Unraveling the nature of magnetism of the 5$\boldsymbol{d^4}$ double perovskite Ba$_2$YIrO$_6$

We report electron spin resonance (ESR) spectroscopy results on the double perovskite Ba$_2$YIrO$_6$. On general grounds, this material is expected to be nonmagnetic due to the strong coupling of the spin and orbital momenta of Ir$^{5+}$ (5$d^4$) ions. However, controversial experimental reports on either strong antiferromagnetism with static order at low temperatures or just a weakly paramagnetic behavior have triggered a discussion on the breakdown of the generally accepted scenario of the strongly spin-orbit coupled ground states in the 5$d^4$ iridates and the emergence of a novel exotic magnetic state. Our data evidence that the magnetism of the studied material is solely due to a few percent of Ir$^{4+}$ and Ir$^{6+}$ magnetic defects while the regular Ir$^{5+}$ sites remain nonmagnetic. Remarkably, the defect Ir$^{6+}$ species manifest magnetic correlations in the ESR spectra at $T\lesssim 20$ K suggesting a long-range character of superexchange in the double prevoskites as proposed by recent theories.