paper

Varying magnetism in the lattice distorted Y2NiIrO6 and La2NiIrO6

arXiv:2310.18641 · doi:10.1103/PhysRevB.108.174428

Abstract

We investigate the electronic and magnetic properties of the newly synthesized double perovskites Y $_{2}$ NiIrO $_{6}$ and La $_{2}$ NiIrO $_{6}$ , 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 Ni $^{2 +}$ $t_{2 g}$ $^{6} e_{g}$ $^{2}$ ( $S = 1$ ) and the low-spin Ir $^{4 +}$ $t_{2 g}$ $^{5}$ ( $S = 1/2$ ) configurations. We address that their lattice distortion induces $t_{2 g}$ - $e_{g}$ orbital mixing and thus enables the normal Ni $^{+}$ -Ir $^{5 +}$ charge excitation with the electron hopping from the Ir ` $t_{2 g}$ ' to Ni ` $e_{g}$ ' orbitals, which promotes the AFM Ni $^{2 +}$ -Ir $^{4 +}$ coupling. Therefore, the increasing $t_{2 g}$ - $e_{g}$ mixing accounts for the enhanced $T_{N}$ from the less distorted La $_{2}$ NiIrO $_{6}$ to the more distorted Y $_{2}$ NiIrO $_{6}$ . Moreover, our test calculations find that in the otherwise ideally cubic Y $_{2}$ NiIrO $_{6}$ , the Ni $^{+}$ -Ir $^{5 +}$ charge excitation is forbidden, and only the abnormal Ni $^{3 +}$ -Ir $^{3 +}$ excitation gives a weakly ferromagnetic (FM) behavior. Furthermore, we find that owing to the crystal field splitting, Hund exchange, and broad band formation in the highly coordinated fcc sublattice, Ir $^{4 +}$ ions are not in the $j_{eff} = 1/2$ state but in the $S = 1/2$ state carrying a finite orbital moment by spin-orbit coupling (SOC). This work clarifies the varying magnetism in Y $_{2}$ NiIrO $_{6}$ and La $_{2}$ NiIrO $_{6}$ associated with the lattice distortions.

7 pages, 7 figures, 1 table