Unconventional s-orbital state of Tb and cooperative Ru(4d)-Tb(4f ) spin-ordering in strongly correlated 4d-4f system, Ba3TbRu2O9

Abstract

The 6H-perovskite Ba3RRu2O9 (R = rare-earth), composed of Ru2O9 dimers connected through RO6 octahedra, exhibits an intriguing variety of magnetic ground states – ranging from non-magnetic to ferromagnetic and antiferromagnetic, depending on the specific R ion. In this study, we investigate the compound Ba3TbRu2O9 using magnetic susceptibility measurements and time-of-flight neutron diffraction experiments. Our combined bulk and microscopic analyses reveal that the Tb4+ (4 f7) electronic configuration results in an s-like state with an orbital moment L = 0 and spin-only value of S = 7/2, and Ru4+ exhibits a spin-only value of S = 1, despite the presence of strong spin-lattice coupling in this compound, representing a sharp contrast to other reported members of this family. A cooperative 4d–4f spin ordering is observed below the Néel temperature (TN ∼ 9.5 K), indicating strong Ru(4d)–Tb(4 f) correlations in the system. The Tb-moments order antiferromagnetically in the bc-plane, whereas the Ru-moments are aligned antiferromagnetically along the b-axis. Furthermore, a collinear antiferromagnetic arrangement of spins is observed within the Ru2O9 dimers throughout the structure, unlike other reported members of this series (e.g., Ho and Nd).