Site-dilution in the quasi-one-dimensional antiferromagnet Sr₂(Cu_1−xPd_x)O₃: Reduction of Néel temperature and spatial distribution of ordered moment sizes

We investigate the Néel temperature of Sr₂CuO₃ as a function of the site dilution at the Cu (S=1∕2) sites with Pd (S=0), utilizing the muon spin relaxation (μSR) technique. The Néel temperature, which is T_N=5.4 K for the undoped system, becomes significantly reduced for less than one percent of doping Pd, giving a support for the previous proposal for the good one-dimensionality. The Pd concentration dependence of the Néel temperature is compared with a recent theoretical study [ S. Eggert, I. Affleck and M. D. P. Horton Phys. Rev. Lett. 89 47 202 (2002)] of weakly coupled one-dimensional antiferromagnetic chains of S=1∕2 spins, and a quantitative agreement is found. The inhomogeneity of the ordered moment sizes is characterized by the μSR time spectra. We propose a model that the ordered moment size recovers away from the dopant S=0 sites with a recovery length of ξ≈150−200 sites. The origin of the finite recovery length ξ for the gapless S=1∕2 antiferromagnetic chain is compared to the estimate based on the effective staggered magnetic field from the neighboring chains.