Unraveling the Nature of Magnetism of the 5d4 Double Perovskite Ba2YIrO6

We report electron spin resonance (ESR) spectroscopy results on the double perovskite ${\mathrm{Ba}}_{2}{\mathrm{YIrO}}_{6}$. On general grounds, this material is expected to be nonmagnetic due to the strong coupling of the spin and orbital momenta of ${\mathrm{Ir}}^{5+}$ ($5{d}^{4}$) 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 ${\mathrm{Ir}}^{4+}$ and ${\mathrm{Ir}}^{6+}$ magnetic defects while the regular ${\mathrm{Ir}}^{5+}$ sites remain nonmagnetic. Remarkably, the defect ${\mathrm{Ir}}^{6+}$ species manifest magnetic correlations in the ESR spectra at $T\ensuremath{\lesssim}20\text{ }\text{ }\mathrm{K}$, suggesting a long-range character of superexchange in the double perovskites as proposed by recent theories.