A search for magnetic resonance of ruthenium in octahedral coordination with oxygen

摘要

We have searched, without success, for microwave-frequency magnetic resonance of Ru~(4+) and Ru~(5+) ions in a number of materials for which the Ru are in octahedral coordination with six oxygen. A number of ruthenates including RuO_2, SrRuO_3, Sr_3Ru_2O_7, Ba_3Ru_2NiO_9, Ba_2GdRuO_6, Sr_2YRuO_6, and Ba_2YRuO_6, which include paramagnetic, antiferromagnetic, and ferromagnetic spin configurations, have been examined. We present analysis which shows that the last material provides an optimized opportunity to detect antiferromagnetic Ru resonance for temperatures less than T_N = 39 K; none is detected for frequencies as high as 35 GHz in magnetic fields up to μ_oH = 2T. This result indicates that the antiferromagnetic magnon energy gap exceeds the energy associated with the signal frequency. SrRuO_3 is a known ferromagnetic contaminant phase in the rutheno-cuprates. We report neutron diffraction measurements on SrRuO_3, finding it to have an appreciable local moment at low temperatures, 1.25(0.1 )μs; this moment vanishes near 165 K. We show that it also fails to exhibit ferromagnetic resonance, at least in the range 10-35 GHz. As a result of the diffraction and resonance studies, it is concluded that the reports of ferromagnetic resonance in superconducting rutheno-cuprates are actually due to antiferromagne-tically ordered Cu in these materials, and the presence of even a few percent of SrRuO_3 as a potential contaminant is of little importance.