Metallic Quantum Ferromagnets*

This talk will provide an overview of the current understanding of the quantum phase transition in metallic ferromagnets. In clean systems the transition is generically first order or discontinuous, in sharp contrast to the second-order, or continuous, transition in classical ferromagnets. This is due to a coupling between the magnetization and low-temperature soft modes in the conduction-electron system. At sufficiently high temperature the continuous classical transition is observed. Continuity is provided by a tricritical point in the phase diagram, with associated tricritical wings in an external magnetic field. There is good agreement between theory and experiment. Quenched disorder lowers the tricritical temperature and, for sufficiently strong disorder, leads to a second-order transition even at zero temperature. Strong disorder can also lead to additional complications, such as quantum Griffiths effects. Finally, interesting and complicated phase diagrams are observed in systems where antiferromagnetic order competes with the ferromagnetic one, which leads to effects that are only partially understood.