Localized Spin Centers Research Articles

Microwave Energy Drives "On-Off-On" Spin-Switch Behavior in Nitrogen-Doped Graphene.

The established application of graphene in organic/inorganic spin-valve spintronic assemblies is as a spin-transport channel for spin-polarized electrons injected from ferromagnetic substrates. To generate and control spin injection without such substrates, the graphene backbone must be imprinted with spin-polarized states and itinerant-like spins. Computations suggest that such states should emerge in graphene derivatives incorporating pyridinic nitrogen. The synthesis and electronic properties of nitrogen-doped graphene (N content: 9.8%), featuring both localized spin centers and spin-containing sites with itinerant electron properties, are reported. This material exhibits spin-switch behavior (on-off-on) controlled by microwave irradiation at X-band frequency. This phenomenon may enable the creation of novel types of switches, filters, and spintronic devices using sp2 -only 2D systems.

Diluted paramagnetic impurities in nonmagnetic Ba2YIrO6

The cubic double perovskite Ba$_2$YIrO$_6$ has been investigated by the local probe techniques nuclear magnetic resonance ($^{89}$Y NMR) and muon spin rotation (muSR). Both methods confirm the absence of magnetic long-range order in this compound but find evidence for diluted localized paramagnetic moments. NMR spin-lattice relaxation rate 1/T1 measurements suggest a slowing down of localized spin moments at low temperatures. An increase of the muSR spin-lattice relaxation rate lambda confirms the presence of weak magnetism in Ba$_2$YIrO$_6$ . However, these findings cannot be explained by the recently suggested excitonic type of magnetism. Instead, they point towards tiny amounts of localized paramagnetic spin centers leading to this magnetic response on the background of a simple nonmagnetic ground state of the 5d$^4$ ($J=0)$ electronic configuration of Ir$^{5+}$.

Characterization and Quantification of Polyradical Character.

The decomposition of ⟨Ŝ(2)⟩ into atomic and diatomic contributions (local spin analysis) is used to detect and quantify the polyradical character of molecular systems. A model triradical system is studied in detail, and the local spin analysis is used to distinguish several patterns of local spin distributions and spin-spin interactions that can be found for different electronic states. How close a real molecular system is to an ideal system of k perfectly localized spin centers is utilized to define a measure of its k-radical character. The spin properties and triradical character of the lowest-lying electronic states of a number of all σ, all π, and σ-π organic triradicals are discussed in detail. The local spin contributions exhibit good correlation with experimental triradical stabilization energies.

Diradical character from the local spin analysis

Diradical species are analyzed in light of the local spin analysis. The atomic and diatomic contributions to the overall 〈Ŝ(2)〉 value are used to detect the diradical character of a number of molecular species mostly in their singlet state, for which no spin density exists. A general procedure for the quantification of diradical character for both singlet and triplet states is achieved by using a recently introduced index that measures the deviation of an actual molecule from an ideal system of perfectly localized spin centers. The index is of general applicability and can be easily determined in equal footing from a multireference or an open-shell single-determinant wave function.

Influence of Solute−Solvent Hydrogen Bonding on Intramolecular Magnetic Exchange Interaction in Aminoxyl Diradicals: A QM/MM Broken-Symmetry DFT Study

We have investigated the effect of nitroxide radical-water hydrogen bonding (NO(*)...H(2)O) on the intramolecular magnetic exchange interaction (J) for biologically relevant aminoxyl diradicals. We adopt a combination of broken-symmetry density functional theory and the quantum mechanics/molecular mechanics (QM/MM) approach. We find that the presence of hydrogen bonding reorients the radical spin density on -NO(*). This phenomenon reduces the effective distance between the two interacting localized spin centers that eventually increases the intramolecular magnetic exchange interaction. We have also investigated the functional variation of the magnetic exchange interaction, using various GGA (BLYP, PBE, HCTH407), meta-GGA (TPSS, VXSC), and hybrid (O3LYP, B3LYP, B3P86, B3PW91, and PBE0) functionals.

Measurement of electrical properties of a carbon black

The direct measurement of electrical properties of a high-surface-area turbostratic carbon black by microwave techniques is reported. This approach obviates many difficulties associated with powder samples and allows absolute values of conductivity and mobility to be obtained, from which charge carrier concentrations can be calculated and compared to those obtained by ESR methods. After a treatment in Ar at 1223 K to remove sulfur, this carbon had conductivity σ = 2.4 (Ω · cm) −1, mobility μ = 5.4 cm 2/V · s, and a carrier concentration of 1.5 × 10 18 g −1 at 300 K. The adsorption of O 2 at 300 K increased σ, decreased μ, and increased carrier concentration to a small extent. After oxidizing this carbon in HNO 3, all three parameters were decreased slightly and adsorbed O 2 had little effect. However, the concentration of charge carriers in both carbon blacks was 10–40 times lower than the conduction carrier concentrations determined from a typical analysis of ESR signal intensity obtained at different temperatures. These preliminary results indicate that holes are the primary charge carriers in these carbon materials, which have small crystallite size and a small extent of graphitization. Oxygen chemisorbed on the pre-cleaned carbon black surface increases the concentration of both localized spin centers and holes and creates a higher electrical conductivity. Intensive oxidation by HNO 3 increases the number of localized spin centers but decreases the concentration of holes, thus also decreasing conductivity. A comparison of these microwave and ESR results implies that the traditional way of identifying and quantifying conduction charge carriers from the temperature-dependent susceptibility of ESR signals may not be satisfactory.

EPR Characterization of Carbon Coated Fiber

AbstractAn amorphous carbon coating has been developed to improve the resistance of silica optical fibers to static fatigue and hydrogen permeation. The carbon coated optical fibers are characterized by electron paramagnetic resonance spectroscopy. A Lorentzian lineshape is observed, centered at the g value of 2.002. Intensity measurements as a function of temperature suggest that localized spin centers contribute to the spin resonance. It is shown that atmospheric control of the carbonaceous environment results in the removal of these EPR active sites.

ESR of pyro-graphite irradiated by ions—I. Low energy ion bombardments

Paramagnetic centers in pyrolytic graphite irradiated with low energy ions were studied by ESR. Above nitrogen temperature, only one sharp line was observed in spite of inhomogeneous distribution of defects caused by the irradiation. The g-value and width of the line from the irradiated part decreased monotonously with an increase in the fluence. By annealing, no change was observed in the g-value and little change in width. With decreasing measuring temperature, the g-value remained unchanged, but the width showed a trend to decrease at low temperatures. The temperature dependence of the intensity of the line followed nearly the Curie law, especially for low fluences. At temperatures below 15 K, the line split into two for low fluences. Localized spin centers were assigned to these lines. The correlation between localized spin centers and the pockets of disorder produced by the irradiation was discussed. The extent of the wave function of spin centers was presumed to be influenced by the disorder of the surroundings. The distribution rather than the concentration of defects was found to play an important role in the localization of spins.

ESR of pyrolytic graphite irradiated by neutrons at low temperature

Pyrolytic graphite irradiated by neutrons at a low temperature was studied by esr. The presence of localized spin centers was confirmed from the esr intensity ratio between room temperature and liquid nitrogen temperature. The concentration ratios of localized spin centers to conduction carriers were obtained from g-values for various irradiation doses. The dose dependence of g-values and widths was explained by the rapid exchange interaction between the two spin systems, i.e. localized spin centers and conduction carriers. An increase of the width was observed around 160 K by pulse annealing experiments. The mechanism of the broadening was discussed and the spin-flip cross section was estimated in order of magnitude. The annealing behavior of the line-width above 300 K was explained as due to the annihilation of localized spin centers.

ESR of Pyro-Graphite Irradiated by High Energy Ions

In pyro-graphite bombarded with high energy chlorine ions, two kinds of paramagnetic centers are observed by ESR. These centers are interpreted as conduction carriers in a sample containing a large amount of acceptors produced by the bombardment and localized spin centers in an amorphous state. It is proposed that the transition from conduction carriers to localized spin centers will be studied in detail by high energy ton bombardments.

ESR of carbons in the transition range—II

Whereas in Part I[1] the attention was focused on ESR broadening effects caused in the transition range (900–1600°C) by impurities and/or by structural transformations, in Part II an effort is made to obtain information as to the absolute intensity of the ESR line and its composition. The contributions of surface localized, internal localized and conduction carrier spins are separated by use of air sensitivity effects and temperature dependence data, before and after reacting the carbons with spin-activating gases. Many types of carbon blacks and some chars and cokes were so investigated. It was found that new localized spin centers ( A-centers) are created by such reactions only as long as a carbon contains some other organic components attached to the surface. After all organics have departed, no A-centers are formed, but gas reactions continue to affect the width of the ESR line throughout the rest of the transition range and above. The general picture of relationships existing in the transition range and the problems of interaction between the various types of spin centers and the carbon lattice are discussed.

Electrical conductivity and electron-spin resonance in oxidatively stabilized polyacrylonitrile subjected to elevated temperature

The development of electrical conductivity in oxidized polyacrylonitrile (PAN) fiber as a function of heat-treatment temperature has been studied for heat-treatment temperatures in the range 710–950 K. By means of a study of conductivity versus measurement temperature, it is shown that PAN heat treated in vacuum in this temperature range is a semiconductor. The room-temperature conductivity is dominated by the contribution of impurities. The activation energy for the formation of the impurities that contribute to the conductivity is 88 kcal/mole. Electron-spin resonance (ESR) measurements have also been made on these fibers. Studies of the variation of the ESR absorption intensity with temperature reveal that the ESR signal can be analyzed in terms of two components: localized spin centers and charge carriers. The charge-carrier component of the ESR signal is interpreted as being due to the formation of metallic conduction pathways in the polymer. Both the electrical conductivity and the ESR absorption intensity decrease when the samples are aged in air.

Reactions of evolved gases with chars and cokes: A study of electron spin resonance

A simple technique has been developed to observe the changes in the ESR absorption of chars and cokes resulting from chemical attack of various gases at pressures below atmospheric and temperatures below 1100°C. A gas evolved in the process of heattreatment of a polyvinylidene chloride polymer-300°C char (the gas being composed mostly of HCl and Cl 2) has been found to be at pressures of a few millimeters of Hg and temperatures exceeding 700°, a most effective agent for formation of artificial localized spin centers ( A-centers) on the accessible surface of chars. Some of such A-centers are highly resistant to reheat in vacuum and to oxidation in air. Before the new spin centers are formed, however, in some cases a partial destruction of the originally present spin centers (centers formed in carbonization process) by the PVDC-gas occurs at lower temperature (~600°C). Sugar-char gas, phenolbenzaldehyde resin char-gas and pure hydrogen have been found also to create A-centers on chars, the spin concentration being controlled at a given temperature by a dynamic equilibrium (the final concentration increasing with pressure and decreasing with temperature). Neither C 3O 2 polymer-gas, nor water vapor, nor their mixture produce A-centers under the same pressure-temperature conditions. Presence of nitrogen was found to have a deleterious effect on some of the spin formation reactions. Observations concerning the dependence of the width of the ESR line on various factors are also reported.

La résonance paramagnétique électronique dans les carbones partiellement graphites: Exemple d'étude sur une série de pyrocarbones irradies

The EPR of graphite and well organised carbons is reviewed. The effects of motional averaging, anisotropy, and skin-effect on the EPR of bulk and powdered samples are discussed with the heat-treatment and neutron irradiation dependences of the g-factor and the linewidth. As an illustration, the paramagnetic susceptibility, the g-factor and linewidth anisotropies of a series of neutron irradiated graphites measured in the temperature range 42-300 K are given. They are shown to be consistent with previous studies, the existence of a Curie like component of the paramagnetism is evidenced. A model involving the interaction between localized spins and free charge carriers, reminiscent of these used for interpreting the EPR of diluted alloys, is described. Assuming the adiabatic approximation to hold, it is shown to account for the only single line always observed and for the empirical relation proposed far ago for the g-factor of carbons. It predicts a similar relation for the line width. Assuming the g-factor and linewidth anisotropies of free carriers to have the same behavior as in graphite single crystals, we show that these relations fit fairly well with experimental data. Incidentally such a model allows us to explain the decrease of the g-factor anisotropy of the non irradiated PGCCL at low temperature by assuming the existence of a small number of localized spin centers at defects. The values of the parameters provided by the fitting are discussed. Their behavior under irradiation is found to be similar to the ones evidenced by previous studies on carbon blacks and neutron irradiated carbon blacks. The “degraphitization” effect of neutron irradiation is seen to be of limited extend. From the three sets of results, we can infer that a linear relationship between the g-factor Δ g 0 and linewidth Δ S 0 anisotropies at zero temperature holds wlthm experimental and fitting errors. Finally, although the degeneracy temperatures T′ 0 and T″ 0 are empirical parameters in those non crystalline carbons, they must be related to the degeneracy temperature T 0 of the free electron gas model for they have the same behavior as the structural disorder increases.

Low temperature specific heat anomalies and spin-spin interactions in soft carbons and in neutron irradiated polycrystalline graphite

In continuation of previous studies of the specific heat of carbons and graphite in the He 4 and He 3 temperature range (0.4–4.5°K) work was extended down to 0.07°K using a dilution refrigerator at the NASA Lewis Center in Cleveland. The presence of a second specific heat peak at around 0.3°K was ascertained and its relation to the first one at around 0.65°K was studied in dependence on heat treatment, neutron irradiation and post irradiation anneal. Both peaks, as well as the linear component, seem to be due to the presence of localized spin centers, interacting in an antiferromagnetic manner. A general explanation of the relationships observed is suggested.

From char to carbon: A study of A-centers

In order to obtain more information about the interaction between localized and conduction carrier spin centers in chars and carbons, artificial spin centers (A-centers) were made on sugar and PVDC chars as well as on P-33 carbon black using the oxidation technique developed by Antonowicz. The temperature dependence of ESR has been studied for materials heattreated in the HTT range 700–1800°C. Whereas for heattreatments 700–900°C the formation of A-centers results in a more Curie-like temperature behavior, for heattreatments of 1100°C and above the behavior does not depend on the concentration of A-centers. An explanation of this surprising result is suggested.

From char to carbon: A study of the electron spin resonance

The ESR intensity and its temperature dependence were investigated for sugar and polivinylidene chloride (PVDC) chars, as well as for Texas. Kendall and Resin C raw cokes for heattreatments up to 1000°C. No evidence of any deviations from the Curie law could be found in the temperature range from 78 to 290°K for heattreatments below 800°C. At higher heattreatments after elimination of most organic components (except hydrogen) an ESR temperature independent part comes into being which seems to be due to conduction carriers. It is shown on the example of PVDC, that the primary polymerization-carbonization reactions occurring in the charred materials lead to a single maximum in ESR intensity (and corresponding single minimum in width) and that the secondary maximum observed in the past at higher heattreatments is caused by gases (evolved from the material or introduced from outside) reacting with the material of the char and forming so called A-centers. Problems involved in the interaction of various types of localized spin centers with hopping and conduction type carriers are discussed.

Electron Spin Resonance of Carbon Films Prepared from Graphite Oxide (II) A graphite oxide film and the heat-treated reduced graphite oxide films

ESR study was performed on graphite oxide films before and after reduction. The concentration of localized spin centers in the graphite oxide film drastically increased by more than twenty times as a result of reduction, while the concentration of delocalized spin centers remained unchanged. The reduction of the film led to the elimination of oxygen atoms from peripheral and/or inner part of the film and, probably, to the formation of conjugated double bonds between carbon atoms in the carbon network plane, increasing the concentration of localized spin centers. The reduced films did not show any ESR signal, however, when heattreated at temperatures between 600°C and 2400°C. The gvalue anisotropy, which was observed for the reduced film treated at 2800°C, was not found in any samples heattreated below 600°C. The gvalue anisotropy seems to be correlated with the appearance of three dimensional (112) diffraction line.

Electron spin resonance in turbocrystalline carbons—II

In continuation of the work on ESR in turbostratic carbons, experiments on introduction of boron acceptors and sodium donors were performed using P33 carbon black samples heat treated to various temperatures mainly in the range 1600–2400°C. The experiments with boronation show a complicated behavior and lead to the conclusion that boron atoms can act not only as acceptors, but neutralize some of the existing localized spin centers and also can under special conditions become spin centers themselves. Experiments with sodium doping have shown that the stability of the sodium against oxidation increases with decrease in HTT. With increasing doping the “Curie-like” behavior (the factor R = I LN I RT ) goes through a maximum and the intensity of the ESR line through a minimum, as expected from the simple band model with the Fermi level originally located below, and moving across and then above the band overlap with increase in doping. Using two spectrometers 9.2 Gc and 35 Gc and a variety of samples it was found that while for low heat treatments the widths are the same (in Gauss), for well graphitized materials the use of the higher frequency results in a considerably broader line. Considerations concerning the temperature and heat treatment dependence of the Δg cond and its relation to the diamagnetic susceptibility are presented. The nature of the band structure in turbostratic carbons and of the localized spin centers are discussed.

Effect of Pressure on Graphitization of Carbon. V. Electron Spin Resonance in Hard Carbon Heat-Treated under High Pressure

Abstract By means of the electron spin resonance (ESR), the graphitization process of a hard carbon (phenolformaldehyde resin char) under a pressure of 5 kbar was investigated in relation to the results of the X-ray diffraction analysis. In the ESR measurement, there is observed only a sharp Lorentzian line having g-value of about 2.003 for the products heat-treated at relatively low temperatures. On the other hand, gradual increase in the heat treatment temperature (HTT) leads to the appearance of new 15–20 G wide line having g-value of about 2.01, overlapping on the sharp one, and its absorption intensity tends to increase further with HTT. The appearance of the broad ESR line has been found always being accompanied by an abrupt increase of the content of graphitic component deduced from the analysis of (002) diffraction profile. From further investigation of features of these ESR lines as a function of ambient temperature, the sharp one is concluded that it originates from the so-called localized spin centers, while the broad one from the delocalized. The coexistence of these two signals suggests that the turbostratic and graphitic parts have enough extent to prevent the motional mixing of these spin centers, which has also been verified by the X-ray diffraction analysis.