Nearly Minimal Magnetogenesis
  We propose a new mechanism for magnetic field generation from inflation, by which strong magnetic fields can be generated on cosmological scales. These fields may be observable by cosmic microwave background radition measurements, and may have a dynamical impact on structure formation. The mechanism is based on the observation that a light nearly minimally coupled charged scalar may be responsible for the creation of a negative photon mass-squared (provided the scalar field coupling to the curvature scalar is small but negative), which in turn results in abundant photon production - and thus in growing magnetic fields - during inflation. 
Tomislav Prokopec, Ewald Puchwein