A combined ESR/X-ray diffraction study of the Y2O3–BaO–CuO phase diagram

Measurements at both X- and Q-band frequencies of various phases in the Y–Ba–Cu–O system are reported. Signals close to the free-spin region are attributed to localised Cu2+ species with g-value anisotropies noticeably dependent upon the local crystal structure. These materials include BaCuO2, Y2BaCuO5, Y2Cu2O5; characteristic g values are discussed in terms of crystal-field models for electronic structure.

Importantly, we find that high purity YBa2Cu3O7–x(x 0.1) superconducting samples (Tc= 92 K), prepared at lower temperature by solution or sol–gel techniques, have no intrinsic ESR signals in the free-spin region. We propose that this ‘ESR silence’ is, in fact, not due to the absence of intrinsic paramagnetism in these superconducting oxides, but rather due to the extremely efficient spin–lattice relaxation of the d-band conduction electrons. Here, electron spin relaxation in these metallic systems for T > Tc proceeds via an Elliott-type mechanism involving the modulation of spin–orbit interactions via itinerant conduction electrons. This efficient relaxation mechanism would lead to extremely large ESR linewidths, typically in the range 50–100 kG.