Direct observation of oxygen polarization in Sr2IrO4 by O K -edge x-ray magnetic circular dichroism

X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD) measurements at the oxygen (O) $K$ edge were performed to investigate the magnetic polarization of ligand O atoms in the weak ferromagnetic (WFM) phase of the Ir perovskite compound ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$. With the onset of the WFM phase below ${T}_{\mathrm{N}}\ensuremath{\simeq}240$ K, XMCD signals corresponding to XAS peaks respectively identified as originating from the magnetic moments of apical and planar oxygen (${\mathrm{O}}_{\mathrm{A}}$ and ${\mathrm{O}}_{\mathrm{P}}$) in the ${\mathrm{IrO}}_{6}$ octahedra were observed. The observation of magnetic moments at ${\mathrm{O}}_{\mathrm{A}}$ sites is consistent (except for the relative orientation) with that suggested by prior muon spin rotation ($\textmu{}\mathrm{SR}$) experiment in the noncollinear antiferromagnetic (NC-AFM) phase below ${T}_{\mathrm{M}}\ensuremath{\approx}100$ K. Assuming that the ${\mathrm{O}}_{\mathrm{A}}$ magnetic moment observed by $\textmu{}\mathrm{SR}$ is also responsible for the corresponding XMCD signal, the magnetic moment of ${\mathrm{O}}_{\mathrm{P}}$ is estimated to be consistent with the previous $\textmu{}\mathrm{SR}$ result. Since the ${\mathrm{O}}_{\mathrm{A}}$ XMCD signal is mainly contributed by Ir $5d\phantom{\rule{4pt}{0ex}}zx$ and $yz$ orbitals which also hybridize with ${\mathrm{O}}_{\mathrm{P}}$, it is inferred that the relatively large ${\mathrm{O}}_{\mathrm{P}}$ magnetic moment is induced by Ir $5d\phantom{\rule{4pt}{0ex}}xy$ orbitals. Moreover, the inversion of ${\mathrm{O}}_{\mathrm{A}}$ moments relative to Ir moments between the two magnetic phases revealed by XMCD suggests the presence of competing magnetic interactions for ${\mathrm{O}}_{\mathrm{A}}$, with which the ordering of ${\mathrm{O}}_{\mathrm{A}}$ moments in the NC-AFM phase may be suppressed to ${T}_{\mathrm{M}}$.