We report the ${}^{139}\mathrm{La}$ nuclear magnetic resonance (NMR), electron spin resonance (ESR), and complementary magnetometry results on the Ir-based double perovskite compounds ${\mathrm{La}}_{2}B{\mathrm{IrO}}_6$ with strongly magnetic ${\mathrm{Co}}^{2+}$ ($3d^7$, $S=3/2$) and nonmagnetic ${\mathrm{Zn}}^{2+}$ ($3d^{10}$, $S=0$) ions at the $B$ site. ESR signals from ${\mathrm{Ir}}^{4+}$ ions in ${\mathrm{La}}_2{\mathrm{ZnIrO}}_6$ and a collective ESR mode of coupled ${\mathrm{Co}}^{2+}$ spins $S=3/2$ and ${\mathrm{Ir}}^{4+}$ pseudospins $j_{\mathrm{eff}}=1/2$ were observed and studied as a function of temperature and excitation frequency. These signals gradually evolve into a resonance mode of the uncompensated ferromagnetic moments in the antiferromagnetically (AFM) ordered state below $T_{\mathrm{N}}\approx 11\mathrm{K}$ and 98 K for ${\mathrm{La}}_2{\mathrm{ZnIrO}}_6$ and ${\mathrm{La}}_2{\mathrm{CoIrO}}_6$, respectively. The temperature evolution of the ${}^{139}\mathrm{La}$ NMR spectra and the nuclear spin relaxation $T_1^{-1}\left(T\right)$ reveal three distinct regimes for ${\mathrm{La}}_{2}B{\mathrm{IrO}}_6$ ($B=\text{Co}$, Zn): (i) a paramagnetic state at high temperatures; (ii) a short-range AFM order in an extended temperature region above $T_{\mathrm{N}}$ where the ESR modes first appear; (iii) magnetic long-range order at lower $T$. The extended short-range-ordered regime at temperatures much higher than $T_{\mathrm{N}}$ constitutes evidence of substantial magnetic frustration which appears to be a salient feature of both studied materials.

• Received 22 June 2018

DOI:https://doi.org/10.1103/PhysRevB.98.174401