Cosmological Structure Formation Creates Large-Scale Magnetic Fields
  This paper examines the generation of seed magnetic fields due to the growth of cosmological perturbations. In the radiation era, different rates of scattering from photons induce local differences in the ion and electron density and velocity fields. The currents due to the relative motion of these fluids generate magnetic fields on all cosmological scales, peaking at a magnitude of ~ 10^{-24} Gauss at the epoch of recombination. Magnetic fields generated in this manner provide a promising candidate for the seeds of magnetic fields presently observed on galactic and extra-galactic scales. 
E. R. Siegel, J. N. Fry