Exploring Low Energy Excitations in the d5 Iridate Double Perovskites La2BIrO6 (B = Zn, Mg)

We experimentally investigate the structural, magnetic, transport, and electronic properties of two d(5) iridate double perovskite materials La2BIrO6 (B = Mg, Zn). Notably, despite similar crystallographic structure, the two compounds show distinctly different magnetic behaviors. The M = Mg compound shows an antiferromagnetic-like linear field-dependent isothermal magnetization below its transition temperature, whereas the M = Zn counterpart displays a clear hysteresis loop followed by a noticeable coercive field, indicative of ferromagnetic components arising from a non-collinear Ir spin arrangement. The local structure studies authenticate perceptible M/Ir antisite disorder in both systems, which complicates the magnetic exchange interaction scenario by introducing Ir-O-Ir superexchange pathways in addition to the nominal Ir-O-B-O-Ir super-superexchange interactions expected for an ideally ordered structure. While spin-orbit coupling (SOC) plays a crucial role in establishing insulating behavior for both these compounds, the rotational and tilting distortions of the IrO6 (and MO6) octahedral units further lift the ideal cubic symmetry. Finally, by measuring the Ir-L-3 edge resonant inelastic X-ray scattering (RIXS) spectra for both the compounds, giving evidence of spin-orbit-derived low-energy inter-J-state (intra t(2g)) transitions (below similar to 1 eV), the charge transfer (O 2p -> Ir 5d), and the crystal field (Ir t(2g) -> e(g)) excitations, we put forward a qualitative argument for the interplay among effective SOC, non-cubic crystal field, and intersite hopping in these two compounds.