Production
Figure 1: Froduction of magnetic field from ref. (1).
At small k, the spectra of the fields generated at phase transitions in a causal way follow powerlaw with the slope ns = 2. Inflationary mechanisms typically result in the slope ns = 1. Inflation could in principle generate a scale-invariant spectrum with ns =-3. At large k all the spectra follow a universal slope formed by turbulence.
References:
(1) R.Durrer, A.Neronov, A and A. Rv. 21 (2013) 62 .