Relationship between A-site cation and magnetic structure in 3d-5d-4f double perovskite iridates Ln2NiIrO6 (Ln = La, Pr, Nd)

We report a comprehensive investigation of Ln(2)NiIrO(6) (Ln = La, Pr, Nd) using thermodynamic and transport properties, neutron powder diffraction, resonant inelastic x-ray scattering, and density-functional theory (DFT) calculations to investigate the role of A-site cations on the magnetic interactions in this family of hybrid 3d-5d-4f compositions. Magnetic structure determination using neutron diffraction reveals antiferromagnetism for La2NiIrO6, a collinear ferrimagnetic Ni and Jr state that is driven to long-range antiferromagnetism upon the onset of Nd ordering in Nd2NiIrO6, and a noncollinear ferrimagnetic Ni and Ir sublattice interpenetrated by a ferromagnetic Pr lattice for Pr2NiIrO6. For Pr2NiIrO6, heat-capacity results reveal the presence of two independent magnetic sublattices, and transport resistivity indicates insulating behavior and a conduction pathway that is thermally mediated. A first principles DFT calculation elucidates the existence of the two independent magnetic sublattices within Pr2NiIrO6 and offers insight into the behavior in La2NiIrO6 and Nd2NiIrO6. Resonant inelastic x-ray scattering is consistent with spin-orbit coupling splitting the t(2g) manifold of octahedral Ir4+ into a J(eff) = and J(eff) = 3/2 state for all members of the series considered.