Magnetic structures of nearly isostructural Tb3RuO7 and Nd3RuO7 : Appearance of a partially disordered state only in the Tb compound

We report on the magnetic structures of ${\\mathrm{Tb}}_{3}{\\mathrm{RuO}}_{7}$ and ${\\mathrm{Nd}}_{3}{\\mathrm{RuO}}_{7}$ determined from powder neutron-diffraction experiments. In ${\\mathrm{Tb}}_{3}{\\mathrm{RuO}}_{7}$, alternating-bond spin-$\\frac{3}{2}$ Ru1-Ru2 chains are formed. The Ru1 and Ru2 moments are ordered and disordered (paramagnetic), respectively, below 15 K. This result indicates the appearance of a partially disordered (PD) state, although the two Ru sites are very similar to each other. In ${\\mathrm{Nd}}_{3}{\\mathrm{RuO}}_{7}$, the Ru1 and Ru2 ions form independent uniform chains. Both the Ru1 and Ru2 moments are ordered below 17.5 K. Probably, the main source of the PD state in ${\\mathrm{Tb}}_{3}{\\mathrm{RuO}}_{7}$ is the magnetic frustration in the exchange interactions. The internal magnetic field generated by the Tb ordered moments at the Ru1 sites is different from that at the Ru2 sites. We speculate that the different (nonuniform) internal magnetic field enhances the difference in the properties between the Ru1 and Ru2 moments. We also report on the magnetic entropy changes of ${\\mathrm{Tb}}_{3}{\\mathrm{RuO}}_{7}$ and ${\\mathrm{Gd}}_{3}{\\mathrm{RuO}}_{7}$.