Origin of magnetic moments and presence of spin-orbit singlets in Ba2YIrO6

While it was speculated that 5d4 systems would possess nonmagnetic J=0 ground state due to strong spin-orbit coupling (SOC), all such systems have invariably shown the presence of magnetic moments so far. A puzzling case is that of Ba2YIrO6, which in spite of having a perfectly cubic structure with largely separated Ir5+(d4) ions, has consistently hosted weak magnetic moments. We show from muon spin relaxation (μSR) measurements that a change in the magnetic environment of the implanted muons in Ba2YIrO6 occurs as the temperature is lowered below 10 K indicating magnetic response has a temperature dependence unlikely for a J=0 system. Interestingly, the estimated value of SOC obtained by fitting the resonant inelastic x-ray scattering (RIXS) spectrum of Ba2YIrO6 with an effective atomic many-body Hamiltonian is found to be as high as 0.39 eV, comparable to the reported value of SOC in other magnetic d4 iridates with noncubic distortion. We argue that in addition to strong SOC, the presence of intersite hopping triggers delocalization of holes, resulting in the spontaneous generation of magnetic moments. Our theoretical calculations further indicate that these moments favor the formation of spin-orbital singlets in the case of Ba2YIrO6, which is manifested in μSR experiments measured down to 60 mK.