Dominant ferromagnetism in the spin-1/2 half-twist ladder 334 compounds, Ba ₃Cu ₃In ₄O ₁₂ and Ba ₃Cu ₃Sc ₄O ₁₂

The magnetic properties of polycrystalline samples of Ba ₃Cu ₃In ₄O ₁₂ (In-334) and Ba ₃Cu ₃Sc ₄O ₁₂ (Sc-334) are reported. Both 334 phases have a structure derived from perovskite, with CuO ₄ squares interconnected to form half-twist ladders along the c-axis. The CuOCu angles, 90°, and the positive Weiss temperatures indicate the presence of significant ferromagnetic (FM) interactions along the Cu ladders. At low temperatures, T<20K, sharp transitions in the magnetic susceptibility and heat capacity measurements indicate three-dimensional (3D) antiferromagnetic (AFM) ordering at T _N. T _N is suppressed on application of a field and a complex magnetic phase diagram with three distinct magnetic regimes below the upper critical field can be inferred from our measurements. The magnetic interactions are discussed in relation to a modified spin-1/2 FMAFM model and the 334 half-twist ladder is compared to other two-rung ladder spin-1/2 systems.