Spin-vacancy-induced long-range order in a new haldane-gap antiferromagnet

Magnetic susceptibility, high-field magnetization, and inelastic neutron sc attering experiments are used to study the magnetic properties of a new S = 1 quasi-one-dimensional antiferromagnet PbNi2V2O8. Interchain interactions are shown to be almost, but not quite, strong enough to destroy the nonmag netic singlet ground state and the energy gap in the magnetic excitation sp ectrum. Substituting nonmagnetic Mg2+ (S = 0) ions for Ni2+ (S = 1) induces a magnetically ordered state at low temperatures. To our knowledge this is the first observation of doping-induced long-range order in a Haldane-gap system.