Quantum critical behavior of itinerant ferromagnets

We investigate the quantum phase transition of itinerant ferromagnets. It i s shown that correlation effects in the underlying itinerant electron syste m lead to singularities in the order parameter field theory that result in an effective long-range interaction between the spin fluctuations. This int eraction turns out to be generically antiferromagnetic for clean systems. I n disordered systems analogous correlation effects lead to even stronger si ngularities. The resulting long-range interaction is, however, generically ferromagnetic. We discuss two possibilities for the ferromagnetic quantum phase transition . In clean systems, the transition is generically of first order, as is exp erimentally observed in MnSi. However, under certain conditions the transit ion may be continuous with non-mean field critical behavior. In disordered systems, one finds a very rich phase diagram showing first order and contin uous phase transitions and several multicritical points.