Magnetic ordering in spin-orbit Mott insulator Ba₂IrO ₄ probed by μsR

Magnetic properties in the novel spin-orbit Mott insulator Ba ₂IrO₄ were studied using muon spin rotation (μSR) technique. Zero-field μSR experiments revealed that Ba₂IrO ₄ shows an antiferromagnetic transition at T_N ∼ 240 K without any spontaneous magnetization. The most stable μ⁺ site was determined by the electrostatic (Madelung) potential calculation. The effective magnetic moment of the iridium ions (|μ|) in the antiferromagnetic ordered state was calculated using a dipolar-field model, with an internal field obtained by μSR experiments. The magnetic moment is significantly reduced (|μ| ∼ 0.34 μ_B) due to a low-dimensional quantum spin fluctuation with a large intra-plane correlation. The magnetic ground state of the spin-orbit Mott insulator Ba₂IrO₄ is quite similar to those in parent materials of high-T_C cuprate superconductors such as La₂CuO₄.