Impurity-induced antiferromagnetic order in the Haldane-gap compound PbNi2-xMgxV2O8 (x=0.24)

Abstract

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 TN, 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 TN =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)μB. 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<TN. The large number of detected spontaneous muonprecession frequencies (0.5<vμ<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 TN, consistent with the picture of an antiferromagnet with magnetic anisotropy and competing exchange interactions.