Unusual magnetic ground states and the effect of spin-orbit coupling in pentavalent iridates

Pentavalent iridium compounds are predicted to be stabilized in an unusual J=0 nonmagnetic ground state in the ambit of j-j coupling regime. However, we show using detailed magnetic and thermodynamic studies and theoretical calculations that the ground state of 6H hexagonal Ba₃ZnIr₂O₉ does develop magnetic moments which do not order magnetically (confirmed by low temperature Neutron Powder Diffraction and muon spin relaxation measurements) and are instead stabilized into a novel spin-orbital liquid ground state. Our results reveal that this system with Ir⁵⁺ (5d⁴) ions and strong spin-orbit coupling (SOC) arrives very close to the elusive J = 0 state but each Ir ions still possess a weak moment through complex pathways. It is observed that there is a complex interplay between SOC and crystal field effects which finally decides the fate of this compound. I would also like to touch upon a controversial double perovskite compound Ba₂YIrO₆ about which absolutely contradictory views exist in the literature. One view suggests a perfect J=0 state without any magnetic contribution while the other emphasizes breakdown of the J=0 state as well as magnetic ordering at low temperatures. Here I would like to show that this may be very close to the coveted J=0 state while tiny moments do form but fail to order down to 60 mK. (author)