Magnetization reversal and negative volume thermal expansion in Fe doped   Ca2RuO4
  We report physical and structural properties of single-crystal Ca2Ru1-xFexO4 (0<x<0.20) as functions of temperature, magnetic field and pressure. Ca2RuO4 is a structurally-driven Mott insulator with a metal-insulator (MI) transition at TMI = 357 K, which is well separated from antiferromagnetic order at TN = 110 K. Fe substitution for Ru in Ca2RuO4 causes a pronounced magnetization reversal and giant negative volume thermal expansion (NVTE). The magnetization reversal is a result of a field-induced antiferromagnetic coupling between the Ru- and Fe-magnetic sublattices that have different temperature dependence. The NVTE is closely associated with the orthorhombic distortion, and becomes smaller as the orthorhombicity weakens due to either Fe doping or application of pressure. The study highlights an intriguing interplay between lattice, orbital and spin degrees of freedom that is at the root of the novel phenomena in Ca2RuO4. 
S. J. Yuan, T. F. Qi, J. Terzic, Hao Zheng, Zhao Zhao, Songxue Chi,   Feng Ye, Hua Wei, S. Parkin, Xuerong Liu, Wendy L. Mao...
cond-mat.str-el