Transport and thermal properties of weakly ferromagnetic Sr ₂IrO₄

We have synthesized polycrystalline Sr₂IrO₄ and measured its magnetic susceptibility, electrical resistivity, specific heat, Seebeck coefficient, and thermal conductivity. The magnetic susceptibility χ(T) shows a ferromagnetic transition at 250K while the behaviour above the transition temperature is well described by a Curie-Weiss fit with a small effective moment μ_eff ≤ 0.33μ_B and a paramagnetic Curie-Weiss temperature, _CW ≤ +251K, consistent with previous studies on this compound. However, specific heat, Seebeck coefficient, and thermal conductivity are all dominated by the phonon contribution and show no anomalies at the ferromagnetic transition. Electrical resistivity, unlike the single crystal, shows a huge increase, three orders of magnitude, with decreasing temperature. The temperature dependence of resistivity is logarithmic at high temperatures (210K<T<350K), Arrhenius type in intermediate temperatures (110K<T<190K) with a small energy gap of Δ ≤ 37meV, and variable-range-hopping type at low temperatures (35K<T<80K), in contrast to the data reported for single crystals. These results can be understood if localized states are present near the Fermi level due to structural oxygen disorder in this low dimensional compound.