Transport properties and Raman scattering studies of superconducting ferromagnet Ru_1-xAl_xSr₂GdCu₂O₈

Transport properties and Raman spectra of Ru₁-_x Al_xSr₂ GdCu₂ O₈ (Al-doped Ru-1212) with x=0, 0.05, 0.1 and 0.15 have been investigated. The superconducting transition temperature of the Al-doped Ru-1212 increases with increasing Al content up to 10%, and then decreases with increasing Al content. The observed result seems to indicate that excess holes induced by Al doping could transfer from RuO₂ plane to CuO₂ plane through apical oxygen for 0≤ x ≤ 0.1. Whereas for x ≥ 0.1, the holes might be trapped or localized in the RuO₂ plane due to a local disorder induced by Al doping. The temperature dependence of the critical field can be fitted as H_c2 (T) = 34 (1-T/Tc)^4.8, suggesting that the vortex dynamics of Ru-1212 might be intrinsically different from that of the other high-T_c cuprates. The largest magnetoresistance (MR of the Al-doped Ru-1212 occurs around magnetic ordering temperature, indicating that the exchange interaction between the carriers in CuO₂ planes and the spins in RuO₂ planes dominates the magneto-transport properties of the Ru-1212 system. The peak position of the 150 cm^-1 involving Cu vibration along the c-axis hardly changes as x increases. It reveals that Al doesn't occupy Cu site. A new Raman active mode around 550 cm^-1 observed in the Al-doped compounds might be attributed to doping-induced A_1g-like apical oxygen mode.