Critical magnetic fluctuations in the layered ruthenates Ca₂RuO₄ and Ca₃Ru₂O₇ studied by neutron spectroscopy

The unconventional magnetic and electronic properties of ruthenates and other transition metal oxides with 4d valence electrons often arise from a complex interplay between spin-orbit coupling, crystal field effects, and Hund's coupling. In single-layer Ca₂RuO₄ recent neutron spectroscopy experiments revealed a novel type of soft-magnetism with strong single-ion anisotropy and ‘Higgs’ amplitude fluctuations in the spin-wave spectrum. On the other hand, bilayer Ca₃Ru₂O₇ exhibits competing antiferromagnetic (AFM) Mott-insulating and AFM-metallic phases, and even minute doping by non-magnetic Ti-ions can alter the magnetic correlations significantly. Here we report on high-resolution neutron triple-axis spectroscopy measurements of critical AFM fluctuations in proximity to the Néel temperatures of Ca₂RuO_4, Ca₃Ru₂O₇, and 1% Ti-doped Ca₃Ru₂O₇. Such fluctuations are fundamentally related to the nature of the magnetic correlations and can provide new insights into the complex magnetism of single and bilayer ruthenates.