  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 below 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.
