Partial antiferromagnetism in spin-chain Sr5 Rh4 O12, Ca5 Ir3 O12, and Ca4 Ir O6 single crystals

We report a structural, thermodynamic, and transport study of the newly synthesized Sr5 Rh4 O12, Ca5 Ir3 O12, and Ca4 Ir O6 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. Sr5 Rh4 O12 is antiferromagnetically ordered below 23 K with strong evidence for an Ising character. Isothermal magnetization of Sr5 Rh4 O12 exhibits two steplike transitions leading to a ferrimagnetic state at 2.4 T and a ferromagnetic state at 4.8 T, respectively. Ca5 Ir3 O12 and Ca4 Ir O6 are also antiferromagnetically ordered below 7.8 and 12 K, respectively, and show an unusually large ratio of the Curie-Weiss temperature to the Néel temperature. In particular, Ca5 Ir3 O12, 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.