Magnetic structure of the double perovskite Formula Presented investigated using neutron diffraction

A polycrystalline sample of Formula Presented has been synthesized and characterized using magnetization and neutron powder diffraction to explore the magnetic ground state. Three anomalies are evident in the bulk magnetization measurements at 28, 50, and 74 K, of which only the 74 K transition is associated with long-range ordering of Formula Presented and Formula Presented spins. The neutron thermodiffractogram recorded between 1.5 and 90 K and Rietveld refinements of the temperature difference data 1.5-90 K, 40-90 K, and 62-90 K confirm that both the Ni and Ir moments order at the same temperature below 74 K. No change in the magnetic peak intensities or new peaks have been found further below 50 and 28 K indicating that the spin structure remains the same down to 1.5 K. The magnetic propagation vector is Formula Presented and the estimated magnetic moment values at 1.5 K are Formula Presented and Formula Presented. Contrary to a recent publication by Ferreira et al. [Phys. Rev. Mater. 5, 064406 (2021)10.1103/PhysRevMaterials.5.064408] on the same compound, the improved data quality allows now to ascertain the presence of an ordered moment on the iridium sites. The magnetic structure proposed by Ferreira et al. is discussed and it is shown that there are several magnetic structures compatible with the data but indistinguishable by neutron powder diffraction methods.