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 Sr2IrO4. With the onset of the WFM phase below T-N similar or equal to 240 K, XMCD signals corresponding to XAS peaks respectively identified as originating from the magnetic moments of apical and planar oxygen (O-A and O-P) in the IrO6 octahedra were observed. The observation of magnetic moments at O-A sites is consistent (except for the relative orientation) with that suggested by prior muon spin rotation (mu SR) experiment in the noncollinear antiferromagnetic (NC-AFM) phase below T-M approximate to 100 K. Assuming that the O-A magnetic moment observed by mu SR is also responsible for the corresponding XMCD signal, the magnetic moment of O-P is estimated to be consistent with the previous mu SR result. Since the OA XMCD signal is mainly contributed by Ir 5d zx and yz orbitals which also hybridize with O-P, it is inferred that the relatively large O-P magnetic moment is induced by Ir 5d xy orbitals. Moreover, the inversion of O-A moments relative to Ir moments between the two magnetic phases revealed by XMCD suggests the presence of competing magnetic interactions for O-A, with which the ordering of O-A moments in the NC-AFM phase may be suppressed to T-M.