# Intricacies of the Co$^{3+}$ spin state in Sr$_2$Co$_{0.5}$Ir$_{0.5}$O$_4$: an x-ray absorption and magnetic circular dichroism study

## TL;DR

This study uses x-ray absorption and magnetic circular dichroism to reveal that Co$^{3+}$ ions in Sr$_2$Co$_{0.5}$Ir$_{0.5}$O$_4$ are in a high spin state with an unquenched orbital moment, challenging expectations based on octahedral elongation.

## Contribution

It provides direct spectroscopic evidence of the high spin state of Co$^{3+}$ in a specific oxide, and uses model calculations to analyze its stability against other spin states.

## Key findings

- Co$^{3+}$ ions are in a high spin state with $S=2$
- Significant unquenched orbital moment observed
- High spin state remains stable despite octahedral elongation

## Abstract

We report on a combined soft x-ray absorption and magnetic circular dichroism (XMCD) study at the Co-$L_{3,2}$ on the hybrid 3$d$/5$d$ solid state oxide Sr$_2$Co$_{0.5}$Ir$_{0.5}$O$_4$ with the K$_2$NiF$_4$ structure. Our data indicate unambiguously a pure high spin state $(S=2)$ for the Co$^{3+}$ (3$d^6$) ions with a significant unquenched orbital moment $L_z/2S_z=0.25$ despite the sizeable elongation of the CoO$_6$ octahedra. Using quantitative model calculations based on parameters consistent with our spectra, we have investigated the stability of this high spin state with respect to the competing low spin and intermediate spin states.