  We investigate the role of disorder for field-driven quantum phase
transitions of metallic antiferromagnets. For systems with sufficiently low
symmetry, the combination of a uniform external field and non-magnetic
impurities leads effectively to a random magnetic field which strongly modifies
the behavior close to the critical point. Using perturbative renormalization
group, we investigate in which regime of the phase diagram the disorder affects
critical properties. In heavy fermion systems where even weak disorder can lead
to strong fluctuations of the local Kondo temperature, the random field effects
are especially pronounced. We study possible manifestation of random field
effects in experiments and discuss in this light neutron scattering results for
the field riven quantum phase transition in CeCu_5.8Au_0.2.
