  We have discovered a novel candidate for a spin liquid state in a ruthenium
oxide composed of dimers of $S = $ 3/2 spins of Ru$^{5+}$,Ba$_3$ZnRu$_2$O$_9$.
This compound lacks a long range order down to 37 mK, which is a temperature
5000-times lower than the magnetic interaction scale of around 200 K. Partial
substitution for Zn can continuously vary the magnetic ground state from an
antiferromagnetic order to a spin-gapped state through the liquid state. This
indicates that the spin-liquid state emerges from a delicate balance of inter-
and intra-dimer interactions, and the spin state of the dimer plays a vital
role. This unique feature should realize a new type of quantum magnetism.
