The EPR spectra of the oxygen-deficient La0.5Yb0.5Ba2Cu3Oy mixed phase has been investigated as a function of temperature. At low temperatures (T<15 K), an intense EPR line indicating the presence of magnetic clusters associated with spin-polarized clusters of copper ions through delocalized oxygen holes, has been detected. Both the EPR linewidth and g-factor of the latter EPR spectrum exhibit strong temperature dependence indicative of short-range order effects. At higher temperatures (T>30 K), the EPR spectrum reveal the contribution of Cu2+ defects and a broad EPR spectrum due to Yb3+ ions which is compared with the corresponding one in Y0.5Yb0.5Ba2Cu3Oy compounds.

The magnetic properties of the mixed high-Tc superconductors Ea0.5R0.5Ba2Cu3O6+x (R - rare earth) are studied after prolonged 'aging' at room temperature (RT) for 1 year. Enhancement of the superconducting properties comprising the increase of the inhomogeneous diamagnetic signal for members of the series with the larger R-ion radius is detected after RT 'aging'. For the members with the larger R-ion radius the diamagnetic signal has been come more stronger whereas for the members with the smaller R-ion radius the time evolution of the superconducting properties remained almost the unchanged and the diamagnetic signal slightly decreases after 'aging'. The growth of the EPR intensity of localized Cu2+ centers complying with the rise of the normal-state magnetic susceptibility after prolonged storage at RT, is detected. The time evolution effects are related to slow ion diffusion processes that can be induced by stress-relaxation in the mixed Ea0.5R0.5Ba2Cu3O6+x structure, most pronounced for large R-ion radius. © 1999 Elsevier Science B.V. All rights reserved.

XRD, magnetic and EPR studies of the mixed phase La0.5Gd0.5Ba2Cu3Oy high-Tc superconductor after prolonged (one year) `aging' at room temperature are reported. Within this time period a pronounced change of the superconducting behavior and the homogeneity of the diamagnetic shielding signal accompanied by substantial structural changes, reduction of the impurity phase BaCuO2+x and increase of the EPR spectrum's intensity of Cu2+ magnetic defects is detected. The `aging' effects are related to strain-relaxation taking place during room temperature annealing that induces the formation of Cu2+ defect centers.

The effect of the increase of the critical temperature Tc by the irradiation of the Y123 samples with small doses of fast neutrons has been investigated by the temperature dependence of resistance and EPR studies. The results indicated that Tc and EPR signals of all specimens are increased with increasing irradiation fluences. These results are explained in terms of redistribution of oxygen, which promotes charge ordering in the superconducting CuO2 planes. © 1999 Elsevier Science B.V. All rights reserved.

The polymorphic modifications α, β, and γ phases of Fe2WO6 of the iron (III) tungstate have been investigated by resistivity and electron paramagnetic resonance (EPR) measurements. The high-electrical resistivity for all phases complies with the dominant antiferromagnetic component of the compounds. The EPR spectra comprise a broad exchange narrowed EPR line due to Fe3+ ions. The broadening of the EPR linewidth is explained in terms of the critical spin fluctuations near the antiferromagnetic phase transition. The shift of the resonance field and the temperature dependence of the EPR intensity imply an extended region of short-range order correlations. © 1999 The American Physical Society.

Two binuclear copper(II) complexes with macrocyclic Schiff bases Cu2LI(CH3COO)2·5H2O (complex I) and Cu2LII(CH3COO)2·2H2O (complex II) were synthesized and then characterized by IR, UV, and thermogravimetric analysis (TGA) measurements. TGA was used to investigate the desolvation of lattice water molecules. IR spectra demonstrated the formation of the cyclic compound and together with chemical elemental analysis were used to propose the structure of the complexes. The UV spectra of both complexes are typical for binuclear copper(II) complexes with Robson-type ligands. Variable-temperature magnetic susceptibility measurements corroborated by EPR and low-temperature isothermal magnetization data confirmed the formation of copper dimers with antiferromagnetic exchange coupling constants of -400 and -1250 cm-1 for complexes I and II, respectively, residing outside the usual range for the phenoxide bridged Cu(II) complexes. This implies the possibility that additional superexchange paths through the macrocyclic ligand may affect the intradimer exchange interaction as well as the phenoxide oxygen bridges.