Oxygen isotope effects on the critical and Curie temperatures and Raman modes in the ferromagnetic superconductor RuSr2GdCu2O8

The effect of oxygen isotopic substitution on transition temperature T-c, the Curie temperature T-M, and Raman modes is investigated in the ferromagnetic superconductor RuSr2GdCu2O8. A large positive oxygen isotope effect, alpha(Tc) = 1.6, is observed in the zero resistance T-c value for as-prepared samples but a much smaller value alpha(Tc) = 0.12 +/- 0.04 is found in the true T-c, where the superconductivity order parameter is established as signified by the rapid rise in d rho/dT. Such a value is in keeping with other underdoped high-T-c superconductor cuprates. A rather small (alpha(FM) approximate to 0.02) oxygen isotope effect is observed in the ferromagnetic transition. Raman modes for this material are identified by inspection of the spectra with progressive O-18 exchange, cation substitution, and structural considerations. [S0163-1829(99)51018-7].