Impurity-induced antiferromagnetic order in the Haldane-gap compound PbNi_2-xMg_xV₂O₈ (x=0.24)

An investigation of the effect of nonmagnetic Mg doping in the Haldane gap system PbNi₂V₂O₈ is presented. Specifically, the magnetic properties of the quasi-one-dimensional compound PbNi_2-xMg_xV₂O₈ (x =0.24), located in the heavily doped region of the T_N, x phase diagram, have been examined using neutron powder diffraction, muon-spin relaxation (μ⁺SR) and magnetic susceptibility. Neutron-diffraction measurements show that antiferromagnetic long-range order (propagation vector κ=[000]) is established below T_N =3.2 K. The spins in the Ni sublattice lie either parallel or antiparallel to the c axis, which is the magnetic easy axis of the system. The spin configuration is antiferromagnetic both along the NiO₆ chains, i.e., the c axis of the crystal structure, and within the ab-plane for the nearest-neighbor Ni atoms. The refined ordered moment at the Ni²⁺-site is μ=0.9(1)μ_B. To our knowledge, this is the first diffraction evidence of the antiferromagnetic ordering in this system. The static magnetic correlations ofthe doping-induced magnetic moments along the Ni chains were probed by zero-field μ⁺SR at T<T_N. The large number of detected spontaneous muonprecession frequencies (0.5<v_μ<3 MHz) implies a nonuniform distribution in the magnitude of the ordered moments along the spiral NiO₆ chains. Bulk susceptibility measurements reveal metamagnetic behavior below T_N, consistent with the picture of an antiferromagnet with magnetic anisotropy and competing exchange interactions.