  Due to weak spin-orbit coupling, the magnetic excitations of an itinerant
ferromagnet become magnetic rotons, excitations with degenerate minima on a
hypersphere at finite wavevector. Using self-consistent Hartree and
renormalization group calculations, we study weak fluctuation-driven
first-order quantum phase transitions, a quantum tricritical point controlled
by anisotropy and the non-Fermi liquid behavior associated with the large phase
volume of magnetic rotons. We propose that magnetic rotons are essential for
the description of the anomalous high-pressure behavior of the itinerant
helical ferromagnet MnSi.
