Spin-orbit mott state in the novel quasi-2D antiferromagnet Ba ₂IrO₄

The spin-orbit Mott state in the novel quasi-2D iridate Ba ₂IrO₄ was studied on crystal structure, electrical resistivity, magnetic susceptibility, and μSR experiments. Ba ₂IrO₄ crystallizes in a K₂NiF₄-type structure (I4/mmm, a = 4.030(1) Å and c = 13.333(4) Å) which includes IrO₂ square planer lattices with straight Ir-O-Ir bonds. The minimal Mott-gap size is ∼70 meV The magnetic ground state is antiferromagnetic long-range order (T_N ∼ 240 K) without spin canting. The magnetic moment (|μ| ∼ 0.34 μ_B/Ir) is much reduced by a low-dimensional quantum spin fluctuation with a large intra-plane correlation |J|. The critical exponent (β ∼ 0.18) suggests that the magnetic state is classified into 2-D X-Y or (anisotropic) Heisenberg spin systems with weak 3-D interlayer coupling |J'|.