Impurity-induced antiferromagnetic order in the Haldane-gap compoundPbNi2−xMgxV2O8(x=0.24)

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An investigation of the effect of nonmagnetic Mg doping in the Haldane gap system ${\mathrm{PbNi}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{8}$ is presented. Specifically, the magnetic properties of the quasi-one-dimensional compound ${\mathrm{PbNi}}_{2\ensuremath{-}x}{\mathrm{Mg}}_{x}{\mathrm{V}}_{2}{\mathrm{O}}_{8}$ $(x=0.24),$ located in the heavily doped region of the ${T}_{N},$ x phase diagram, have been examined using neutron powder diffraction, muon-spin relaxation $({\ensuremath{\mu}}^{+}\mathrm{SR})$ and magnetic susceptibility. Neutron-diffraction measurements show that antiferromagnetic long-range order (propagation vector $\mathbf{\ensuremath{\kappa}}=[000])$ is established below ${T}_{N}=3.2\mathrm{K}.$ 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 ${\mathrm{NiO}}_{6}$ 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 ${\mathrm{Ni}}^{2+}$-site is $\ensuremath{\mu}=0.9(1){\ensuremath{\mu}}_{B}.$ 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 ${\ensuremath{\mu}}^{+}\mathrm{SR}$ at $T<{T}_{N}.$ The large number of detected spontaneous muon-precession frequencies $(0.5<{\ensuremath{\nu}}_{\ensuremath{\mu}}<3\mathrm{MHz})$ implies a nonuniform distribution in the magnitude of the ordered moments along the spiral ${\mathrm{NiO}}_{6}$ chains. Bulk susceptibility measurements reveal metamagnetic behavior below ${T}_{N},$ consistent with the picture of an antiferromagnet with magnetic anisotropy and competing exchange interactions.