Spin-gap and antiferromagnetic correlations in low-dimensional PbNi 2-x A x V 2 O 8 compounds (A=Mg, Co)

The effect of the impurity doping in the quantum-spin chains of the PbNi2V2O8 Haldane-gap compound is studied. We report elastic neutron scattering and bulk magnetic susceptibility measurements on the quasi-one-dimensional system PbNi2-xAxV2O8 doped at the Ni2+ (S=1) sites by either non-magnetic A=Mg2+ (S=0) or magnetic A=Co2+ (S=3/2) ions, with x=0.12 and 0.08, respectively. The results reveal that the magnitude of the Neel temperature depends strongly on the spin value of the A cation.