Antiferromagnetism and superconductivity: Magnetic order inYSr2Cu2.1Ru0.9O7.9

Abstract

Superconducting $({T}_{c}\ensuremath{\sim}40\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ $\mathrm{Y}{\mathrm{Sr}}_{2}{\mathrm{Cu}}_{2.1}{\mathrm{Ru}}_{0.9}{\mathrm{O}}_{7.9}$, 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 ${T}_{N\text{\ensuremath{-}}\mathrm{Ru}}\ensuremath{\sim}140\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, as reported by Lynn et al. and Takagiwa et al. ESR data show antiferromagnetic order that persists well above the Ru ordering temperature, with an intensity that peaks close to ${T}_{N\text{\ensuremath{-}}\mathrm{Ru}}$. The ESR data are very similar to those observed in $\mathrm{Gd}{\mathrm{Sr}}_{2}{\mathrm{Cu}}_{2}\mathrm{Ru}{\mathrm{O}}_{8}$ and $\mathrm{Y}{\mathrm{Sr}}_{2}{\mathrm{Cu}}_{2.1}{\mathrm{Nb}}_{0.9}{\mathrm{O}}_{7.9}$. The apparent ferromagnetism observed at ${T}_{N\text{\ensuremath{-}}\mathrm{Ru}}$ 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 ${T}_{N\text{\ensuremath{-}}\mathrm{Ru}}$, indicating that Cu atoms are ordered above that temperature. The neutron diffraction data at $16\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ show additional intensity on the (0,0,1) reflection. Although Takagiwa et al. attribute this to ferromagnetism, the polarized neutron data of Lynn et al. on $\mathrm{Gd}{\mathrm{Sr}}_{2}{\mathrm{Cu}}_{2}\mathrm{Ru}{\mathrm{O}}_{8}$ 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 $0.4(0.2){\ensuremath{\mu}}_{\mathrm{B}}$. 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 $\mathrm{Y}{\mathrm{Sr}}_{2}{\mathrm{Cu}}_{2.1}{\mathrm{Nb}}_{0.9}{\mathrm{O}}_{7.9}$ suggests that cuprate plane ordering is a common feature in this class of materials, which is very closely related to $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$.