Using a fermionic version of the quantum order-by-disorder mechanism, I show that fluctuations self-consistently stabilize incommensurate, spiral magnetic order near ferromagnetic quantum critical points. The theoretical predictions agree remarkably well with neutron scattering, x-ray diffraction, and specific heat measurements on PrPtAl. As a second example, I will use the quantum order-by-disorder approach to explain various features of the enigmatic partially ordered phase of the helimagnet MnSi.
I further show that weak charge disorder destabilizes the ferromagnetic state and enhances the susceptibility towards spiral magnetic ordering. The Goldstone modes of the spiral phase are governed by a 3d-XY model. The induced disorder in the pitch of the spiral generates a random anisotropy for the Goldstone modes, inducing vortex lines in the phase of the helical order and rendering the magnetic correlations short ranged with a strongly anisotropic correlation length. Our theory provides an explanation for the unusual glassy state observed near the avoided ferromagnetic quantum critical point of CeFePO.