Spin-flop and antiferromagnetic phases of the ferromagnetic half-twist ladder compounds Ba3Cu3In4O12 and Ba3Cu3Sc4O1

Abstract

The title compounds have dominant ferromagnetic exchange interactions within one-dimensional (1D) half-twist ladders of s = 1/2 Cu{sup 2+} ions and antiferromagnetic (AFM) interactions between ladders, leading to ordered 3D phases at temperatures below 20 K. 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 and PM phases whose boundaries have sharp features in the experimental magnetization M(T,H) and specific heat C{sub P}(T,H). Exact diagonalization 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, H{sub AF}, obtained from modelling the ordered phases. Isotropic exchange J{sub 1} =- 135 K and g-tensor g = 2.1 within ladders, plus exchange and anisotropy fields H{sub AF} and H{sub A}, describe the ordered phases and are almost quantitative for the PM phase.