Properties of Quasi-Two-Dimensional Ba₇Ru₄Cl₂O₁₅ and Ba₇Ru₄Br₂O₁₅

We have systematically studied newly synthesized quasi-two-dimensional Ba₇Ru₄Cl₂O₁₅ and Ba₇Ru₄Br₂O₁₅ in single-crystal form. These two compounds with a similar crystal structure host in double-layer, face-sharing Ru₂O₉ dimers that are weakly connected with Ba-Cl/Br layers along the c-axis. The long superexchange pathway through a Ru-O-Ba-Cl-Ba-O-Ru chain along the c-axis renders a quasi-two-dimensional lattice and supports a robust antiferromagnetic state with a Néel temperature at 40K. In addition, all results indicate that the onset of the antiferromagnetic order is insensitive to the c-axis lattice parameter, leaving magnetism of these materials an intriguing topic to explore. The magnetic order is also accompanied by a sizable frustration parameter (up to 6) and a rather large Sommerfeld coefficient, suggesting significant frustration.