  We describe the spin distribution in the vicinity of a non-magnetic impurity
in a two-dimensional antiferromagnet undergoing a transition from a
magnetically ordered Neel state to a paramagnet with a spin gap. The quantum
critical ground state in a finite system has total spin S=1/2 (if the system
without the impurity had an even number of S=1/2 spins), and recent numerical
studies in a double layer antiferromagnet (K. H.Hoglund et al.,
cond-mat/0611418) have shown that the spin has a universal spatial form
delocalized across the entire sample. We present the field theory describing
the uniform and staggered magnetizations in this spin texture for two classes
of antiferromagnets: (i) the transition from a Neel state to a paramagnet with
local spin singlets, in models with an even number of S=1/2 spins per unit
cell, which are described by a O(3) Landau-Ginzburg-Wilson field theory; and
(ii) the transition from a Neel state to a valence bond solid, in
antiferromagnets with a single S=1/2 spin per unit cell, which are described by
a deconfined field theory of spinons.
