Varying magnetism in the lattice distorted Y2NiIrO6 and La2NiIrO6

We investigate the electronic and magnetic properties of the synthesized double perovskites Y2NiIrO6 and La2NiIrO6, using density functional calculations, crystal field theory, superexchange pictures, and Monte Carlo simulations. We find that both systems are antiferromagnetic (AFM) Mott insulators, with the high-spin Ni2+ t62ge2g (S = 1) and the low-spin Ir4+ t52g (S = 1/2) configurations. We address that their lattice distortion induces t2g-eg orbital mixing and thus enables the normal Ni+-Ir5+ charge excitation with the electron hopping from the Ir 't2g' to Ni 'eg' orbitals, which promotes the AFM Ni2+-Ir4+ coupling. Therefore, the increasing t2g-eg mixing accounts for the enhanced TN from the less distorted La2NiIrO6 to the more distorted Y2NiIrO6. Moreover, our test calculations find that in the otherwise ideally cubic Y2NiIrO6, the Ni+-Ir5+ charge excitation is forbidden, and only the abnormal Ni3+-Ir3+ excitation gives a weakly ferromagnetic behavior. Furthermore, we find that, owing to the crystal field splitting, Hund exchange, and broad band formation in the highly coordinated fcc sublattice, Ir4+ ions are not in the jeff = 1/2 state but in the S = 1/2 state carrying a finite orbital moment by spin-orbit coupling. This paper clarifies the varying magnetism in Y2NiIrO6 and La2NiIrO6 associated with the lattice distortions.