Comparison of magneto-superconductive properties of RuSr₂GdCu₂O_8-δ and RuSr₂Gd_1.5Ce_0.5Cu₂O_10-δ

Samples of compositions, RuSr₂GdCu₂O_8-δ (Ru-1212) and RuSr₂Gd_1.5Ce_-0.5Cu₂O_10-δ (Ru-1222), were synthesised through solid-state reaction routes. The former crystallized in the space group P4/mmm and the latter in I4/mmm. The magnetization vs. temperature curve measured in 5-Oe field showed a clear branching into zero-field-cooled (ZFC) and field-cooled (FC) curves for Ru-1212 and Ru-1222, respectively, around 140 and 100 K with a cusp at 135 and 80 K and a diamagnetic transition around 20 and 30 K only in the ZFC branch. The two down-turn cusps at 135 and 80 K may be attributed to the antiferromagnetic nature of Ru spins. The isothermal magnetization possessed a non-linear contribution due to a ferromagnetic component at low temperatures below 50 K for both samples. The resistance vs. temperature behaviour of the Ru-1212 samples in applied fields of 0, 3 and 7 T confirmed superconductivity, exhibiting a different type of broadening of the superconductivity transition under magnetic fields from that known for conventional high-T_c superconductors. The magnetoresistance (MR) was negative above the Ru magnetic ordering temperature at 135 K. Below the Ru magnetic ordering temperature MR displayed a positive peak at low fields and became negative at higher fields for Ru-1212. For Ru-1222 MR remained negative both above and below the ordering temperature. A maximum of 2% was observed for the negative MR value at the Ru magnetic ordering temperature. An electron diffraction (ED) pattern from the Ru-1212 sample showed two types of superstructure: one has a weak spot at the centre of the a*-b* rectangle, and the other at the center of the b* edge only. The latter indicates that either Ru/Cu or vacancy ordering of 2b periodicity takes place along the b-direction. Interestingly, Ru-1222 showed only ED patterns with clean a*-b* and a*-c* planes without any superspots.