Structural and magnetic transitions in the planar antiferromagnet Ba4Ir3O10

We report the structural and magnetic ground state properties of the monoclinic compound barium iridium oxide Ba₄Ir₃O₁₀ using a combination of resonant x-ray scattering, magnetometry, and thermodynamic techniques. Magnetic susceptibility exhibits a pronounced antiferromagnetic transition at T_N≈25 K, a weaker anomaly at T_S≈142 K, and strong magnetic anisotropy at all temperatures. Resonant elastic x-ray scattering experiments reveal a second order structural phase transition at T_S and a magnetic transition at T_N. Both structural and magnetic superlattice peaks are observed at L = half integer values. The magnetization anomaly at T_S implies the presence of magnetoelastic coupling, which conceivably facilitates the symmetry lowering. Mean field critical scattering is observed above T_S. The magnetic structure of the antiferromagnetic ground state is discussed based on the measured magnetic superlattice peak intensity. Our study not only presents essential information for understanding the intertwined structural and magnetic properties in Ba₄Ir₃O₁₀ but also highlights the necessary ingredients for exploring novel ground states with octahedra trimers.