Possible ordering of Ru and Cu in the charge-reservoir of magneto-superconductor RuSr[sub 2]GdCu[sub 2]O[sub 8] (Ru-1212): Magnetic, transport, and TEM microstructural studies.

Magnetization vs temperature behavior of RuSr[sub 2]GdCu[sub 2]O[sub 8-δ] (Ru-1212) measured in an field of 5 Oe, shows a clear branching of zero-field-cooled (ZFC) and field-cooled (FC) curves around 140 K, a cusp at 135 K, and a diamagnetic transition around 20 K (in the ZFC branch). The cusp at 135 K is due to the antiferromagnetic ordering of the Ru moments. The magnetization-field isotherms, below 50 K, show a nonlinear contribution from a ferromagnetic component. The resistance vs temperature behavior of the compound, in applied fields of 0, 3, and 7 T, confirms that the sample is superconducting at around 20 K. The superconducting transition exhibits field broadening of a type different than that known for conventional high T[sub c] superconductors. The magnetoresistance (MR) is negative above the Ru magnetic ordering temperature of 135 K, while below this temperature, MR displays a positive peak in low fields and becomes negative in higher fields. A maximum of 2% is observed for the negative MR value at the Ru magnetic ordering temperature. An electron diffraction pattern obtained for this Ru-1212 sample shows two types of superstructure; one with a weak spot at the center of the a-b rectangle and the other only along the b direction. It is possible that either Ru/Cu or Ru[sup 4+] / Ru[sup 5+] ordering of 2b periodicity takes place along the b direction. [ABSTRACT FROM AUTHOR]