Ba₃NiIr₂O₉: a three-dimensional spin liquid with two magnetic sublattices

The quantum spin liquid (QSL) is an exotic phase of magnetic materials where the spins continue to fluctuate without any symmetry breaking down to zero temperature. Over the past 15 years, several quantum spin liquid candidates have been proposed with spin – ½ in 2D frustrated lattices. But it is difficult to realize such a state in 3D systems with higher spins as these factors may suppress quantum fluctuations giving ordering in the system. We devised a new strategy to find such candidates by utilizing two magnetically active ions at different crystallographic locations. We have synthesized Ba₃NiIr₂O₉, which suits our expectations. Here both Ni and Ir ions are magnetically active and form a 2D frustrated triangular lattice. X-ray absorption spectroscopy measurements confirm the presence of Ni²⁺ (S=1). Absence of any spin ordering/freezing and existence of a gapless QSL state at least down to 100 mK is confirmed by magnetic susceptibility, heat capacity, and µ-SR measurements. Ab-initio calculations find a strong magnetic exchange between Ir and Ni sublattices and in-plane antiferromagnetic coupling between the dimers, resulting in dynamically fluctuating magnetic moments on both the Ir and Ni sublattice.