Influence of rare regions on magnetic quantum phase transitions

The effects of quenched disorder on the critical properties of itinerant qu antum magnets are considered. Particular attention is paid to locally order ed rare regions that are formed in the presence of quenched disorder even w hen the bulk system is still in the nonmagnetic phase. It is shown that the se local moments or instantons destroy the previously found critical fixed point in the case of antiferromagnets. In the case of itinerant ferromagnet s, the critical behavior is unaffected by the rare regions due to an effect ive long-range interaction between the order parameter fluctuations.