Probing the $J_{eff}=0$ ground state and the Van Vleck paramagnetism of the Ir$^{5+}$ ions in the layered Sr$_2$Co$_{0.5}$Ir$_{0.5}$O$_4$

TL;DR

This study combines experimental and theoretical x-ray magnetic circular dichroism to investigate the magnetic ground state of Ir$^{5+}$ ions in Sr$_2$Co$_{0.5}$Ir$_{0.5}$O$_4$, revealing deviations from a pure $J_{eff}=0$ state but confirming Van Vleck paramagnetism.

Contribution

It provides a detailed analysis of the Ir$^{5+}$ ground state using XMCD and theory, highlighting the effects of multiplet interactions and crystal fields on the magnetic properties.

Findings

Deviation from pure $J_{eff}=0$ ground state observed

Large energy gap supports Van Vleck paramagnetism

Anisotropic orbital-to-spin moment ratio measured

Abstract

We report a combined experimental and theoretical x-ray magnetic circular dichroism (XMCD) spectroscopy study at the Ir-$L_{2,3}$ edges on the Ir$^{5+}$ ions of the layered hybrid solid state oxide Sr$_2$Co$_{0.5}$Ir$_{0.5}$O$_4$ with the K$_2$NiF$_4$ structure. From theoretical simulation of the experimental Ir-$L_{2,3}$ XMCD spectrum, we found a deviation from a pure $J_{eff}=0$ ground state with an anisotropic orbital-to-spin moment ratio ($L_x/2S_x$ = 0.43 and $L_z/2S_z$ = 0.78). This deviation is mainly due to multiplet interactions being not small compared to the cubic crystal field and due to the presence of a large tetragonal crystal field associated with the crystal structure. Nevertheless, our calculations show that the energy gap between the singlet ground state and the triplet excited state is still large and that the magnetic properties of the Ir$^{5+}$ ions can be well described in terms of singlet Van Vleck paramagnetism.