Y NMR imaging of the staggered magnetization in doped Haldane chain Y(2)BaNi(1-x)Mg(x)O(5)

We present a static, model-independent, experimental determination of the spin-spin correlation length \xi in a quantum spin system. This is achieved in doped Haldane (i.e., S=1 Heisenberg antiferromagnetic) chain Y(2)BaNi(1-x)Mg(x)O(5) by Y NMR imaging of the staggered magnetization induced around the Mg-impurities (i.e., chain boundaries) by magnetic field. The magnitude of this magnetization is found to decay exponentially with \xi equal to the theoretical prediction for an infinite S=1 chain, and the staggered magnetic moment at the edge site showing the Curie behavior of an effective S=1/2 spin.