Magnetization measurements of Sr{14-x}Ca_xCu_{24}O_{41} with 0 <= x <=12 in
magnetic fields up to 16 T show that the low-temperature magnetic response of
the CuO_2 spin chains changes strongly upon Ca doping. For x=0 quantum
statistical simulations yield that the temperature and field dependence of the
magnetization can be well described by an effective Heisenberg model in which
the ground state configuration is composed of spin dimers, trimers, and
monomers. For x>0 a constant contribution to the low-temperature magnetic
susceptibility is observed which cannot be explained in terms of simple chain
models. Alternative scenarios are outlined.Comment: 2 pages, submitted to the proceedings of the ICM, Kyoto, Japan,
August 200
