Antiferromagnetism and superconductivity: Determination of the Cu spin-spin relaxation time T2 and the spin-lattice relaxation time T1 in Gd1.5Ce0.5Sr2Cu2RuO10

Gd1.5Ce0.5Sr2Cu2RuO10 exhibits antiferromagnetic resonance at 23.9 GHz for applied fields less than 1000 Oe with a spin-spin relaxation time T2 of approximately 0.45 ns, and with a spin-lattice relaxation time T1 of at least 320 μs. Since in the homologue, Eu1.5Ce0.5Sr2Cu2RuO10, the Ru atoms evidently fail to exhibit magnetic order, the antiferromagnetic resonance must arise from the cuprate planes. In other homologues, the cuprate planes are known to order ferromagnetically and are stacked in an antiferromagnetic configuration. The large value of T1 suggests that phonon mediation plays no role in high temperature superconductivity. In addition, the presence of ferromagnetic cuprate planes is inconsistent with spin-fluctuation models of high temperature superconductivity.