Magnetic properties of a S=1/2 zigzag spin chain compound (N_2H_5)CuCl_3
  We present a theoretical and experimental study of a quasi-one-dimensional zigzag antiferromagnet (N_2H_5)CuCl_3, which can be viewed as weakly coupled Heisenberg chains with a frustrated interaction. We first discuss generic features of the magnetic properties of the zigzag spin chain between the nearly single chain case and the nearly double chain case, on the basis of the finite temperature density-matrix renormalization group (DMRG) calculations. We next show the experimental results for the magnetic susceptibility and the high-field magnetization of a single crystal of (N_2H_5)CuCl_3 above the Neel temperature T_N=1.55K. By comparing the experimental data with the DMRG results carefully, we finally obtain the ratio of the nearest and next-nearest exchange couplings as J_1/J_2=0.25 with J_2/k_B=16.3K. We also investigate the three-dimensional (3D) coupling J' effect by using the mean-field theory combined with the DMRG calculations. The estimated value J' \sim 0.04J_2 supports good one-dimensionality of (N_2H_5)CuCl_3 above T_N. 
N. Maeshima, M. Hagiwara, Y. Narumi, K. Kindo, T. C. Kobayashi, K.   Okunishi