Partial antiferromagnetism in spin-chain Sr5Rh4O12, Ca5Ir3O12, and Ca4IrO6 single crystals

We report a structural, thermodynamic, and transport study of the newly synthesized Sr5Rh4O12, Ca5Ir3O12, and Ca4IrO6 single crystals. These quasi-one-dimensional insulators consist of a triangular lattice of spin chains running along the c axis, and are commonly characterized by a partial antiferromagnetic (AFM) order, a small entropy removal associated with the phase transitions, and a sizable low-temperature specific heat linearly proportional to temperature. Sr5Rh4O12 is antiferromagnetically ordered below 23 K with strong evidence for an Ising character. Isothermal magnetization of Sr5Rh4O12 exhibits two steplike transitions leading to a ferrimagnetic state at 2.4 T and a ferromagnetic state at 4.8 T, respectively. Ca5Ir3O12 and Ca4IrO6 are also antiferromagnetically ordered below 7.8 and 12 K, respectively, and show an unusually large ratio of the Curie-Weiss temperature to the Neel temperature. In particular, Ca5Ir3O12, which includes both Ir4+ and Ir5+ ions, reveals that only S=1/2 spins of the Ir4+ ions are involved in the magnetic ordering, whereas S=1 spins of the Ir5+ ions remain disordered. All results suggest the presence of a geometrical frustration that causes incomplete long-range AFM order in these quasi-one-dimensional compounds.