Impurity-induced antiferromagnetic order in the Haldane-gap compound PbNi2-xMgxV2O8 (x=0.24) -: art. no. 014428

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 PbNi2-xMgxV2O8 (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 (mu(+)SR) and magnetic susceptibility. Neutron-diffraction measurements show that antiferromagnetic long-range order (propagation vector kappa=[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 mu=0.9(1)mu(B). To our knowledge, this is the first diffraction evidence of the antiferromagnetic ordering in this system. The static magnetic correlations of the doping-induced magnetic moments along the Ni chains were probed by zero-field mu(+)SR at T<T-N. The large number of detected spontaneous muon-precession frequencies (0.5<nu(mu)<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.