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

Abstract

We report electron spin resonance (ESR) spectroscopy results on the double perovskite Ba2YIrO6. On general grounds, this material is expected to be nonmagnetic due to the strong coupling of the spin and orbital momenta of Ir 5+ (5d 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 5d 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 20 K suggesting a long-range character of superexchange in the double perovskites as proposed by recent theories.