Magnetic versus non-magnetic doping effectsin the Haldane chain compound PbNi₂V₂O₈

Study of an impurity-driven phase transition into a magnetically ordered state in the spin-liquid Haldane chain compound PbNi₂V₂O₈ is presented. Both macroscopic magnetization as well as ¹⁵V nuclear magnetic resonance (NMR) measurements reveal that the magnetic nature of dopants has a crucial role in determining the stability of the induced long-range magnetic order. In the case of non-magnetic (Mg²⁺ doping at the Ni²⁺ sites (S = 1), a metamagnetic transition is observed in relatively low magnetic fields. On the other hand, the magnetic order in magnetically (Co²⁺) doped compounds survives at much higher magnetic fields and temperatures. We attribute this feature to a significant anisotropic impurity–host magnetic interaction. The NMR measurements confirm that the staggered magnetic moments liberated next to the impurity sites, are the reason for the magnetic ordering. In addition, differences in the broadening of NMR spectra and the increase of nuclear spin-lattice relaxation in doped samples indicate an impurity-dependent character for the dominant electron-spin correlations; the latter begin to develop at rather high temperatures with respect to the antiferromagnetic phase-transition temperature.