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 $L{n}_{2}{\mathrm{NiIrO}}_{6}$ ($Ln=\text{La}$, 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 $3d\text{\ensuremath{-}}5d\text{\ensuremath{-}}4f$ compositions. Magnetic structure determination using neutron diffraction reveals antiferromagnetism for ${\mathrm{La}}_{2}{\mathrm{NiIrO}}_{6}$, a collinear ferrimagnetic Ni and Ir state that is driven to long-range antiferromagnetism upon the onset of Nd ordering in ${\mathrm{Nd}}_{2}{\mathrm{NiIrO}}_{6}$, and a noncollinear ferrimagnetic Ni and Ir sublattice interpenetrated by a ferromagnetic Pr lattice for ${\mathrm{Pr}}_{2}{\mathrm{NiIrO}}_{6}$. For ${\mathrm{Pr}}_{2}{\mathrm{NiIrO}}_{6}$, 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 ${\mathrm{Pr}}_{2}{\mathrm{NiIrO}}_{6}$ and offers insight into the behavior in ${\mathrm{La}}_{2}{\mathrm{NiIrO}}_{6}$ and ${\mathrm{Nd}}_{2}{\mathrm{NiIrO}}_{6}$. Resonant inelastic x-ray scattering is consistent with spin-orbit coupling splitting the ${t}_{2g}$ manifold of octahedral ${\mathrm{Ir}}^{4+}$ into a ${J}_{\mathrm{eff}}=\frac{1}{2}$ and ${J}_{\mathrm{eff}}=\frac{3}{2}$ state for all members of the series considered.