  We consider a magnetic impurity in two different S=1/2 Heisenberg bilayer
antiferromagnets at their respective critical inter-layer couplings separating
N'eel and disordered ground states. We calculate the impurity susceptibility
using a quantum Monte Carlo method. With intra-layer couplings in only one of
the layers (Kondo lattice), we observe an anomalous Curie constant C*, as
predicted on the basis of field-theoretical work [S. Sachdev et al., Science
286, 2479 (1999)]. The value C*=0.262 +/- 0.002 is larger than the normal Curie
constant C=S(S+1)/3. Our low-temperature results for the symmetric bilayer are
consistent with a universal C*.
