Orbital Magnetism and Spin-Orbit Effects in the Electronic Structure of ${\mathrm{BaIrO}}_3$

The electronic structure and magnetism of Ir $5d^5$ states in nonmetallic, weakly ferromagnetic ${\mathrm{BaIrO}}_3$ are probed with x-ray absorption techniques. Contrary to expectation, the Ir $5d$ orbital moment is found to be $\sim 1.5$ times larger than the spin moment. This unusual, atomiclike nature of the $5d$ moment is driven by a strong spin-orbit interaction in heavy Ir ions, as confirmed by the nonstatistical large branching ratio at Ir $L_{2,3}$ absorption edges. As a consequence, orbital interactions cannot be neglected when addressing the nature of magnetic ordering in ${\mathrm{BaIrO}}_3$. The local moment behavior persists even as the metallic-paramagnetic phase boundary is approached with Sr doping or applied pressure.

Figure 1

XAS (left axis) and XMCD (right axis) spectra at the Ir $L_{2,3}$ edges for ${\mathrm{Ba}}_{1-x}{\mathrm{Sr}}_x{\mathrm{IrO}}_3$. The inset shows a schematic energy diagram of how the $d$ levels split under the influence of crystal-field and spin-orbit interactions.

Figure 2

Comparison between XAS and XMCD experimental data (dotted lines) and numerical calculations for ${\mathrm{BaIrO}}_3$ (XMCD is scaled to the XAS). (a) Dependence of the spectral line shape on the crystal field with $10Dq=2.5$ (brown dashed line), 2.9 (blue line), and 3.3 (red line). (b) Effect of mixing of $E^{\prime \prime }$ and $U^{\prime }$ ground-state symmetries (approximately $j_{\mathrm{eff}}=1/2$ and $3/2$ states) with 0% (blue line), 10% (red line), and 22% (brown dashed line) $U^{\prime }$ component in the ground state. (c) Effect of the effective exchange field: pure $S_z$ (red line), pure $L_z$ (brown dashed line), and proportional to $L_z+5S_z$ (blue line).

Figure 3

(a) Thermal dependence of XMCD intensity at the Ir $L_3$ edge (left axis) and magnetization data [6]. (b) Ground-state expectation values of $L_z$, $S_z$, and the magnetic moment $\mu =-(⟨L_z⟩+2⟨S_z⟩)$ of ${\mathrm{BaIrO}}_3$ as a function of temperature. Inset: $⟨L_z⟩/⟨S_z⟩$ ratio for several Sr dopings.