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

TL;DR

This study uses ESR spectroscopy to show that the magnetism in Ba$_2$YIrO$_6$ arises from a small fraction of magnetic defects, challenging the expectation of nonmagnetic behavior in 5d$^4$ iridates.

Contribution

It provides direct experimental evidence that magnetism in Ba$_2$YIrO$_6$ is due to magnetic defects, not the Ir$^{5+}$ ions, and supports theories of long-range superexchange.

Findings

Magnetism is caused by a few percent of Ir$^{4+}$ and Ir$^{6+}$ defects.

Ir$^{6+}$ defects show magnetic correlations below 20 K.

Regular Ir$^{5+}$ sites remain nonmagnetic.

Abstract

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.