− x)SiO 2.4P 2O 5. xFe 2O 3.3Na 2O ( 2 ≤ x ≤ 10 mol % ) glass–ceramics. The parent glass samples were prepared by quenching the homogenized melt from 1550 ∘C. The glasses were then heat-treated at 1050 ∘C for 3 h and slowly cooled to room temperature to yield glass–ceramics. EPR and high temperature magnetic susceptibility measurements were made to elucidate the state of Fe +3 ions in the glass–ceramics. The EPR absorption line centered at g ≈ 4.3 disappeared at higher concentrations of Fe 2O 3. The intensity and linewidth of the absorption line centered at g ≈ 2.1 increased as the Fe 2O 3 concentration was increased. Information about the structural changes involving iron ions, their valence state and type of magnetic interactions between the Fe ions was obtained using EPR and magnetic susceptibility measurements. The temperature dependence of the magnetization of the glass–ceramics with low Fe 2O 3 concentrations exhibit both ferrimagnetic and paramagnetic contributions. The results obtained have been used to understand the structural changes occurring in the vicinity of iron ions and the nature of magnetic interactions between the iron ions in these glass–ceramics.

Electron paramagnetic resonance investigation of metalloendofullerene derived carbon nanotube peapods

Single Wall Carbon Nanotubes (SWCNT) prepared by the "super growth" method and arc-grown material were used as templates for peapod preparation with La@C(82). A qualitative change of the electron paramagnetic resonance (EPR) properties of La@C(82) is observed after incorporation into SWNT. The loss of lanthanum hyperfine interaction in combination with the observed increase of EPR susceptibility by two orders of magnitude after peapod preparation when comparing with signals from "empty" tubes is indicative for the generation of itinerant spins by charge and spin transfer from La@C(82) to the tubes. This interpretation is supported by the observation of fast spin dephasing, detected with pulsed EPR techniques.

EPR and Magnetic Susceptibility of Na2O–CuO–P2O5 Glasses

(50−x/2)Na2O–xCuO–(50−x/2)P2O5 glasses (x=1, 5, 15, or 30 mol%) have been prepared and characterized by electron paramagnetic resonance (EPR) and magnetic susceptibility measurements. The shape of the Cu2+ EPR spectrum depends on the Cu content, and the corresponding computer simulations suggest that the Cu2+ ions occupy two different sites in these glasses: one of them is preponderant at low Cu content and the other is preponderant at high content, in which the Cu2+–Cu2+ interactions are more important. From EPR parameters, it was found that for the site at low content, the covalency of copper ion bonding with the surrounding ligands is appreciable. The magnetic susceptibility data appear to follow the Curie–Weiss law (χ=C/(T−θp)) with negative paramagnetic Curie temperature θp indicating antiferromagnetic interactions between Cu2+ ions that are more significant in the samples with high Cu content, in agreement with EPR results.

EPR and magnetic susceptibility studies of B 2O 3–Bi 2O 3–Gd 2O 3 glasses

Glasses of the xGd 2O 3 · (100 − x)[B 2O 3 · Bi 2O 3] system with 0.5 ⩽ x ⩽ 10 mol% were studied by electron paramagnetic resonance (EPR) and magnetic susceptibility measurements. Data obtained show that for low gadolinium oxide contents of the samples ( x ⩽ 3 mol%) the Gd 3+ ions are randomly distributed in the host glass matrix and are present as isolated and dipole–dipole coupled species. For higher gadolinium oxide contents of the samples ( x > 3 mol%) the Gd 3+ ions appear as both isolated and antiferromagnetically coupled species. The EPR spectra of the glasses reveal resonance sites with an unexpected high crystalline field in addition to the ‘U’ spectrum, typical for Gd 3+ ions in disordered systems. This absorption line is due to Gd 3+ ions that replace Bi 3+ ions from the host glass matrix and could play the network unconventional former role in the studied glasses.

Magnetic and spectroscopic properties of gadolinium tripodal Schiff base complex

Gadolinium(III) tripodal Schiff base (tris(((5-chlorosalicylidene)amino)ethyl)amine) complex has been obtained and investigated by infrared spectroscopy (IR), magnetic susceptibility, and electron paramagnetic resonance (EPR) methods. Comparison of IR bands in ligand and gadolinium complex confirmed the formation of the gadolinium complex and allowed to propose its structure. Both electron ionization and electron spray molecular spectroscopy spectra confirmed the [1:1] proportion of a ligand to metal in gadolinium tripodal Schiff base complex sample. IR spectroscopy and TG–DTA excluded the presence of water molecule in the metal coordination sphere. X-ray powder analysis applying Fullprof computer program has shown that the investigated sample was monophase with the monoclinic symmetry of the unit cell having the lattice constants: a = 10.028(4) Å, b = 13.282(5) Å, c = 21.20(1) Å and β = 101.58(4)°. Space group P21/ c, Z = 4. EPR spectra of the complex have been registered in the 4–300 K temperature range. Each spectrum has been fitted using EPR–NMR computer program and the values of the spin-Hamiltonian parameters at each temperature have been calculated. Temperature dependence of the integrated intensity of the EPR spectrum allowed revealing the magnetic interactions in the spin system of this compound. Comparison of the temperature dependence of dc magnetic susceptibility ( χ) and EPR susceptibility ( χ EPR) showed significant differences between these quantities due to the presence of short-lived clusters with a non-magnetic ground state.

Syntheses, Crystal Structures, and Spectroscopic and Magnetic Properties of [Mn2III(H2L1)(Cl4Cat)4·2H2O]∞ and [Mn2III(H2L2)(Cl4Cat)4·2CH3CN·2H2O]∞: Temperature-Dependent Valence Tautomerism in Solution

The synthesis, X-ray data, and electronic structures of two manganese(III) 1D polymers ligated by tetrachlorocatechol, [Mn(2)(III)(H(2)L(1))(Cl(4)Cat)(4).2H(2)O](infinity) (1) and [Mn(2)(III)(H(2)L(2))(Cl(4)Cat)(4).2CH(3)CN.2H(2)O](infinity) (2), are reported. The electronic structures of the complexes have been determined by UV-vis-near-IR, IR, electron paramagnetic resonance (EPR), and magnetic susceptibility measurements. Both 1 and 2 are air stable in the solid state and in solution, unlike most of the previously reported o-quinone-chelated transition-metal complexes. Electronic spectroscopy exhibits a strong near-IR band near 1900 nm for both, suggesting the presence of a mixed-valence semiquinone-catecholate oxidation state of the catechol ligands, Mn(2)(III)(Cl(4)Cat)(2)(Cl(4)SQ)(2), together with the pure catecholate forms. The presence of this isomer was further supported by EPR and magnetic susceptibility measurements. The complexes undergo intramolecular electron transfer (valence tautomerism) upon an increase of the temperature involving the equilibrium Mn(2)(III)(Cl(4)Cat)(2)(Cl(4)SQ)(2) <==> Mn(2)(II)(Cl(4)SQ)(4). This phenomenon is reversible and is studied in solution using UV-vis-near-IR spectroscopy.

Electron spin resonance studies in the doped polyaniline PANI-AMPSA: Evidence for local ordering from linewidth features

X-band electron spin resonance (ESR) of polyaniline (PANI) doped with 2-acrylamido-2-methyl-1-propane sulfonic acid (AMPSA), at conductivity values of 90, 4, and $0.2\phantom{\rule{0.3em}{0ex}}\mathrm{S}∕\mathrm{cm}$, has been investigated from $300\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Free-standing films ($30--40\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ thick) of PANI were prepared by solution casting method. The spectra consist of two signals, one is broad $(\ensuremath{\sim}60\phantom{\rule{0.3em}{0ex}}\mathrm{G})$ and the other is narrow $(\ensuremath{\sim}10\phantom{\rule{0.3em}{0ex}}\mathrm{G})$, at nearly the same resonance field. The temperature variation of ESR susceptibility indicates a stronger Pauli component in the broad line signal, as compared to the narrow signal, at all values of conductivity. Both linewidths are independent of the presence of ${\mathrm{O}}_{2}$. The temperature variation of linewidths is explained in terms of the quasi-two-dimensional (Q2D) model. These results suggest that in some regions the doped PANI chains have self-assembled Q2D structure, and the presence of a significant Pauli susceptibility indicates that the charge carriers are highly delocalized in these Q2D regions.

Magnetic study of Cr 3+ ion in M2CrV 3O 11−x ( M[dbnd6]Zn, Mg) compounds

Electron paramagnetic resonance (EPR) and magnetic susceptibility measurements on the recently synthesized vanadates M 2CrV 3O 11− x ( M=Zn, Mg) have been analyzed. Two absorption lines with g≈2.0 (type I) and g ≈ 1.98 (type II) were recorded in the EPR spectra, which can be attributed to V 4+ ions and Cr 3+ ion clusters (pairs), respectively. The exchange constant J between Cr 3+ ions has been calculated, using both EPR and magnetic susceptibility data. Fitting of the EPR and magnetic susceptibility data has been carried out. The sign of J is a negative one for all samples and indicates antiferromagnetic interactions. Some difference in the J constant value among samples has been obtained. Volumetric titration confirms distinctly the presence of vanadium V 4+ ions in the investigated compounds.