We report an unusual magnetic ground state in single-crystal, double-perovskite $\mathrm{B}{\mathrm{a}}_2\mathrm{YIr}{\mathrm{O}}_6$ and Sr-doped $\mathrm{B}{\mathrm{a}}_2\mathrm{YIr}{\mathrm{O}}_6$ with $\mathrm{I}{\mathrm{r}}^{5+}\left(5d^4\right)$ ions. Long-range magnetic order below 1.7 K is confirmed by dc magnetization, ac magnetic susceptibility, and heat-capacity measurements. The observed magnetic order is extraordinarily delicate and cannot be explained in terms of either a low-spin $S=1$ state, or a singlet $J_{\mathrm{eff}}=0$ state imposed by the spin-orbit interactions (SOI). Alternatively, the magnetic ground state appears consistent with a SOI that competes with comparable Hund's rule coupling and inherently large electron hopping, which cannot stabilize the singlet $J_{\mathrm{eff}}=0$ ground state. However, this picture is controversial, and conflicting magnetic behavior for these materials is reported in both experimental and theoretical studies, which highlights the intricate interplay of interactions that determine the ground state of materials with strong SOI.

• Received 1 September 2016
• Revised 4 May 2017

DOI:https://doi.org/10.1103/PhysRevB.96.064436