Unusual heavy-mass nearly ferromagnetic state with a surprisingly large Wilson ratio in the double layered ruthenates (Sr1-x Cax) 3 Ru2 O7

We report an unusual nearly ferromagnetic heavy-mass state with a surprisingly large Wilson ratio Rw (e.g., Rw ∼700 for x=0.2) in double layered ruthenates (Sr1-x Cax) 3 Ru2 O7 with 0.08<x<0.4. This state does not evolve into a long-range ferromagnetically ordered state despite considerably strong ferromagnetic correlations, but it freezes into a cluster-spin glass at low temperatures. In addition, evidence of non-Fermi-liquid behavior is observed as the spin-freezing temperature of the cluster-spin glass approaches zero near x 0.1. We discuss the origin of this unique magnetic state from the Fermi-surface information probed by Hall-effect measurements.