Magnetic ordering and enhanced electronic heat capacity in insulatingL2RuO5(L=Pr,Nd, Sm, Gd, and Tb)

Abstract

Polycrystalline samples of the lanthanide ruthenate series ${L}_{2}{\mathrm{RuO}}_{5}$ $(L=\mathrm{Pr},$ Nd, Sm, Gd, and Tb) magnetically order in the range $8<T<24\mathrm{K}$ with the ordering involving both the $L$ and Ru cations. The series has a complex orthorhombic structure (space group $Pnma)$ possessing chains of ${\mathrm{RuO}}_{5}$ 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 $H<5\mathrm{T}$ 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, \ensuremath{\gamma}, for four compounds is anomalously large, ranging from $\ensuremath{\gamma}=229$ to 774 mJ/mole ${\mathrm{K}}^{2}$, for $L=\mathrm{Nd}$ and $L=\mathrm{Gd},$ respectively. These values are comparable to those for heavy fermion systems, yet all five members of the ${L}_{2}{\mathrm{RuO}}_{5}$ series exhibit semiconducting to insulating behavior. Isomorphic ${\mathrm{Gd}}_{2}{\mathrm{TiO}}_{5},$ where Ti with no unpaired d electrons ${(4d}^{0}$ configuration) replaces Ru ${(4d}^{4}$ configuration), has a vanishingly small \ensuremath{\gamma}, indicating that the thermodynamic properties of ${L}_{2}{\mathrm{RuO}}_{5}$ are dominated by unpaired $4d$ electrons. The $4d$ electrons significantly enhance the $L\ensuremath{-}L$ interaction, as demonstrated by the dramatic increase in ordering temperature for ${\mathrm{Gd}}_{2}{\mathrm{RuO}}_{5}$ ${(T}_{M}=10\mathrm{K})$ compared to ${\mathrm{Gd}}_{2}{\mathrm{TiO}}_{5}$ which does not appear to order magnetically.