Unusual ferromagnetism and strong spin-orbit coupling in Post-Perovskite $CaIrO_3 $*

Strong spin-orbit coupling (SOC) and strong correlations have been considered essential in understanding the unusual physical properties of the 4d and 5d transition-metal oxides, such as the SOC driven Mott insulating state in $Sr_2 IrO_4$. Recently, an unusual atomic-like orbital moment and strong SOC have been confirmed experimentally in $9R-BaIrO_3$ through analysis of the branching ratio at the Ir $L_{2,3}$ absorption edges as obtained from x-ray absorption and x-ray magnetic circular dichroism (XMCD) measurements. We have applied the same techniques to probe unusual ferromagnetism and SOC in the post-perovskite (pPv) $CaIrO_3$, which is an insulator and exhibits weak ferromagnetism below $T_C \approx 110K$. The branching ratio at the Ir $L_{2,3}$ absorption edges, which is close to unity in pPv $CaIrO_3$, appears to indicate an even stronger spin-orbit interaction in the pPv $CaIrO_3$ than in $9R-BaIrO_3$. However, it has been challenging to model the Ir 5d orbital moment, as probed by the XMCD measurements, due to the understood local octahedral-site distortions.