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

TL;DR

This study uncovers an unconventional S-orbital state of Tb and reveals a cooperative 4d-4f spin-ordering in Ba3TbRu2O9, highlighting unique magnetic properties driven by strong electron correlations in a 4d-4f system.

Contribution

It demonstrates the existence of an s-like state of Tb4+ with zero orbital moment and a novel cooperative spin-ordering between Ru and Tb in Ba3TbRu2O9, contrasting with other similar compounds.

Findings

Tb4+ exhibits an s-like state with L=0 and S=7/2.

Strong Ru(4d)=Tb(4f) correlations induce cooperative spin ordering.

Distinct antiferromagnetic arrangements of Tb and Ru moments.

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+ (4f7) 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 Neel temperature (around 9.5 K), indicating strong Ru(4d)=Tb(4f) 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).