Magnetic properties and crystal structure of Sr₃CoIrO₆ and Sr₃NiIrO₆

We have studied the magnetic properties and crystal structure of Sr₃CoIrO₆ and Sr₃NiIrO₆ as a function of temperature. Two characteristic temperatures, T₁ = 90 K and T₂ = 25 K for Sr₃CoIrO₆, and T₁ = 85 K and T₂ = 15 K for Sr₃NiIrO₆, were observed. Below T₁ a significant increase of magnetization and below T₂ a weak temperature dependence of magnetization in the field-cooled and practically zero magnetization values in the zero-field-cooled mode were detected for both compounds. The existence of Ir⁴⁺ in Sr₃CoIrO₆ was confirmed by an Ir-L_III x-ray absorption measurement. Magnetoelastic effects have been observed in the temperature dependence of the lattice parameters of Sr₃CoIrO₆ and Sr₃NiIrO₆. The magnetic structure of Sr₃CoIrO₆ in zero fields can be described as a commensurate modulated antiferromagnet with a propagation vector k = (0,0,1). Neutron powder diffraction with polarized neutrons gave evidence of short-range magnetic order, above and below the magnetic ordering temperature.