Impurity-induced antiferromagnetic order in the Haldane-gap compound (formula presented) (formula presented)

An investigation of the effect of nonmagnetic Mg doping in the Haldane gap system (formula presented) is presented. Specifically, the magnetic properties of the quasi-one-dimensional compound (formula presented) (formula presented) located in the heavily doped region of the (formula presented) x phase diagram, have been examined using neutron powder diffraction, muon-spin relaxation (formula presented) and magnetic susceptibility. Neutron-diffraction measurements show that antiferromagnetic long-range order (propagation vector (formula presented) is established below (formula presented) 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 (formula presented) 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 (formula presented)-site is (formula presented) 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 (formula presented) at (formula presented) The large number of detected spontaneous muon-precession frequencies (formula presented) implies a nonuniform distribution in the magnitude of the ordered moments along the spiral (formula presented) chains. Bulk susceptibility measurements reveal metamagnetic behavior below (formula presented) consistent with the picture of an antiferromagnet with magnetic anisotropy and competing exchange interactions.