Boundary effects in finite Heisenberg antiferromagnetic S=1 chains: Y-89 NMR in Y2BaNi1-xMgxO5

By means of Y-89 nuclear magnetic resonance (NMR) spectra direct evidence o f staggered magnetization induced by a uniform field has been obtained in t he Heisenberg antiferromagnetic S = 1 chain Y2BaNi1-3MgxO5. A correspondenc e between the resonance lines and the lattice positions is established, pro viding an image of the alternating magnetic moments that develop around the Mg impurities at the chain boundaries. The amplitude of these moments is f ound to decrease exponentially from the edges. with a characteristic decay distance equal to the magnetic correlation length numerically evaluated for an infinite chain. While the magnetization pertaining to ions far from the boundaries behaves as in typical gapped systems, the edge spins exhibit an S = 1/2 Curie-like deviation. These results promote the NMR approach to ac cess the spin-spin correlation function in antiferromagnetic quantum spin s ystems at finite temperatures.