Quantum Versus Geometric Disorder in a Two-Dimensional Heisenberg Antiferromagnet

We present a numerical study of the spin-[Formula presented] bilayer Heisenberg antiferromagnet with random interlayer dimer dilution. From the temperature dependence of the uniform susceptibility and a scaling analysis of the spin correlation length we deduce the ground state phase diagram as a function of nonmagnetic impurity concentration [Formula presented] and bilayer coupling [Formula presented]. At the site percolation threshold, there exists a multicritical point at small but nonzero bilayer coupling [Formula presented]. The magnetic properties of the single-layer material [Formula presented] near the percolation threshold appear to be controlled by the proximity to this new quantum critical point.