Thermal and magnetic properties of the spin-chain material Pr 3 Ru O 7

We present measurements of the magnetic moment and specific heat of single crystals of insulating ${\mathrm{Pr}}_{3}\mathrm{Ru}{\mathrm{O}}_{7}$, with octahedral $\mathrm{Ru}{\mathrm{O}}_{6}$ and pseudocubic $\mathrm{Pr}{\mathrm{O}}_{8}$ chains, separated by layers of sevenfold-coordinate praseodymium ions. The susceptibility indicates that antiferromagnetic order sets in at ${T}_{N}=54\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, with polarization approximately along the chain direction, but the susceptibility anisotropy above ${T}_{N}$ indicates that the exchange interaction is strongly anisotropic, with competing ferromagnetic interactions. This competition presumably gives rise to the observed metamagnetic behavior for applied fields along the chain direction. The specific heat anomaly at ${T}_{N}$ is unusually mean field in shape and roughly consistent with ordering of the moments of the ruthenium ions and the ``chain'' praseodymium ions, whereas the remaining interlayer praseodymium ions appear to be magnetically inert. A Schottky-type anomaly at low temperature, however, suggests that the ``ordered'' chain praseodymium ions are still sensitive to a crystal field.