  We study S=1/2 dimers which are weakly coupled by three-dimensional
antiferromagnetic interactions J'. It is commonly known that doping with
non-magnetic impurities immediately induces a long-range Neel order. We show
that application of an external magnetic field H may drive the system back to a
disordered phase. We discuss the zero temperature phase diagram in the (H,J')
plane and we propose suggestions for experiments.
