Electron Spin Resonance Linewidth Research Articles

13C Dynamic Nuclear Polarization using SA-BDPA at 6.7 T and 1.1 K: Coexistence of Pure Thermal Mixing and Well-Resolved Solid Effect.

SA-BDPA is a water-soluble, narrow-line width radical previously used for dynamic nuclear polarization (DNP) signal enhancement in solid-state magic angle spinning NMR spectroscopy. Here, we report the first study using SA-BDPA under dissolution DNP conditions (6.7 T and 1.15 K). Longitudinal-detected (LOD)-electron spin resonance (ESR) and 13C DNP measurements were performed on samples containing 8.4 M [13C]urea dissolved in 50:50 water:glycerol (v/v) doped with either 60 or 120 mM SA-BDPA. Two distinct DNP mechanisms, both "pure" thermal mixing and a well-resolved solid effect could clearly be identified. The radical's ESR line width (30-40 MHz), broadened predominantly by dipolar coupling, excluded any contribution from the cross effect. Microwave frequency modulation increased the enhancement by DNP at the lower radical concentration but not at the higher radical concentration. These results are compared to data acquired with trityl radical AH111501, highlighting the unusual 13C DNP properties of SA-BDPA.

Magnetic Field Induced Berezinskii-Kosterlitz-Thouless Correlations in 3-Dimensional Manganites

Ideal two-dimensional (2D) Heisenberg magnets lack long range magnetic order. However, the XY model with spins confined to a plane shows a topological phase transition at a finite temperature corresponding to binding and unbinding of vortices. Experimental evidence for such Berezinskii-Kosterlitz-Thouless (BKT) transitions has been difficult to obtain in condensed matter systems, where, even a weak interlayer coupling that is invariably present leads to long-range order, pre-empting the BKT transition. The BKT signatures are still discernible above the long-range ordering temperature, however, in the characteristic exponential temperature dependence of the coherence length of the fluctuations. In this work we report that an applied magnetic field can induce such BKT correlations not only in quasi 2-dimensional systems but also in nominally 3-dimensional manganites undergoing antiferromagnetic transitions. We arrive at this unexpected conclusion based on our studies of temperature dependence of electron paramagnetic resonance (EPR) linewidth ΔH(T) of Cr3+ doped bismuth strontium manganite Bi0.5Sr0.5Mn1-xCrxO3 (x= 0.04, 0.1) (BSMCO04 and BSMCO10).

Signatures of field-induced Berezinskii-Kosterlitz-Thouless correlations in the three-dimensional manganite Bi0.5Sr0.5Mn0.9Cr0.1O3

Despite experimental evidence for the two-dimensional topological Berezinskii-Kosterlitz-Thouless (BKT) transition in superconducting films and $^{4}\mathrm{He}$ superfluid films, the observation of a BKT transition in condensed matter systems has proven to be difficult. Potential signatures of BKT transitions were reported in two-dimensional magnets, however, the observation of a classical BKT transition in nominally three-dimensional systems has naturally not been investigated as the transition requires an underlying $XY$ model with spins confined to a plane. Here we report that the temperature dependence of the electron paramagnetic resonance (EPR) linewidth observed in ${\mathrm{Bi}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Cr}}_{x}{\mathrm{O}}_{3}$ $(x=0.1 \mathrm{and} 0.04)$ as well as in certain other three-dimensional (3D) manganites undergoing antiferromagnetic transitions is described satisfactorily by the BKT model. We explain this unexpected observation of signatures of a two-dimensional topological phase transition in 3D systems in terms of an effective two-dimensional $XY$ easy plane anisotropy induced by the magnetic field applied in the EPR experiment that allows for the mediation of long-range vortex-like correlations between spin clusters formed due to phase segregation. This conclusion is supported by a re-analysis of EPR results reported earlier in La-doped ${\mathrm{CaMnO}}_{3}$ and a nonmanganite compound ${\mathrm{BaNi}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{8}$. We infer that field-induced BKT correlations in a 3D system provide a step towards the observation of a BKT transition in a suitably chosen condensed matter system by the application of an appropriate magnetic field.

Radiation induced defects in timolol: temperature dependent electron paramagnetic resonance spectroscopy analysis

Ionizing radiation-induced antihypertensive drug timolol in powder form was studied by means of electron paramagnetic resonance (EPR) technique as a function of temperature. The effect of temperature on EPR linewidth, intensity and spectra was investigated. The room temperature EPR spectra of ionizing radiation-induced timolol were recorded in dimethyl sulfoxide solution at a frozen state. The first-order kinetic mechanism rate constant of timolol was calculated. The findings on all EPR experimental data indicated that ionizing radiation-induced stable-free radical species on timolol are characterized by g = 2.0027.

Charge-Order Phase Transition in the Quasi One-Dimensional Organic Conductor {hbox {(TMTTF)}}_2 {hbox {NO}}_3

Low-dimensional organic conductors show a rich phase diagram, which has, despite all efforts, still some unexplored regions. Charge ordered phases present in many compounds of the {hbox {(TMTTF)}}_2X family are typically studied with their unique electronic properties in mind. An influence on the spin arrangement is, however, not expected at first glance. Here, we report temperature and angle dependent electron spin resonance (ESR) measurements on the quasi one-dimensional organic conductor {hbox {(TMTTF)}}_2 {hbox {NO}}_3. We found that the {hbox {(TMTTF)}}_2 {hbox {NO}}_3 compound develops a peculiar anisotropy with a doubled periodicity (ab'-plane) of the ESR linewidth below about T_{text {CO}}= ({250pm 10})~hbox {K}. This behavior is similar to observations in the related compounds {hbox {(TMTTF)}}_2X (X = {hbox {PF}}_6, {hbox {SbF}}_6 and {hbox {AsF}}_6), where it has been attributed to relaxation processes of magnetically inequivalent sites in the charge-ordered state. For the structural analogous {hbox {(TMTTF)}}_2 {hbox {ClO}}_4, known for the absence of charge order, such angular dependence of the ESR signal is not observed. Therefore, our ESR measurements lead us to conclude that a charge-order phase is stabilized in the title compound below T_{text {CO}} approx 250~hbox {K}.

Ultralong Spin Lifetime in Light Alkali Atom Doped Graphene.

Today’s great challenges of energy and informational technologies are addressed with a singular compound, Li- and Na-doped few-layer graphene. All that is impossible for graphite (homogeneous and high-level Na doping) and unstable for single-layer graphene works very well for this structure. The transformation of the Raman G line to a Fano line shape and the emergence of strong, metallic-like electron spin resonance (ESR) modes attest the high level of graphene doping in liquid ammonia for both kinds of alkali atoms. The spin-relaxation time in our materials, deduced from the ESR line width, is 6–8 ns, which is comparable to the longest values found in spin-transport experiments on ultrahigh-mobility graphene flakes. This could qualify our material as a promising candidate in spintronics devices. On the other hand, the successful sodium doping, this being a highly abundant metal, could be an encouraging alternative to lithium batteries.

Investigations of Magnetic Properties Hg0.865Mn0.135Te by the Electron Spin Resonance Method

Mercury chalcogenide (Hg 0.865 Mn 0.135 Te) monocrystal was grown by crystallization from a two-phase mixture with replenishment of the melt from a tellurium solution and has n-type conductivity. The electron spin resonance (ESR) spectra of a single crystal were studied before and after X-irradiation. Only one intense anisotropic ESR line was observed at 4.2 K before X-irradiation. The anisotropy parameters were established from the angular dependence of the resonance field of the ESR line. After X-irradiation, three ESR lines were registered in the spectrum. The first and second lines with a large g -factor and unusual temperature dependencies were associated with charge carriers localized on defects with the strong exchange interaction with the manganese ion spins. The third ESR narrowed line was identified as signal from Mn 2+ ions. Estimated anisotropic exchange parameters from angular dependencies of magnetic resonance field depend on temperature. The temperature dependencies of ESR linewidths are well described by the Korringa law and spin-lattice relaxation contributions. The discontinuities in temperature dependencies of resonance field and linewidth at 18 and 65 K were observed.

Spin reorientation transition and spin dynamics study of perovskite orthoferrite TmFeO3 detected by electron paramagnetic resonance.

The temperature-dependent spin-reorientation transition (SRT) and spin interaction mechanism of bulk TmFeO3 were studied by the electron paramagnetic resonance (EPR) method. The combined experimental results of magnetic curves and EPR spectra confirmed that there is an antiferromagnetic transition at 85 K with a reentering ferromagnetic state due to the spin-reorientation behavior. In the high-temperature region of T > 90 K, there are three distinct resonance peaks in the EPR spectrum, which indicates the presence of multiple magnetic phases (canted antiferromagnetic, weak ferromagnetic, and paramagnetic phases). In the low-temperature region (T < 85 K), the temperature dependence of the EPR linewidth, effective g-factor, and intensity can be used to infer a strong spin-lattice correlation. Different magnetic interactions such as Fe3+-Fe3+, Fe3+-Tm3+, and Tm3+-Tm3+ lead to a paramagnetic-canted antiferromagnetic phase at T > 85 K, with SRT between 85-65 K and ferromagnetic interaction at the lower temperature, respectively. Above 90 K, we find that the spin relaxation mechanism is determined by the mixture of spin-spin and spin-lattice interactions. Below 85 K, the transverse relaxation rate increases with the decrease in temperature, which is consistent with the weakening of the fluctuating internal field in this temperature region. This EPR detection provides a new method to clarify the strong spin coupling in antiferromagnetic materials.

Electron spin resonance study of spin relaxation in the strong-leg spin ladder with nonmagnetic dilution

We have studied electron spin resonance (ESR) absorption spectra for the nonmagnetically diluted strong-leg spin ladder magnet ({C}$_{7}$H$_{10}$N)$_{2}$Cu$_{(1-x)}$Zn$_{x}$Br$_{4}$ (abbreviated as DIMPY) down to 450 mK. Formation of the clusters with non-zero net magnetization is confirmed; the cluster-cluster interaction is evidenced by the concentration dependence of ESR absorption. High-temperature spin-relaxation time was found to increase with non-magnetic dilution. The ESR linewidth analysis proves that the Dzyaloshinskii-Moriya (DM) interaction remains the dominant spin-relaxation channel in diluted DIMPY. Experimental data indicate that the dilution results in the weakening of the effective DM interaction, which can be interpreted as total suppression of DM interaction in the close vicinity of impurity atom.

Spectroscopic investigation of new manganese tellurite glasses synthesized by sol-gel method

New tellurite glasses of the xMnO(100-x)TeO2 system with x = 0–26.2 mol % were prepared by sol-gel method. X-ray diffraction and FTIR, UV–Vis, electron paramagnetic resonance (EPR) and photoluminescence (PL) spectroscopies were used to investigate the structural and behavioural properties of the samples. X-ray analysis confirms the amorphous nature of samples. FTIR spectra reveal the presence of TeO3, TeO4 and TeO6 vibrational modes. Addition of manganese ions to the samples produces the change of tellurium ions coordination, namely the conversion of TeO4 into TeO3 structural units up to x = 12.4 mol% MnO. UV-VIS spectroscopy data show the presence of Te=O bonds from [TeO3] structural units and of transitions characteristic of Mn2+ and Mn3+ ions. The EPR spectra of the xMnO(100-x)TeO2 glasses are due to the presence of Mn2+ ions and consist of two resonance absorptions, a weak one located at geff ≈ 4.3 and a strong one at geff ≈ 2.0. The nature of EPR absorption lines and the influence of manganese content on the evolution of EPR lines intensity and linewidth are discussed. The PL spectra are due to the presence of manganese and tellurium ions in the studied samples.

Motional narrowing under Markovian and non-Markovian hopping transitions in inhomogeneous broadened absorption line shape.

Inspired by recent experiments showing a minimum of electron spin or paramagnetic resonance (ESR and EPR) line width as a function of inverse temperature, we studied the motional narrowing effect by considering a combined model of carrier transitions and static dispersion of the angular frequency giving rise to an inhomogeneous broadening in the spectrum. The dispersion of the angular frequency results from the distribution of the local field. The transition between the sites under inhomogeneous static local field induces adiabatic relaxation of the spin. We also considered the on-site inherent (nonadiabatic) relaxation of the spin. We obtained the exact solution of the spin correlation function by explicitly considering transitions between two sites for both Markovian and non-Markovian transition processes. The absorption line shape is expressed in terms of the Voigt function, which is a convolution of a Gaussian function and a Lorentzian function. Using the known properties of the Voigt function, we discuss the correlation between the change in the full-width at half-maximum and the change in line shape, both of which are induced by motional narrowing. By assuming thermal activation processes for both the hopping transition and the on-site inherent relaxation, we show that the minimum of the width appears as a function of inverse temperature as observed experimentally in organic materials. Contrary to the general belief, we also show that the narrowing of the Gaussian line shape under a local random field did not necessarily lead to a Lorentzian line shape in particular under the presence of heavy tail property in the waiting time distribution of hopping transitions.

Magnetic anisotropy and spin-polarized two-dimensional electron gas in the van der Waals ferromagnet Cr2Ge2Te6

We report a comprehensive experimental investigation on the magnetic anisotropy in bulk single crystals of Cr$_2$Ge$_2$Te$_6$, a quasi-two-dimensional ferromagnet belonging to the family of magnetic layered transition metal trichalcogenides that have attracted recently a big deal of interest with regard to the fundamental and applied aspects of two-dimensional magnetism. For this purpose electron spin resonance (ESR) and ferromagnetic resonance (FMR) measurements have been carried out over a wide frequency and temperature range. A gradual change in the angular dependence of the ESR linewidth at temperatures above the ferromagnetic transition temperature $T_{\rm c}$ reveals the development of two-dimensional spin correlations in the vicinity of $T_{\rm c}$ thereby proving the intrinsically low-dimensional character of spin dynamics in Cr$_2$Ge$_2$Te$_6$. Angular and frequency dependent measurements in the ferromagnetic phase clearly show an easy-axis type anisotropy of this compound. Furthermore, these experiments are compared with simulations based on a phenomenological approach, which takes into account results of static magnetization measurements as well as high temperature $g$ factors obtained from ESR spectroscopy in the paramagnetic phase. As a result the determined magnetocrystalline anisotropy energy density (MAE) $K_U$ is $(0.48 \pm 0.02) \times10^6$ erg/cm$^3$. This analysis is complemented by density functional calculations which yield the experimental MAE value for a particular value of the electronic correlation strength $U$. The analysis of the electronic structure reveals that the low-lying conduction band carries almost completely spin-polarized, quasi-homogeneous, two-dimensional states.

The influence of Ho3+ doping on 13C DNP in the presence of BDPA.

Polarization transfer from unpaired electron radicals to nuclear spins at low-temperature is achieved using microwave irradiation by a process broadly termed dynamic nuclear polarization (DNP). The resulting signal enhancement can easily exceed factors of 104 when paired with cryogenic cooling of the sample. Dissolution-DNP couples low temperature polarization methods with a rapid dissolution step, resulting in a highly polarized solution that can be used for metabolically sensitive magnetic resonance imaging (MRI). Hyperpolarized [1-13C]pyruvate is a powerful metabolic imaging agent for investigation of in vitro and in vivo cellular metabolism by means of NMR spectroscopy and MRI. Radicals (trityl OX063 and BDPA) with narrower EPR linewidths typically produce higher nuclear polarizations when carbon-13 is the target nucleus. Increased solid-state polarization is observed when narrow line radicals are doped with lanthanide ions such as Gd3+, Ho3+, Dy3+, and Tb3+. Earlier results have demonstrated an incongruence between DNP experiments with trityl and BDPA, where the optimal concentrations for polarization transfer are disparate despite similar electron spin resonance linewidths. Here, the effects of Ho-DOTA on the solid-state polarization of [1-13C]pyruvic acid were compared for 3.35 T (1.4 K) and 5 T (1.2 K) systems using BDPA as a radical. Multiple concentrations of BDPA were doped with variable concentrations of Ho-DOTA (0, 0.2, 0.5, 1, and 2 mM), and dissolved in 1 : 1 (v/v) of [1-13C] pyruvic acid/sulfolane mixture. Our results reveal that addition of small amounts of Ho-DOTA in the sample preparation increases the solid-state polarization for [1-13C] pyruvic acid, with the optimum Ho-DOTA concentration of 0.2 mM. Without Ho-DOTA doping, the optimum BDPA concentration found for 3.35 T (1.4 K) is 40 mM, and for 5 T (1.2 K) system it is about 60 mM. In both systems, inclusion of Ho-DOTA in the 13C DNP sample leads to a change in the breadth (ΔDNP) of the extrema between the P(+) and P(-) frequencies in microwave spectra. At no combination of BDPA and Ho3+ did polarizations reach those achievable with trityl. Simplified analysis of increased polarization as a function of decreased electron T1e used to explain results in trityl are insufficient to describe DNP with BDPA.

Preparation and evaluation of trityl-loaded lipid nanocapsules as oxygen sensors for electron paramagnetic resonance oximetry

Oxygen is essential in physiology and pathophysiology. Electron paramagnetic resonance (EPR) oximetry, using oxygen sensitive paramagnetic materials, could be attractive for measuring oxygen in tissues. The aim of the present study was to assess the properties of lipid nanocapsules (LNCs) loaded with the nitroxide tempo-benzoate (TB) or tetrathiatriarylmethyl (TAM) radicals. LNCs loaded with the EPR probes were successfully prepared by the phase inversion process leading to nanocapsules of about 60 nm. LNCs protected the TB radical against reduction in vitro. The calibration of the EPR line width (LW) as a function of the pO2 showed a two-fold increase in sensitivity with TAM-LNC compared to hydrophilic trityl radical. The TAM-LNCs were evaluated in vivo. Contrarily to unencapsulated TAM, for which a rapid decrease in EPR signal was observed, the half-life of TAM-LNCs administered in muscles or in tumours exceeded an hour. Carbogen-challenges in mice demonstrated that the TAM-LNCs responded well to changes in oxygen environment. However, the apparent pO2 values acquired were higher than the expected physiological values. These results warrant further investigation in the formulation of stable nano-objects encapsulating EPR oxygen sensitive probes.

Determination of the g-factors measured by EPR based on theoretical crystal field and superposition model analyses for lanthanide-based magnetically concentrated crystals – case study: double tungstates and molybdates

ABSTRACTThe methods developed for theoretical determination of g-factors measured by electron paramagnetic resonance (EPR) for single transition ions are extended to lanthanide-based magnetically concentrated crystals. The crystal field (CF) and superposition model (SPM) analyses are employed for optoelectronic materials: double tungstates and molybdates. Using the crystallographic data for Nd3+ ions in RbNd(WO4)2 and Yb3+ ions in KYb(MoO4)2 as input for SPM modelling, the CF parameters (CFPs) are predicted for the respective ions. The matrix for g tensor is calculated by the complete diagonalisation method. By matching three principal g values (gxx, gyy, gzz) to those measured by EPR, three adjustable SPM intrinsic parameters:, and are obtained. The reliability of the related CFPs is verified by comparative analysis of available data. The results demonstrate that the methods suitable for single lanthanide ions in crystal can be successfully applied to magnetically concentrated systems. The predicted g tensor anisotropy enables consideration of EPR linewidths arising from dipolar broadening in ALn(MO4)2 (A = alkali ions, Ln = lanthanide ions, M = W or Mo) systems based on the moment method, which requires accurate g-factors. The model parameters determined here may be utilised for SPM/CFP calculations for Nd3+ and Yb3+ ions in single-molecule magnets and single-ion magnets.

Structure and electronic properties of biomorphic carbon matrices and SiC ceramics prepared on their basis

Biomorphic carbon matrices (BCMs) were produced by pyrolysis from wood species of different forest and garden trees, after which the as-prepared BCMs were converted to SiC ceramics through their impregnation with liquid silicon and further heat-treatment. Both types of obtained samples were studied by scanning electron microscopy (SEM), Raman scattering (RS), and electron spin resonance (ESR) methods. The SEM data reveal that all BCM samples contain large (10–50 μm) and small (1–5 μm) micro-pores with surface densities ∼109 m−2 and 1011 m−2, respectively. Analysis of RS allowed to estimate carbon cluster sizes of about 5–11 nm depending on the sample type. The study of the electronic structure using ESR spectroscopy is carried out for BCM and SiC ceramics samples. Using theoretical analysis of the ESR spectra, it was found that spin resonance in BCMs is due to the contribution of three spin systems: free electron spins, “pseudo-free” electron spins from the tail of density states below the conduction band, and localized spins at dangling carbon bonds (DCBs). Their contributions depend on the ratio of different structural phases such as sp2-hybridized graphite-like carbon network and amorphous carbon phase. For most BCM samples, the large ESR line width is dramatically narrowed when samples are pumped out due to the exclusion of the broadening effect of molecular oxygen. The transformation of BCM into SiC by impregnation with liquid silicon can be clearly traced in the Raman spectra and in the ESR spectra. It is established that the electronic properties of synthesized SiC ceramics are due to the presence of residual graphite-like carbon nanoclusters.

Nonmagnetic impurity effect on magnetic correlation in the triangular-lattice antiferromagnet Ba3Mn1−xZnxSb2O9(0 < x < 0.16)

The nonmagnetic impurity effects on the triangular-lattice antiferromagnet (TLAF) Ba3Mn1−xZnxSb2O9 () by Zn2+ () dilutions are investigated based on magnetic susceptibility, specific heat and electron spin resonance (ESR) measurements. The ac susceptibility and specific heat result reveal that the two-dimensional (2D) antiferromagnetic (AFM) ground state is stable rather than forming a spin-glass (SG) phase for Zn2+ substitution up to , where the AFM transition temperature (TN) shifts slowly from 10.2 K () to 8.2 K (). For all samples, the divergence of the temperature-dependent linewidth of ESR can be well described in terms of the Berezinskii-Kosterlitz-Thouless (BKT) scenario. The evolution of ESR spectra suggests that the nonmagnetic impurity does not induce magnetic defects decoupled with AFM matrix but rather change the bulk AFM properties.

Characterization of a clinically used charcoal suspension for in vivo EPR oximetry.

Electron paramagnetic resonance (EPR) oximetry using particulate materials allows repeatable measurements of oxygen in tissues. However, the materials identified so far are not medical devices, thus precluding their immediate use in clinical studies. The aim of this study was to assess the magnetic properties of Carbo-Rep®, a charcoal suspension used as a liquid marker for preoperative tumor localization. Calibration curves (EPR linewidth as a function of pO2) were built using 9-GHz EPR spectrometry. The feasibility of performing oxygen measurements was examined in vivo by using a low-frequency (1GHz) EPR spectrometer and by inducing ischemia in the gastrocnemius muscle of mice or by submitting rats bearing tumors to different oxygen-breathing challenges. Paramagnetic centers presenting a high oxygen sensitivity were identified in Carbo-Rep®. At 1GHz, the EPR linewidth varied from 98 to 426µT in L-band in nitrogen and air, respectively. The sensor allowed repeated measurements of oxygen over 6months in muscles of mice. Subtle variations of tumor oxygenation were monitored in rats when switching gas breathing from air to carbogen. The magnetic properties of Carbo-Rep® are promising for its future use as oxygen sensor in clinical EPR oximetry.

Dynamic nuclear polarization of carbonyl and methyl 13C spins of acetate using 4-oxo-TEMPO free radical.

Hyperpolarization of 13C-enriched biomolecules via dissolution dynamic nuclear polarization (DNP) has enabled real-time metabolic imaging of a variety of diseases with superb specificity and sensitivity. The source of the unprecedented liquid-state nuclear magnetic resonance spectroscopic or imaging signal enhancements of >10 000-fold is the microwave-driven DNP process that occurs at a relatively high magnetic field and cryogenic temperature. Herein, we have methodically investigated the relative efficiencies of 13C DNP of single or double 13C-labeled sodium acetate with or without 2H-enrichment of the methyl group and using a 4-oxo-TEMPO free radical as the polarizing agent at 3.35 T and 1.4 K. The main finding of this work is that not all 13C spins in acetate are polarized with equal DNP efficiency using this relatively wide electron spin resonance linewidth free radical. In fact, the carbonyl 13C spins have about twice the solid-state 13C polarization level of methyl 13C spins. Deuteration of the methyl group provides a DNP signal improvement of methyl 13C spins on a par with that of carbonyl 13C spins. On the other hand, both the double 13C-labeled [1,2-13C2] acetate and [1,2-13C2, 2H3] acetate have a relative solid-state 13C polarization at the level of [2-13C] acetate. Meanwhile, the solid-state 13C T1 relaxation times at 3.35 T and 1.4 K were essentially the same for all six isotopomers of 13C acetate. These results suggest that the intramolecular environment of 13C spins plays a prominent role in determining the 13C DNP efficiency, while the solid phase 13C T1 relaxation of these samples is dominated by the paramagnetic effect due to the relatively high concentration of free radicals.

Concentration dependence of the Cr3+ ESR linewidth in barium titanate

The concentration dependence of the electron spin resonance (ESR) linewidth of Cr3+ ions in chromium-doped barium titanate (BaTiO3) was investigated at room temperature for chromium concentrations between 0.10 and 2.00 mol%. According to previous studies, chromium substitutes Ti4+ sites in the lattice and its preferred valence state is Cr4+, which is ESR silent in the X-band. In the present work, the Cr3+ state was enhanced by codoping with Nb, as it has been done previously in a similar compound, SrTiO3. The results show that the concentration dependence of the ESR peak-to-peak linewidth and the relative intensity of the Cr3+ spectrum can be approximated by the theoretical equations ΔHpp = 3.30 + C1(1 − f)80 mT and I = C2f(1 − f)80, where f is the chromium concentration and C1 and C2 are constants. This suggests that the range of the exchange interaction between Cr3+ ions in barium titanate is about 0.96 nm.