Confirmation of complex non-collinear Fe(II) spin ordering in the iron oxychalcogenide La$_2$O$_2$Fe$_2$OSe$_2$ by broad range $^{139}$La-NQR
The transition metal oxychalcogenide La$_2$O$_2$Fe$_2$OSe$_2$ exhibits a non-collinear spin-ice like antiferromagnetic ordering below $T_N \approx $ 90 K due to complex magnetic exchange interactions and axial single-ion anisotropy of Fe$^{2+}$ spins. The magnetic structure was deduced/determined from combined $^{57}$Fe Mössbauer spectroscopy and $^{139}$La-NMR on polycrystalline specimen. In this work, we present zero-field $^{139}$La-NQR investigations on polycrystalline La$_2$O$_2$Fe$_2$OSe$_2$ in a broad frequency and temperature range between 2.8 - 20 MHz and 4.2 - 100 K. The measured resonances at $ν_Q$, $2 \, ν_Q$, and $3 \, ν_Q$ are fully consistent with the previous NMR studies and also with the magnetic structure. A substantial modification of the electric field gradient at the La-position below $T_N$ is inconsistent with the current investigations.