Field Dependence of the Magnetization for the spin-ladder material   Cu_2(C_5H_{12}N_2)_2Cl_4
  We have developed a series expansion method for calculating the uniform magnetization, M, as a function of the applied field h for Heisenberg systems at T=0. The method involves introducing Ising anisotropy along the z-axis together with an applied uniform field along the x-axis. On extrapolation to the isotropic limit, one recovers the magnetization for the Heisenberg system with an applied field along the x-axis. This method circumvents the difficulties in developing perturbation theory associated with the commuting nature of the uniform field. The results developed for two-chain ladders appropriate for the material Cu_2(C_5H_{12}N_2)_2Cl_4 are in good agreement with the experimental data. In addition, uniform susceptibility is calculated by high temperature expansions and also compared with the experimental data. 
Zheng Weihong (1), Rajiv R.P. Singh (2), and J. Oitmaa (1) ((1) Univ.   of NSW, (2) University of California)