The samples of well-oxidized REBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-δ</sub> with different trivalent RE (rare earth) ions, were studied with use of EPR method at the temperature above of liquid nitrogen (77 K). The measured samples were obtained by the solid phase synthesis method. The aim of these measurements was to find and describe the relation between critical temperature, critical magnetic field, and the shape of the resonance signal. It was expected that samples possess the HTSC transition at about 90 K. For most samples, both a nonresonant absorption and emerging of HTSC state were visible in the EPR experiment. The evolution of these signals allowed determining the critical temperature, as well as the evolution of the critical magnetic fields as a function of temperature. Additionally, EPR signal of Cu (II) ions was detected and explained by insufficiently oxidized samples or the existence of contaminated phases.
Cobalt doped La<sub>3</sub>Ga<sub>5.5</sub>Ta<sub>0.5</sub>O<sub>14</sub> single crystal (0.5 mol.% in the melt with respect to Ga) grown by the Czochralski method has been investigated using electron paramagnetic resonance and optical absorption measurements. X- band EPR spectra observed at 8 K under rotation around <i>c</i> crystal axis consists of two asymmetric lines: a narrow line at ~150 mT magnetic field and a second wider line at ~350 mT. These lines could be ascribed to the high spin S=3/2 Co<sup>2+</sup> ions located at two different crystallographic positions. The low field signal arises from cobalt occupying Ga positions with C<sub>3</sub> symmetry (octaheder), whereas dominating in spectrum, more complicated signal at higher fields arises from cobalt located at Ga positions (tetraheder) with C<sub>2</sub> point symmetry. The whole spectrum repeats every 60° as is expected for trigonal crystal symmetry. Optical measurements have confirmed the conclusion especially for γ-irradiated crystals which reveal additional Co<sup>3+</sup> content after the treatment. Co<sup>3+</sup> ions are also seen in "as-grown" (not irradiated) crystals as <sup>5</sup>T<sub>2</sub>-<sup>5</sup>E transition giving absorption bands in the range 700-1100 nm. So in the "as-grown" crystals there arise Co<sup>2+</sup> ions at octahedral or tetrahedral Ga sites and Co<sup>3+</sup> ions at octahedral Ga sites.