Effect of vacancy doping on the Haldane spin-liquid state inPbNi2−xMgxV2O8

Abstract

The effect of spin-vacancy doping on the ground state of the quasi-one-dimensional $S=1$ quantum antiferromagnet ${\mathrm{PbNi}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{8}$ is studied by X band as well as high-field ESR. A very broad ESR line is observed at low temperatures in powdered ${\mathrm{PbNi}}_{2\ensuremath{-}x}{\mathrm{Mg}}_{x}{\mathrm{V}}_{2}{\mathrm{O}}_{8}$ for $0l~xl~0.24.$ The frequency dependence of the observed ESR line rules out the possibility that the low-temperature ESR signal comes from the isolated $S=\frac{1}{2}$ spins. Instead, it is proposed that a magnetic coupling (most likely of second order) between the chain-end spins on both sides of the ${\mathrm{Mg}}^{2+}$ impurity plays an important role in this system. Impurity-induced spin states, due to the strong interchain coupling, also induce a staggered magnetization on the neighboring chains with a characteristic length \ensuremath{\xi}. \ensuremath{\xi} increases with decreasing temperature, as suggested by the additional broadening and the shift of the ESR signal at low temperatures. On the other hand the temperature dependence of the low temperature susceptibility suggests a much more complicated picture in which impurity-induced spins are exposed to coupling to thermal excitations on each chain as well as to three-dimensional antiferromagnetic correlations.