Influences of alkaline earth metal substitution on the crystal structure and physical properties of magnetic RuSr_1.9A_0.1GdCu ₂O₈ (A = Ca, Sr, and Ba) superconductors

We have investigated the effect of alkaline earth metal substitution on the crystal structure and physical properties of magnetic superconductors RuSr _1.9A_0.1GdCu₂O₈ (A = Ca, Sr, and Ba) in order to probe an interaction between the magnetic coupling of the RuO ₂ layer and the superconductivity of the CuO₂ layer. X-ray diffraction and X-ray absorption spectroscopic analyses demonstrate that the isovalent substitution of Sr ions with Ca or Ba ions makes it possible to tune the interlayer distance between the CuO₂ and the RuO₂ layers. From the measurements of electrical resistance and magnetic susceptibility, it was found that, in contrast to negligible change of magnetization, both of the alkaline earth metal substitutions lead to a notable depression of zero-resistance temperature T_c (ΔT_c ∼ 17-19 K). On the basis of the absence of a systematic correlation between the T_c and the interlayer distance/magnetization, we have concluded that the internal magnetic field of the RuO₂ layer has insignificant influence on the superconducting property of the CuO₂ layer in the ruthenocuprate.