Direct observation of oxygen polarization in Sr$_2$IrO$_4$ by O $K$-edge x-ray magnetic circular dichroism

Abstract: 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 Sr$2$IrO$_4$. With the onset of the WFM phase below $T{\rm N}\simeq240$ K, XMCD signals corresponding to XAS peaks respectively identified as originating from the magnetic moments of apical and planar oxygen (O${\rm A}$ and O${\rm P}$) in the IrO$6$ octahedra were observed. The observation of magnetic moments at O${\rm A}$ sites is consistent (except for the relative orientation) with that suggested by prior muon spin rotation ($\mu$SR) experiment in the non-collinear antiferromagnetic (NC-AFM) phase below $T_{\rm M}\approx100$ K. Assuming that the O${\rm A}$ magnetic moment observed by $\mu$SR is also responsible for the corresponding XMCD signal, the magnetic moment of O${\rm P}$ is estimated to be consistent with the previous $\mu$SR result. Since the O${\rm A}$ XMCD signal is mainly contributed by Ir 5$d$ $zx$ and $yz$ orbitals which also hybridize with O${\rm P}$, it is inferred that the relatively large O${\rm P}$ magnetic moment is induced by Ir 5$d$ $xy$ orbitals. Moreover, the inversion of O${\rm A}$ moments relative to Ir moments between the two magnetic phases revealed by XMCD suggests the presence of competing magnetic interactions for O${\rm A}$, with which the ordering of O${\rm A}$ moments in the NC-AFM phase may be suppressed to $T_{\rm M}$.