Temperature-dependent Electron Spin Resonance Spectra Research Articles

Magnetic decoupling in the triangular-lattice antiferromagnet Cu2(OH)3CHO2 studied by high-field magnetization and ESR

The high-field magnetization and electron spin resonance (ESR) are reported for the two-dimensional triangular-lattice antiferromagnet Cu2(OH)3HCO2. The compound has an antiferromagnetic (AFM) order at TN = 4.9 K with the Curie-Weiss temperature θCW = − 3.8 K. The high-field magnetization up to 50 T exhibits a metamagnetic transition at 1.7 T followed by a half-saturated ferromagnetic (FM)-like magnetization process, showing the magnetic decoupling between the Cu1 FM chain and the Cu2 AFM chain. The unsaturated magnetization at 50 T serves as an evidence of much larger AFM exchange in the Cu2 chain. The temperature-dependent ESR spectra are in line with the development of magnetic correlations and AFM ordering. The observed ESR modes below TN originate from the AFM resonances of the antiferromagnetically coupled Cu1 FM chains. The zero-field gap of 111 GHz and the zero-frequency field of 2.4 T suggest the presence of significant magnetic anisotropy.

Two-sublattice description of the dimer-trimer chain compound Li2Cu5Si4O14 : High-field magnetization and ESR studies

We carried out high magnetic field magnetization and electron spin resonance (ESR) measurements for the alternating dimer-trimer chain compound ${\mathrm{Li}}_{2}{\mathrm{Cu}}_{5}{\mathrm{Si}}_{4}{\mathrm{O}}_{14}$. The magnetization curve measured at 2 K exhibits a spin-flop transition at 2.6 T followed by a nonlinear increase in magnetization at high field up to 55 T. The antiferromagnetic ordering at ${T}_{\mathrm{N}}=22\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ is well characterized by the temperature-dependent ESR spectra, and the anisotropic $g$ factors at the paramagnetic phase are determined. Intriguingly, our frequency-dependent ESR spectra at 2 K reveal the development of two collinear antiferromagnetic sublattices with biaxial anisotropy, which is quite unusual because ${\mathrm{Li}}_{2}{\mathrm{Cu}}_{5}{\mathrm{Si}}_{4}{\mathrm{O}}_{14}$ contains five sublattices. Thus, ${\mathrm{Li}}_{2}{\mathrm{Cu}}_{5}{\mathrm{Si}}_{4}{\mathrm{O}}_{14}$ is three-dimensionally ordered and its magnetism might be described by a pseudo-two-sublattice model composed of antiferromagnetically coupled dimer-trimer groups (\ensuremath{\uparrow}\ensuremath{\uparrow}, \ensuremath{\uparrow}\ensuremath{\downarrow}\ensuremath{\uparrow}) and (\ensuremath{\downarrow}\ensuremath{\downarrow}, \ensuremath{\downarrow}\ensuremath{\uparrow}\ensuremath{\downarrow}).

High Magnetic Field ESR in S = 1 Skew Chain Antiferromagnet Ni2V2O7 Single Crystal

We report electron spin resonance (ESR) in S = 1 skew chain antiferromagnet Ni2V2O7, which exhibits a spin-flop transition and a well-defined 1/2 magnetization plateau. The antiferromagnetic (AFM) ordering at TN = 7 K can be reflected by the temperature-dependent ESR spectra at low frequency for the easy axis. At 2 K, at the spin-flop transition fields along the easy a and b axes, anomalies are observed from the frequency‒field relationship. However, these modes cannot be understood by the conventional two-sublattice AFM resonance theory with uniaxial anisotropy. For the easy b axis, an unusual resonance mode is observed and its resonance field increases with decreasing frequency. This ESR mode becomes softening at ~8 T, corresponding to the onset of the 1/2 magnetization plateau.

Magnetization, ESR and large magnetocaloric effect in zigzag chain SrGd2O4

We performed the magnetization and electron spin resonance (ESR) measurements in the zigzag chain antiferromagnet SrGd2O4. First, the temperature-dependent ESR spectra suggest the presence of spin–spin correlations well above the ordering temperature due to geometric frustration. Second, the ESR data reveal the field-induced magnetic transition from an ordered state to a paramagnetic state, in accordance with the magnetization data. Third, the magnetization process exhibits a large magnetocaloric effect, which is associated with the alignment of paramagnetic spins along the direction of magnetic field. The maximum magnetic entropy change (−ΔSM) near 3 K and refrigerant capacity (RCP) are 27 J · Kg−1 · K−1 and 325 J · Kg−1, respectively, for a field change of 0–7 T. With negligible thermal and field hysteresis, SrGd2O4 can be considered as a cryogenic refrigerant material.

Voltage-Tuned Transport Properties and Ferromagnetic Resonance in Lanthanum-Strontium-Manganite/Lead-Magnesium–Niobate-Lead-Titanate Multiferroic Heterostructures

We demonstrate piezostrain-induced modulation of transport and ferromagnetic resonance (FMR) properties in the La 0.5Sr0.5MnO3 (LSMO5)/0.7Pb(Mg1/3Nb2/3)O3 -0.3PbTiO3 (PMN-PT) (0 0 1) multiferroic heterostructures. The poling strain leads to the enhancement of the double exchange interaction and the reduction of Jahn–Teller electron-lattice distortion in the LSMO5 film, showing a maximum resistance decrease of 27% at 105 K, around which temperature the maximum magnetoresistance change of 10% is tuned by the piezostrain. The FMR response to the strain changes is probed by temperature-dependent electron spin resonance spectra, showing the largest FMR field shift of 220 Oe at the Neel temperature of 193 K. The magnetoelectric coupling behavior at the antiferromagnetic phase of LSMO5 film was dominated by the two-magnon scattering effect, which provides a new route for voltage-controllable magnetism in future spintronic devices.

Magnetic Defects in Transitional Metal Di-Chalcogenide Semiconducting Layers

ABSTRACTIn this work, we report on the electron spin resonance (ESR) studies performed on few-layered nanocrystalline (NCs) MoS2, WS2, and TiS2 prepared using hydrothermal and vapor transport methods. From the temperature dependent ESR spectra collected from MoS2 NCs, we have identified adsorbed oxygen species, sulphur vacancies, thio- and oxo-Mo5+ related paramagnetic defect centers. WS2 NCs have exhibited W+3 and oxo-W+5 paramagnetic defect spin species. TiS2 NCs showed defects such as Fe3+ (unwanted), oxygen and sulfur vacancies. This work demonstrates the usage of spin-sensitive spectroscopy such as ESR in unravelling the defects which contain unpaired electron spin centers in layered NCs two-dimensional materials.

Intrinsic and extrinsic defects in a family of coal-derived graphene quantum dots

In this letter, we report on the high frequency (239.2 and 336 GHz) electron spin resonance (ESR) studies performed on graphene quantum dots (GQDs), prepared through a wet chemistry route from three types of coal: (a) bituminous, (b) anthracite, and (c) coke; and from non-coal derived GQDs. The microwave frequency-, power-, and temperature-dependent ESR spectra coupled with computer-aided simulations reveal four distinct magnetic defect centers. In bituminous- and anthracite-derived GQDs, we have identified two of them as intrinsic carbon-centered magnetic defect centers (a broad signal of peak to peak width = 697 (10−4 T), g = 2.0023; and a narrow signal of peak to peak width = 60 (10−4 T), g = 2.003). The third defect center is Mn2+ (6S5/2, 3d5) (signal width = 61 (10−4 T), g = 2.0023, Aiso = 93(10−4 T)), and the fourth defect is identified as Cu2+ (2D5/2, 3d9) (g⊥ = 2.048 and g‖ = 2.279), previously undetected. Coke-derived and non-coal derived GQDs show Mn2+ and two-carbon related signals, and no Cu2+ signal. The extrinsic impurities most likely originate from the starting coal. Furthermore, Raman, photoluminescence, and ESR measurements detected no noticeable changes in the properties of the bituminous GQDs after one year. This study highlights the importance of employing high frequency ESR spectroscopy in identifying the (magnetic) defects, which are roadblocks for spin relaxation times of graphene-based materials. These defects would not have been possible to probe by other spin transport measurements.

Antiferromagnetic ordering in spin-chain multiferroic Gd2BaNiO5 studied by electronic spin resonance

High-field electron spin resonance (ESR) has been employed to study the antiferromagnetic (AFM) ordering state (T < TN = 55 K) of spin-chain multiferroic Gd2BaNiO5. The spin reorientation at TSR = 24 K is well characterized by the temperature-dependent ESR spectra. The magnetization data evidence a field-induced spin-flop transition at 2 K. The frequency-field relationship of the ESR data can be explained by conventional AFM resonance theory with uniaxial anisotropy, in good agreement with magnetization data. Related discussion on zero-field spin gap is presented.

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

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

Formation of p-Phenylenediamine−Crown Ether−[PMo12O40]4- Salts

Electron transfer from the electron donor of p-phenylenediamine (PPD) to the electron acceptor of (H+)3[PMo12O40]3- forms a one-electron-reduced Keggin cluster of [PMo12O40]4-, bearing a S = 1/2 spin, while proton transfer from the proton donor of (H+)3[PMo12O40]3- to the proton acceptor of PPD yielded mono- and diprotonated cations of 4-aminoanilinium (HPPD+) and p-phenylenediammonium (H2PPD2+). By introduction of crown ether receptors during the crystallization process, supramolecular cations of (HPPD+)(crown ethers) and/or (H2PPD2+)(crown ethers) were successfully introduced into three new alpha-[PMo12O40]4- salts of (H2PPD2+)2([12]crown-4)4[PMo12O40]4- (1), (HPPD+)4([15]crown-5)4[PMo12O40]4- (2), and (HPPD+)2(H2PPD2+)([18]crown-6)4[PMo12O40]4- (3) as the countercation. The protonated states of PPD and molecular-assembly structures of the supramolecular cations depended on the size of the crown ethers. In salt 3, a novel mixed-protonated state of HPPD+ and H2PPD2+ was confirmed to be complexed in the cation structure. According to the changes in the cation structures, the anion arrangements were modulated from those of the two-dimensional layer for salt 1 to the isolated cluster for salts 2 and 3. The temperature-dependent magnetic susceptibilities of salts 1-3 were consistent with the isolated spin arrangements of [PMo12O40]4-. The electronic spectra of salts 1-3 indicated the intervalence optical transition from pentavalent Mo(V) to hexavalent Mo(VI) ions within the [PMo12O40]4- cluster. Temperature-dependent electron spin resonance spectra of salt 2 revealed the delocalization-localization transition of the S = 1/2 spin at 60 K. The spin on the [PMo12O40]4- cluster was localized on a specific Mo(V) site below 60 K, which was thermally activated with an activation energy of 0.015 eV.

Motional heterogeneity and phase separation of semi-interpenetrating networks and mixtures based on functionalised polyurethane and polymethacrylate prepolymers

Semi-interpenetrating polymer networks (SIPNs) and polymer mixtures (1:1 mass ratio) based on segmented polyester polyurethane (PU) with carboxylic groups and methacrylic copolymer (PM) with tertiary amine groups were studied by the electron spin resonance (ESR) spin label method. The concentration of functional groups varied from 0 to 0.45 mmol g −1 in both prepolymers. The ESR spectra of spin labelled PM component were used to characterise the heterogeneity of segmental motion and transitions due to the additional polymer interactions imposed by complementary functional groups. The results were deduced from the temperature dependent ESR spectra. Two component spectra reflect the effect of PU chains on segmental motion of the PM component below the macroscopic glass transition temperature, T g. The ratio of the fast and slow component was related to the complex polymer–polymer interaction or extent of miscibility. Restrictions of segmental motion of PM chains increase with functional groups concentration and above certain concentration (0.25 mmol g −1) PM segments in the network assess faster motion suggesting a change in the local packing density and domain structure. An increased miscibility and disorganisation of the ordered domains are confirmed by the loss of spherulitic morphology and crystallinity at higher functional groups concentration. PU/PM mixtures reveal similar motional behaviour as SIPNs of the same composition. However, the differences in the fractional amount of fast and slow motions confirm better interpenetration and interaction of the two polymers in the SIPNs. The results of motional heterogeneity and polymer interactions were complemented with the T gs.

Magnetism and electron spin resonance in single crystalline β-AgNpO 2(SeO 3)

We report magnetization, susceptibility, electrical transport, and electron spin resonance (ESR) studies of single crystals of β-AgNpO 2(SeO 3). Here the valence of the Np sites is expected to be Np(V). We observe a magnetic transition below 8 K, where the transition temperature is dependent on the effective magnetic moment. Although the transition appears to be ferromagnetic, no hysteresis is seen in the magnetization, and the saturation moment above 0.1 T is found to be about 60% of the free NpO 2 ion moment. The decrease in the Np moments determined experimentally is thought to arise from crystal field and spin–orbit effects. We observe temperature dependent ESR spectra at ∼44 GHz (for fields above the saturation field) that show slight shifts in the g-factor and line width at low temperatures. Our results provide evidence that both Np(V) and Np(IV) valences are present, where the latter may be a minority population. The crystals, although dark in appearance, are electrically insulating ( ρ>10 10 Ωcm) at room temperature.

Electron Spin Resonance Study on Dynamic Heterogeneity in Miscible Blend of Poly(cyclohexyl methacrylate) and Poly(cyclohexyl acrylate)

Electron spin resonance (ESR) was applied to estimate the segmental dynamics of poly(cyclohexyl methacrylate) (PCHMA) and poly(cyclohexyl acrylate) (PCHA) in their miscible blend. The PCHMA and PCHA chains were labeled with nitroxide radicals, and the temperature-dependent ESR spectra of the spin-labels selectively reflected the segmental dynamics of the labeled chains in the blend. The ESR result showed different mobility between PCHMA and PCHA chains even though their blend showed a single glass transition in any composition by a calorimetric measurement. The WLF treatment demonstrated that a transition temperature, T5.0mT, at which the extreme separation width due to 14N anisotropic hyperfine splitting was 5.0 mT, reflected the glass transition observed at a frequency of the ESR. The T5.0mT of poly(CHMA-random-CHA) was dependent on the composition; namely, the mobility of the spin-labels was sensitive to the composition around them. The self-concentration model suggested by Lodge and McLeish [Lodge, T....

A Site-Specific ESR Spin-Labeling Study of Molecular Motion in Microphase-Separated Polystyrene-block-poly(methyl acrylate) with Lamellar Morphology

Molecular motion at the specific sites of polystyrene-block-poly(methyl acrylate) (PS-block-PMA) forming a lamellar morphology was studied by electron spin resonance (ESR). PS-block-PMA's having nearly the same molecular weight and composition were selectively spin-labeled with stable nitroxide radicals at different specific sites (each free end, inside segments of each block, or the chemical junction point between the blocks). The molecular motion was analyzed from temperature-dependent ESR spectra of the spin-labels. Enhancement of molecular mobility of the chain end in comparison with that of the inside was detected. The enhanced molecular mobility of the both ends of the PS-block-PMA was more remarkable than those of the both homopolymers. This was brought from the much large free volume around the chain ends of the PS-block-PMA because of the local high concentration of the chain ends in the lamellar microdomains. The molecular mobilities of the PS-block-PMA and polystyrene-block-poly(ethyl acrylate) (PS-block-PEA) labeled at the junction points were related to thermal behavior and local environment of polymer chains in the interfacial region of the lamellar microdomains. This demonstrated that the structure in the interface was heterogeneous, although the both components were mixing compatibly in the interfacial region.

Graft polymerization of methacrylic acid onto polytetrafluoroethylene initiated by alkyllithium/electron-donating solvents

Graft polymerization of methacrylic acid (MAA) onto polytetrafluoroethylene (PTFE), initiated by the mixed solution of alkyllithium/hexamethylphosphoramide (HMPA) at low temperature, was studied. Using electron spin resonance (ESR), nuclear magnetic resonance, X-ray photoelectron and vibrational (infrared) spectroscopies, the chemical structure of the grafted poly(methacrylic acid) (PMAA) onto the PTFE (PTFE-g-PMAA) was investigated. The grafted PMAA was found to be located deeper than 2.5 nm from the surface of the PTFE. The molecular motion of the PTFE-g-PMAA was also studied by means of the temperature-dependent ESR spectra of the spin-labeled PTFE-g-PMAA. The results suggested that the molecular motion of the grafted PMAA chain was mainly controlled by that of the principal PTFE chain, and the evaluated activation energy was lower for the grafted PMAA chain (e.g., 6.5 kJ/mol at temperature range below 295 K) than that for the PMAA homopolymer (13.2 kJ/mol). © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 923–931, 2004

Motional heterogeneity of segmented polyurethane–polymethacrylate mixtures: an influence of functional groups concentration

The motional heterogeneity in the polymer mixtures of segmented polyether polyurethane (PU) with carboxylic groups and methacrylic copolymer (PM) with tertiary amine groups was studied by the electron spin resonance (ESR)—spin probe and spin label methods. Pure polymer components containing varying amounts of functional groups and their 1:1 mass ratio mixtures have been analysed. The results of motional heterogeneity and polymer interaction were complemented with the glass transition temperatures, structural and morphological characteristics. Spin probed PU/PM mixtures indicate that the probe motion and the phase separation deduced from the temperature-dependent ESR spectra are sensitive to a free volume determined by the polymer–polymer interactions. The interaction between the two components in PU/PM mixture with the highest functional groups concentration disorganizes hard segment domains with spherulitic character at the microscopic level as compared with the ordered hard phase in the corresponding pure PU sample. The influence of PU hard and soft segments on the motional dynamics of PM chains is analysed from the ESR spectra of spin labelled PM chains in the mixtures with unlabelled PU components. The fractional amount of the PM fast motion depends on the temperature and concentration of functional groups.

Electron spin resonance analysis of motional effects of hyperoxide (O2-) absorbed in calcium-strontium- and barium-Y zeolites

Electron spin resonance (ESR) spectra of ultraviolet photogenerated O2 in alkaline-earth-Y zeolites have been studied from 4 K to room temperature. A temperature-dependent ESR spectrum is observed for O2 only in Ba-Y zeolite. Analysis of the g tensor of O2 in Ba-Y, Ca-Y, and Sr-Y zeolites indicates that the crystal field interaction is weakest for Ba-Y zeolite and suggests that this is the reason why only this zeolite shows a temperature-dependent spectrum for O2 . The temperature-dependent ESR spectra were simulated by the modified Bloch equations in terms of g-anisotropy averaging of O2 . The biggest change occurs in the g1 tensor component which lies along the O-O bond direction. The motional model that fits the temperature-dependent spectra best corresponds to 180 rotational jumps about an axis between BaS and one oxygen of O2 . The O-O bond of O2 is nearly parallel to the interior zeolite surface, making an angle with the surface of only 16. The activation energy for the motion is 25 kJ mol . Other motion models are also discussed. 17 references, 10 figures, 2 tables.

ESR studies of barriers to ring inversion in cyclic monocarboxylic acid radicals

Activation energies for ring inversion have been estimated for a number of cyclic monocarboxylic acid radicals, of structure (CH2)n≳ĊCOOH (I) with n=3,4, and 5, using electron spin resonance (ESR) methods. These radicals were produced from the corresponding cyclic monocarboxylic acids, by γ irradiation (at 195 K) and subsequent annealing. The ESR spectra of these radicals vary with temperature. The variations are attributed to ring inversion of the cyclic group in these radicals. Computer simulations of the temperature dependent ESR spectra were employed using the solutions of modified Bloch equations for a two-jump process. From these simulations the rate of ring inversion (τ) as well as the ESR parameters were estimated as a function of temperature (T). Arrhenius parameters found for the ring inversion in cyclohexane carboxylic acid radical (i.e., I with n=5) are Ea=4.0±0.3 kcal/mol and ln(1/τ0) =29.0±0.7 (τ0 in sec). The radical species with five-membered ring (n=4) was studied in a number of matrices: (i) sodium cyclopentane carboxylate, (ii) 12M LiCl(D2O), (iii) 8M NaOD, (iv) cycloleucine and (v) cyclopentane carboxylic acid. In the last two matrices, the five-membered ring was found to have twisted configuration yielding four nonequivalent β-proton splittings. In the other matrices a symmetrical configuration was found revealing two pairs of equivalent splittings. The barrier (Ea) to ring inversion for this radical (n=4) varies only slightly (from 1 to 1.5 Kcal/mole) in different matrices.