Antiferromagnetism and superconductivity:: Magnetic order in YSr2Cu2.1Ru0.9O7.9

Superconducting (T-c similar to 40 K) YSr2Cu2.1Ru0.9O7.9, prepared by a high-temperature high-pressure process, has been studied by electron spin resonance (ESR), neutron diffraction and superconducting quantum interference device magnetometry. Neutron diffraction confirms the antiferromagnetic ordering of the Ru atoms at TN-Ru similar to 140 K, as reported by Lynn and Takagiwa ESR data show antiferromagnetic order that persists well above the Ru ordering temperature, with an intensity that peaks close to TN-Ru. The ESR data are very similar to those observed in GdSr2Cu2RuO8 and YSr2Cu2.1Nb0.9O7.9. The apparent ferromagnetism observed at TN-Ru cannot arise from the strongly coupled c-axis-ordered Ru moments, which exhibit no canting; rather it appears to be due to a metamagnetic response of the cuprate planes induced by the Ru order. Magnetization data show the system as consisting of more than one magnetically ordered species; the magnetization is nonlinear well above TN-Ru, indicating that Cu atoms are ordered above that temperature. The neutron diffraction data at 16 K show additional intensity on the (0,0,1) reflection. Although Takagiwa attribute this to ferromagnetism, the polarized neutron data of Lynn on GdSr2Cu2RuO8 rule out this explanation. These data, however, can be successfully modeled with ferromagnetic cuprate planes that are antiferromagnetically stacked. The refined Cu moment is found to be 0.4(0.2)mu(B). The (0,0,1) magnetic Bragg peak is broadened relative to the nuclear peak, presumably due to a short correlation length of the magnetic domains along the c axis. The similarity of the magnetization curves in this compound and in YSr2Cu2.1Nb0.9O7.9 suggests that cuprate plane ordering is a common feature in this class of materials, which is very closely related to YBa2Cu3O7-delta.