ESR Absorption Research Articles

Polarization Effect in ESR of Co Doped CuGeO3

Electron spin resonance of a single crystal of CuGeO3 doped with 2% of Co has been studied at f = 99 GHz in temperature range 1.8–50 K. Contributions to ESR absorption from Cu2+ chains and from Co2+ ions were derived. It is found that functions obtained for ESR integrated intensities: Curie-Weiss for Cu2+ (χCu ∼ CCu/(T + Θ), with Θ = 92 K) and Curie for Co2+ (χCo ∼ CCo/T) are well consistent with temperature dependence of static magnetic susceptibility. Strong dependence of ESR absorption on polarization of oscillating magnetic field was discovered for Co2+ contribution. Polarization effect was studied for magnetic field applied along a, b and c directions. Values of g-factors of resonance lines are presented.

High-frequency ESR studies of colossal magnetoresistance system Cu(Cr 1−xZr x) 2S 4

The magnetic properties of colossal magnetoresistance materials Cu(Cr 1− x Zr x ) 2S 4 have been studied by the ESR measurements. A single ESR absorption line which has a Lorentzian shape was observed at high-temperatures for x=0.50. The estimated g-value is 1.98 at 198 K, which well agrees with the g-value of the free ion of Cr 3+. With decreasing temperature, the absorption at high-temperatures splits into two absorptions below 100 K. As a possible mechanism, we suggest the coexistent magnetic states of ferromagnetism and paramagnetism for x=0.50. Any qualitative differences have not been observed in the ESR measurements for the sample with 0.45⩽ x⩽0.60. This fact indicates that the magnetic state does not change upon Zr substitution for x=0.50 in the range of at least ±0.05.

High frequency ESR measurements of antiferromagnetic state in quantum spin system [formula omitted

The coexistence of a nearly spin-singlet state and antiferromagnetic long-range order (AFLRO) were found in Cu 2 CdB 2 O 6 . The high frequency ESR measurements have been performed by using a millimeter wave vector network analyzer (MVNA; ABmm) with a small cavity in the frequency of 35–100 GHz. The external magnetic field was applied by using a superconducting magnet. Below the temperature of T N = 9.8 K , ESR absorptions were observed. The frequency–field diagram of ESR absorption lines was obtained by the multiple frequency ESR measurements at the temperature of 1.5 K. We analysed these frequency–field relations based on antiferromagnetic resonance (AFMR) of the two-sublattice antiferromagnet with uniaxial type anisotropy. The AFMR mode at zero magnetic field has a finite antiferromagnetic gap of about 38 GHz. The spin-flop magnetic field H c = 1.38 T was estimated. These results are consistent with the coexistence model of a nearly spin-singlet state and AFLRO in Cu 2 CdB 2 O 6 .

High field ESR measurements on the low dimensional S = 1/2 Heisenberg antiferromagnet (CPA)2CuBr4

ESR measurements have been performed on poly crystalline sample of an S = 1/2 Heisenberg antiferromagnetic ladder-like system (C5H9NH3)2CuBr4. We estimated the g-value as g = 2.20 from analysis of a broad absorption line at high temperatures. Multiple ESR absorption lines were observed below 10 K. The ESR modes show non-linear frequency dependence of the resonance field at 4 K, and the temperature dependence of the line width with a thermal activation type formula. The ESR modes observed are well explained by the theoretical model of the breather excitation.

Environment of Fe 3+ ion and its influence on dielectric and spectroscopic properties of Pbo–Nb 2O 5–P 2O 5 glasses

PbO–Nb 2O 5–P 2O 5 glasses containing different concentrations of Fe 2O 3 (ranging from 0 to 3 mol%) were prepared. A number of studies viz., chemical durability, differential thermal analysis, spectroscopic (infrared, optical absorption ESR spectra) and dielectric studies (constant ε′, loss tan δ, AC conductivity σ AC over a range of frequency and temperature) of these glasses have been carried out. The results of these studies have been analyzed in the light of different oxidation states of iron ions. The analysis indicates that the iron ions (when present in the concentration range, 0.2–1.0 mol%) exist mostly in trivalent state and occupy both tetrahedral and octahedral substitutional positions and are found to increase the rigidity of the glass network. However, in the concentration range of 1.0–2.5 mol%, the iron ions in the glass network seem to be existing by and large in Fe 2+ state, occupying predominantly octahedral positions and are acting as modifiers.

Gamma-induced absorption spectra as a new method for RE-ion environment study in fluorophosphate glasses

Abstract Doped fluorophosphate glasses based on pseudo-binary system: barium metaphosphate Ba(PO3)2– usovite MgCaSrBaAl2F14, are studied by means of fluorescence, induced ESR and optical absorption spectra. Doping the glass, containing 5 and 7 mol% Ba(PO3)2 with TbF3 and EuF3 results in drastic changes of color and paramagnetic center numbers correspondingly. Post-irradiation process results in separation of the spectra analyzed into groups in dependence of Tb3+ ion concentration. The crucial concentration, dividing two concentration ranges is not more than 0.02 mol%. It is supposed that the local environment of Tb3+ ion doped up to concentration mentioned is preferably phosphate. Estimates of Eu3+ ion concentration that have preferably phosphate groupings in their environment result from concentration dependence of PO 4 2 − centers and produce the value of approximately 0.01 mol%. Close values of two crucial concentrations is interpreted in terms of doped ion segregation model.

Spectroscopic investigation on the chemical forms of Cu during the synthesis of zeolite X at low temperature

The direct synthesis of zeolites in polluted soils has proved to be a promising process for the stabilization of metals inside these minerals. Nevertheless, more detailed information about this process is still needed in order to better foresee the fate of metals in treated soils. In this work, zeolite X has been synthesized under alkaline conditions in an aqueous solution containing 2500 mg kg −1 of Cu, starting from Na silicate and Al hydroxide at 60 °C. Aluminium, Si and Cu concentrations in the aqueous phase, during zeolite synthesis, were measured over a period of 160 h. The solid products have been characterized over time by XRD, SEM-EDX, ESR, FT-IR, and synchrotron radiation X-ray microbeam absorption near edge structure (μ-XANES) and extended X-ray absorption fine structure (μ-EXAFS) spectroscopy. It appears that the marked reduction of Cu concentration in solution is not only due to a simple precipitation effect, but also to processes connected with the formation of zeolite X which could entrap, inside its porous structure, nano- or micro-occlusions of precipitated Cu hydroxides and/or oxides. In addition, EXAFS observations strengthen the hypothesis of the presence of different Cu phases even at a short-range molecular level and suggest that some of these occlusions could be even bound to the zeolite framework. The results suggest that zeolite formation could be used to reduce the availability of metals in polluted soils.

ESR and optical study of Mn 2+ doped ammonium selenate single crystals

ESR and optical absorption studies of manganese doped ammonium selenate single crystals are performed at X-band and room temperature to ascertain the site symmetry and location of impurity ions in the lattice. Manganese ions are expected to enter the lattice interstitially. Various spin Hamiltonian parameters are determined. Optical absorption study is also done and associated distortion in the crystal lattice is estimated.

Complexation and ligand exchange in aqueous of Cu(II) and Ni(II) with hydrazides of some aromatic acids

Solvation and complexation of Cu(II) and Ni(II) with benzoic (L′), p-methoxybenzoic (L″), and isonicotinic (L) acid hydrazides in aqueous-ethanol solutions (ethanol mole fraction 0.07–0.68) were studied by pH-metry, spectrophotometry, and nuclear magnetic relaxation. The formation constants of the species M(L′)2+, M(L′)22+, M(L″)2+, M(L″)22+, M(LH)3+, M(L)2+, M(L)(LH)3+, and M(L)22+, where M = Cu2+ and Ni2+, were determined. With isonicotinic acid hydrazide, a change in the coordination mode was observed in an isomer of Cu(L)22+, with one of the ligands coordinating in the bidentate fashion, and the other, in the monodentate fashion via the pyridine nitrogen atom. The suggested structures were confirmed by analysis of the parameters of the ESR and electronic absorption spectra of the complexes. The rate constants of ligand exchange and formation of the complexes Cu(L′)2+, Cu(L′)22+, Cu(L″)2+, and Cu(L″)22+ in aqueous solutions were determined from nuclear magnetic relaxation measurements; the reactions occur by the associative mechanism. A cyclic process of reduction of Cu(II) to colloidal copper in the presence of L″ and atmospheric oxygen is described.

Synthesis and guest exchange reactions of inclusion compounds of cucurbit[8]uril with nickel(II) and copper(II) complexes

Inclusion compounds of the macrocyclic cavitand cucurbit[8]uril (CB[8]) with the nickel(II) complex, {trans-[Ni(en)2(H2O)2]@CB[8]}Cl2 · 23.5H2O, the copper(II) complex, {2[Cu(dien)(bipy)(H2O)]@CB[8]}(ClO4)4 · 11H2O, and the organic molecules, 2(pyCN)@CB[8]} · 16H2O and {2(bpe)@CB[8]} · 17H2O, where bipy is 4,4′-bipyridyl, pyCN is 4-cyanopyridine, and bpe is trans-1,2-bis(4-pyridyl)ethylene, were synthesized. The inclusion compounds with organic molecules were synthesized starting from inclusion compounds of cucurbit[8]uril with cyclam and ethylenediamine complexes of copper(II) and nickel(II) by the guest exchange method, which is based on the replacement of one guest with another in the cavity of the cavitand The resulting compounds were characterized by X-ray diffraction, ESR, 1H NMR, IR, and electronic absorption spectroscopy, and electrospray mass spectrometry. Photochemically induced [2+2]-cycloaddition of two 1,2-bis(4-pyridyl)ethylene molecules included in cucurbit[8]uril was studied.

A Study of the Incommensurate Phase of Rb 2 ZnCl 4 by ESR

The ESR absorption spectrum of Mn2+ ions at Zn positions has been studied in the incommensurate phase of Rb2ZnCl4. It is known that Rb2ZnCl4 crystals exhibit an incommensurate phase between temperatures Ti = 303 K and Tl = 192 K. The form and the temperature dependence of ESR spectra are in good agreement with the predictions of the phase soliton theory. The parameters h1 and h2 determining the magnetic resonance field show, as expected, a power law as a function of the temperature. A modified superposition model was used to estimate the modulation amplitude of the incommensurate phase, and the principal axis of the zero field splitting D.

Numerical analysis of electron-spin resonance in the spin-12XYmodel

This paper presents numerical results of the ESR absorption spectrum for S= ½ XY model in one dimension. We obtain detailed numerical results for a wide range of temperature and magnetic field for a large system of 400 sites, using the method developed by Derzhko et al. In the high-temperature regime, a similar behavior is observed for both the ferromagnet and the antiferromagnet; these line shapes have only one peak. On the other hand, in the low-temperature regime, line shapes show very different behaviors depending on the sign of the exchange coupling. While the line shape for the ferromagnet has only one peak as in the high-temperature regime, the line shape for antiferromagnet has two peaks and shows very strong magnetic-field dependence. We also discuss the physical meaning by comparing our numerical results with several theoretical results: the exact result in the high-temperature limit, Kubo-Tomita theory, field theory, mean-field theory, and elementary excitations.

High magnetic field ESR study of field induced antiferromagnetic ordering in CsFeBr 3 at low temperature

The triangular antiferromagnet CsFeBr 3, which has the spin singlet ground state due to the large single ion anisotropy of Fe 2+ ions, shows the field-induced magnetic ordering for B|| c. Submillimeter and millimeter wave ESR measurements of CsFeBr 3 have been performed at 2 K . Several absorption lines were observed below the ordering field 5 T which is in the paramagnetic region. Temperature dependence of ESR absorption lines for paramagnetic region shows the decrease of intensity as the temperature is increased. It suggests that the observed ESR absorption lines come from the ground state. Changes of the ESR modes at 2 K were observed at the field corresponding to the antiferromagnetic gap of 2.89 T . The results are discussed in connection with the theory of the gap system.

High field ESR measurements of pyrochlore slab antiferromagnets Ba 2Sn 2Ga 3+ xZnCr 7− xO 22

High field ESR measurements of pyrochlore slab antiferromagnets Ba 2Sn 2Ga 3+ x ZnCr 7− x O 22 have been performed in the millimeter wave region at temperatures from 4.2 to 200 K . The concentration dependence of Cr 3+ ions was observed from x=0 to 5.0. For Ba 2Sn 2Ga 3ZnCr 7O 22 ( x=0), the pyrochlore slab is fully filled with Cr 3+ ions, the line width of the ESR absorption lines increased and the resonance fields shifted as the temperature was decreased. These tendencies were suppressed as the Cr 3+ concentration was decreased.

High Field ESR Measurements Using Gyrotron FU Series as Radiation Sources

Submillimeter wave ESR spectrometer using Gyrotron FU series as electromagnetic wave radiation sources and the pulse magnet for high field up to 40 T has been developed. The ESR spectrometer has been successfully applied to several ESR measurements. ESR measurements were performed under the pulsed magnetic fields up to 40 T at the temperature of 4.2 K in CsFeCl 3 single crystal. The frequency-field diagram of ESR absorptions was obtained.

High Field ESR of the Frustrated Antiferromagnet Ba2Sn2Ga3ZnCr7O22on a Pyrochlore Slab

High field ESR measurements of pyrochlore slab antiferromagnets Ba 2 Sn 2 Ga 3+ x ZnCr 7- x O 22 ( x =0,1,3,5,6.9) have been performed in the millimeter wave region at temperatures from 4.2 to 200 K. For Ba 2 Sn 2 Ga 3 ZnCr 7 O 22 ( x =0), the pyrochlore slab is fully filled with Cr 3+ ions, the line width of the ESR absorption line increased and the resonance fields shifted as the temperature was decreased. These tendencies were suppressed as the Cr 3+ concentration was decreased. The ESR results were compared with those of another frustrated magnet and mechanism of the resonance field shift was discussed.

Preparation and structural characterization of a novel dicopper(ii) complex with a terminal hydroxide: a structural model of an active site in phosphohydrolasesElectronic supplementary information (ESI) available: the effective magnetic moments and the molar magnetic susceptibilities of 1, the positive ion electrospray ionization mass spectrum of [CuII2(TNPDO)(OH)(F)]+ and the structure of HTNPDO. See http://www.rsc.org/suppdata/dt/b3/b303787h/

As an intermediate model for active dinuclear metal sites in phosphohydrolases, a novel dicopper(II) complex with a terminal hydroxide has been prepared using a new dinucleating ligand with intramolecular hydrogen-bonding N–H sites and sterically hindered tert-butyl groups. It has been characterized by X-ray structure analysis, magnetic susceptibility, and ESR and electronic absorption spectroscopies.

Electron spin relaxation in CMR manganites: absence of critical acceleration

Using original modulation technique, the longitudinal electron spin-lattice relaxation time T 1 has been measured in a series of nearly cubic La 1− x Ca x MnO 3 ( x=0.1; 0.2; 0.25; 0.33) and layered La 2−2 x Sr 1+2 x Mn 2O 7 ( x=0.4 and 0.5) manganites both in paramagnetic state and around the temperature ( T c) of the magnetic ordering. The data are compared with the evolution of the transverse relaxation time T 2 as determined from the ESR linewidth. Well above T c, the T 1= T 2 equality is confirmed, whereas a pronounced slowing down of T 1 is observed in all the materials as T c is approached. The temperature dependence of T 1 is found to be consistent with that of Tχ( T), where χ( T) represents the temperature dependence of the electron-spin susceptibility determined from the ESR absorption area. The interpretation suggests freezing the local field fluctuations due to polarization of superparamagnetic clusters in the external magnetic field.

Electron spin resonance studies ofCo(tbp)·C60 single crystal

ESR measurements have been made on a newly synthesizedcocrystallite of C60 with the CoII complex of 5,10, 15, 20-tetrakis[3, 5-(di-tert-butyl)-phenyl]porphyrin(tbp) single crystal to study the strong intermolecularinteractions which are suggested by x-ray structural analyses. Asingle anisotropic ESR absorption line is observed at roomtemperature with the principal g-values g⊥ = 2.423 andg∥ = 2.005, which are typical values for the low-spinCoII ions (S = 1/2). The antiferromagnetic superexchangeinteraction between CoII ions via C60 is confirmedfrom the observation of the finite Weiss temperature, which isfurther supported by the absence of the hyperfine splittingdue to 59Co (I = 7/2) nuclei. As the temperature is lowered,the g⊥-value decreases monotonically suggesting an increaseof the crystal-field splitting energy. This result is consistentwith the fact that the intermolecular distance between CoIIand C60 becomes shorter as the temperature decreases. Thepeak-to-peak ESR linewidths ΔHpp along bothprincipal axes increase with temperature due to the shorterspin-lattice relaxation time.

High-temperature thermoluminescence manifestations of anion interstitials in neutron-irradiated pure and doped single crystals of MgO

The thermal stability of anion interstitials (H centers) was studied in neutron irradiated single crystals of MgO. The ESR spectra of H centers were investigated, the annealing of their ESR absorption and the optical absorption of F+ centers was measured. Both defects were found to disappear in the 650—750 K temperature region. Thermal decay of H centers was investigated using fractional thermoactivation spectroscopy and a common high-temperature (300—775 K) thermoluminescence, combined with the method of controlled X-colouration. A possible mechanism for the thermal destruction of complex anion Frenkel defects in MgO, including the hopping diffusion of neutral oxygen interstitials to F centres, is proposed.