Magnetism of low-doped spin chains in Lax(Sr,Ca)14-xCu24O41

We present magnetization data of Lax(Ca,Sr)14-xCu24O41 (x⩾5) in high magnetic fields up to 16 T. The data reflect the magnetic response of lightly doped CuO2 spin chains containing ⩽10% non-magnetic holes. Our data correspond to a strongly anisotropic magnetic phase diagram. Using Monte-Carlo simulations, we present and analyse a two-dimensional Ising model with mobile holes, describing the experimental observations qualitatively.