Effects of rare regions on magnetic quantum phase transitions

Abstract. The Griffiths region that results from rare regions in disordered itinerant quantum magnets, and its influence on the critical behavior, is considered within the framework of an effective field theory. It is shown that local moments destroy the previously found critical fixed point for antiferromagnets. In contrast, the critical behavior of itinerant ferromagnets is unaffected by the rare regions due to an effective long-range interaction between the order parameter fluctuations.