Spin-Vacancy-Induced Long-Range Order in a New Haldane-Gap Antiferromagnet

Magnetic susceptibility, high-field magnetization, and inelastic neutron scattering experiments are used to study the magnetic properties of a new $S=1\mathrm{}$ quasi-one-dimensional antiferromagnet ${\mathrm{PbNi}}_2{\mathrm{V}}_2{\mathrm{O}}_8$. Interchain interactions are shown to be almost, but not quite, strong enough to destroy the nonmagnetic singlet ground state and the energy gap in the magnetic excitation spectrum. Substituting nonmagnetic ${\mathrm{Mg}}^{2+}$ ( $S=0\mathrm{}$) ions for ${\mathrm{Ni}}^{2+}$ ( $S=1\mathrm{}$) 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.