Magnetic Properties of an S = 1 Heisenberg Antiferromagnet TMNIN with Bond Impurity

We have carried out magnetic susceptibility and heat capacity measurements on a typical Haldane antiferromagnet (CH₃)₄NNi(NO)₃ (TMNIN) doped with nonmagnetic Zn²⁺ ion impurities at the concentrations of 7.4%, 11.8% and 20.4% and on a pure system down to 0.2 K. At low temperatures below 1.2 K, the paramagnetic susceptibility expected in the Haldane system with the open chain is found to be suppressed. A broad maximum of the heat capacity is observed around 0.7 K for each impurity system. The observed results strongly suggest that there exists an antiferromagnetic interaction between the edge S = 1/2 spins adjacent to Zn²⁺ ions via the next-nearest exchange interaction J_nn which can be regarded as a bond impurity in the Haldane system. This is the first experiment on bond impurity models for the Haldane system.