Effect of Sn, Nb, and La substitution on superconductivity and magnetic order in RuSr2RCu2o8 (R=Eu,Gd)

초록

We report the results from a resistance, and magnetization study on Ru<inf>1-x</inf>A<inf>x</inf>Sr<inf>2-z</inf>B<inf>z</inf>RCu<inf>2</inf> O<inf>8</inf> (A = Sn,Nb, B = La, R = Eu,Gd) where the parent compound has previously been reported to display superconductivity and predominately low field antiferromagnetic order. We show that, contrary to a study on Ru<inf>1-x</inf>Sn<inf>x</inf>Sr<inf>2</inf>GdCu<inf>2</inf>O<inf>8</inf>, there is no significant change in the superconducting transition temperature when Sn is substituted for Ru, implying that there is no significant Sn-induced charge transfer to the CuO<inf>2</inf> planes. Furthermore, increasing the Sn concentration has the effect of increasing the temperature where zero resistance and the onset of bulk ac diamagnetic shielding occurs, which we interpret in terms of a decreasing ferromagnetic component leading to an increase in the temperature where the spontaneous vortex phase occurs. Contrary to previous studies on the pure compounds, we find an initial negative dM/dB at low temperatures in magnetically ordered Ru<inf>1-x</inf>Sn<inf>x</inf>Sr<inf>2</inf>RCu<inf>2</inf>O<inf>8</inf> for Sn concentrations at, and greater than, 0.1. The substitution of Ru by nuclei that have a larger (Nb5+) or smaller (Sn4+) valence results in a similar decrease in the magnetic ordering temperature and an increase in the average Ru valence. La substitution for Sr causes charge transfer to the CuO<inf>2</inf> planes and an increase in the magnetic ordering temperature that is not expected.