Abstract
Magnetic orderings of the Cu, Gd, and Ru moments in nonsuperconducting Ba 2 GdRu 1-u Cu u O 6 and of the Cu and Ru moments in superconducting Sr 2 YRu 1-u Cu u O 6 (whose superconducting onset temperature is ∼45 K) have been studied using dc susceptibility, microwave magnetic resonance, and neutron diffraction (on Sr 2 YRu 0.85 Cu 0.15 O 6 only). In both homologues, Cu exhibits antiferromagnetism with an ordering temperature of ∼86 K (much greater than the resistive superconductivity onset transition of ∼45 K), and a magnon energy gap ℏω magnon (q=0) that exceeds the microwave photon frequency of ω/2π= 13 GHz. The Cu moment extracted from neutron data for Sr 2 YRu 1-u Cu u O 6 is ∼1.7μ B at low temperature. Gd, in Ba 2 GdRu 1-u Cu u O 6 , is paramagnetic and displays a g = 2 electron spin resonance at temperatures above ∼48 K, which also persists well below ∼48 K (but with a very much broadened line), and orders antiferromagnetically at ∼12 K. Ru in Ba 2 GdRu 1-u Cu u O 6 orders at ∼48 K, but in Sr 2 YRu 1-u Cu u O 6 orders at ∼23 K and has a moment of ∼1.6μ B , extracted from neutron scattering data. In both Sr 2 YRu 1-u Cu u O 6 and Ba 2 GdRu 1-u Cu u O 6 the Ru orders ferromagnetically in the a-b planes with the sheet magnetization alternating in direction as one moves along the c axis, forming a net antiferromagnetic structure. We find no evidence of a Ru signature in the magnetic resonance data anywhere in the range from 3 to 300 K, a result which is consistent with the electrons being itinerant. Attempts to detect Ru magnetic resonances in various other materials have also failed. Since in Sr 2 YRu 1-u Cu u O 6 the magnetic moments of the Ru and the Cu are ordered at low temperatures, its superconductivity is inconsistent with a spin-fluctuation pairing model.