Magnetic ordering and enhanced electronic heat capacity in insulating L2RuO5 (L = Pr, Nd, Sm, Gd, and Tb) -: art. no. 144427

Polycrystalline samples of the lanthanide ruthenate series L2RuO5 (L = Pr, Nd, Sm, Gd, and Tb) magnetically order in the range 8 < T < 24 K with the ordering involving both the L and Ru cations. The series has a complex orthorhombic structure (space group Pnma) possessing chains of RuO5 five oxygen coordinated square pyramids which are corner-sharing and two inequivalent seven coordinated L sites which are edge-sharing. At T-M a weak ferromagnetic moment emerges upon cooling, most likely due to a canted antiferromagnetic spin configuration. Isothermal dc and pulsed field magnetization for T < T-M reveal field induced magnetic transitions for H < 5 T but the approach to full free ion magnetic saturation is incomplete even at fields to 60 T due to the influence of crystalline electric fields on the L ions. The low temperature linear specific heat coefficient, gamma, for four compounds is anomalously large, ranging from gamma = 229 to 774 mJ/mole K-2, for L = Nd and L = Gd, respectively. These values are comparable to those for heavy fermion systems, yet all five members of the L2RuO5 series exhibit semiconducting to insulating behavior. Isomorphic Gd2TiO5, where Ti with no unpaired d electrons (4d(0) configuration) replaces Ru (4d(4) configuration), has a vanishingly small gamma, indicating that the thermodynamic properties of L2RuO5 are dominated by unpaired 4d electrons. The 4d electrons significantly enhance the L-L interaction, as demonstrated by the dramatic increase in ordering temperature for Gd2RuO5 (T-M =10 K) compared to Gd2TiO5 which does not appear to order magnetically.