Magnetic properties of the RuSr2Ln1.5Ce0.5Cu2O10-δ (Ln = Y, Ho and Dy) and RuSr2YCu2O8-δ rutheno-cuprate families:: a comparative study

The magnetic behavior of polycrystalline samples of the magneto-superconductors RuSn(2)Ln(1.5)Ce(0.5)Cu(2)O(10-delta) (LnRu-1222, Ln = Y, Ho and Dy) and RuSr2YCu2O8-delta (YRu-1212) is analyzed, and compared. Clear evidences for the occurrence of a spin glass phase on all LnRu-1222 samples are provided by ac susceptibility (chi(ac)) and do magnetization measurements. A frequency-dependent peak in chi(ac) vs. T measurements, at low magnetic field, is observed. The strong suppression of this peak in the presence of magnetic fields as low as 500 Oe, as well as the results of thermoremanent magnetization (TRM) and isothermal remanent magnetization (IRM) measurements, suggest a spin glass behavior. Interestingly, the same measurements on the YRu-1212 sample show no indication of glassy behavior, contrasting with the results for LnRu-1222. We suggest that frustration of the Ru magnetic moments due to the presence of oxygen vacancies would be favored only in the LnRu-1222 compounds, thus causing the glassy behavior. This idea is supported by the observation of large oxygen deficiency that affects the RuO2 planes in Ru-1222, but does not occur in Ru-1212. (C) 2004 Elsevier B.V. All rights reserved.