Upper critical field of the magnetic superconductor RuGd_1.4Ce_0.6Sr₂ Cu₂O_10-δ

We performed measurements of magnetic susceptibility, electrical resistivity, and magnetoresistance in magnetic fields of up to 18 T in the magnetic superconductor RuGd_1.4Ce_0.6Sr₂Cu₂O_{10 - δ} synthesized in oxygen pressures up to 95 atm. The magnetic-susceptibility data show the occurrence of an antiferromagnetic state below T_N∼ 175 K, followed by the development of a weak ferromagnetic state near T_M ∼ 100 K, and followed further by the onset of superconductivity (SC) at T_c ∼ 42 K. The electrical resistivity as a function of temperature shows an evolution from nonmetal-to-SC behavior in samples prepared in a flux of O₂ to a well defined metal-like behavior in samples prepared under 95 atm pressure of O₂. The electron-phonon coupling constant was calculated from transport data to be λ_tr∼0.17, a value comparable with other cuprates, indicating weak electron-phonon coupling in these ruthenates. The values of the upper critical field H_c2 for the O₂ high-pressure treated samples were obtained from the magnetoresistivity data yielding H_c2^ab(0)∼39 T, and the out-of-plane superconducting coherence length ξ_c(0) ∼ (0) ∼ 28 Å. Based on the similarities between these ruthenates and the superconductor YBa₂Cu₃O_{7 - δ}, we estimated H_c2^c(0) ∼ 8 T and ξ_ab(0) ∼ 140 Å. We used these parameters to discuss the coexistence of long-range magnetic order and superconductivity on a microscopic scale on these materials.