The title compounds have dominant ferromagnetic (FM) exchange interactions within one-dimensional (1D) half-twist ladders of s =1/2 Cu2^{+} ions and antiferromagnetic(AFM) interactions between ladders, leading to ordered 3D phases at temperatures bel
ow 20K. Here we show that a microscopic 1D model of the paramagnetic (PM) phase combined with a phenomenological model based on sublattice magnetization describes the observed temperature and field dependent magnetism. The model identifies AFM, spin-flop (SF) and PM phases whose boundaries have sharp features in the experimental magnetization M(T,H) and specific heat CP(T,H). Exact diagonalization (ED) of the 1D model, possible for 24 spins due to special structural features of half-twist ladders, yields the magnetization and spin susceptibility of the PM phase. AFM interactions between ladders are included at the mean-field level using the field, HAF, obtained from modeling the ordered phases. Isotropic exchange J1 = -135K and g-tensor g = 2.1 within ladders, plus exchange and anisotropy fields HAF and HA, describe the ordered phases, and are almost quantitative for the PM phase.
The magnetic properties of polycrystalline samples of Ba3Cu3In4O12 (In-334) and Ba3Cu3Sc4O12 (Sc-334) are reported. Both 334 phases have a structure derived from perovskite, with CuO4 squares interconnected to form half-twist ladders along the c-axis
. The Cu-O-Cu angles, ~ 90o, and the positive Weiss temperatures indicate the presence of significant ferromagnetic (FM) interactions along the Cu ladders. At low temperatures, T < 20 K, sharp transitions in the magnetic susceptibility and heat capacity measurements indicate three-dimensional (3D) antiferromagnetic (AFM) ordering at TN. TN 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 FM-AFM model and the 334 half-twist ladder is compared to other 2-rung ladder spin-1/2 systems.
A quantum magnet, LiCuSbO4, with chains of edge-sharing S = 1/2 CuO6 octahedra is reported. While the Curie-Weiss constant is ferromagnetic, theta = 30 K, no phase transition or spin freezing occurs down to 100 mK. Specific heat indicates a distinct
high field phase near the 12 T saturation field. Neutron scattering shows incommensurate spin correlations with q = 0.47pm0.01{pi}/a and places an upper limit of 70 mueV on a potential spin gap. Exact diagonalization of easy plane S = 1/2 chains with competing ferro- and antiferromagnetic interactions (J1 = - 75 K, J2 = 34 K) accounts for the T > 2 K data.
