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 $T_{\mathrm{N}}\simeq 240$ K, XMCD signals corresponding to XAS peaks respectively identified as originating from the magnetic moments of apical and planar oxygen (O${}_{\mathrm{A}}$ and O${}_{\mathrm{P}}$) in the IrO${}_6$ octahedra were observed. The observation of magnetic moments at O${}_{\mathrm{A}}$ 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 $T_{\mathrm{M}}\approx 100$ K. Assuming that the O${}_{\mathrm{A}}$ magnetic moment observed by $\mu$SR is also responsible for the corresponding XMCD signal, the magnetic moment of O${}_{\mathrm{P}}$ is estimated to be consistent with the previous $\mu$SR result. Since the O${}_{\mathrm{A}}$ XMCD signal is mainly contributed by Ir 5$d$ $zx$ and $yz$ orbitals which also hybridize with O${}_{\mathrm{P}}$, it is inferred that the relatively large O${}_{\mathrm{P}}$ magnetic moment is induced by Ir 5$d$ $xy$ orbitals. Moreover, the inversion of O${}_{\mathrm{A}}$ moments relative to Ir moments between the two magnetic phases revealed by XMCD suggests the presence of competing magnetic interactions for O${}_{\mathrm{A}}$, with which the ordering of O${}_{\mathrm{A}}$ moments in the NC-AFM phase may be suppressed to $T_{\mathrm{M}}$.

6 pages, 6 figures

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