  We use the Zel'dovich approximation to analyse the amplification of magnetic
fields in gravitational collapse of cold dark matter during the mildly
nonlinear regime, and identify two key features. First, the anisotropy of
collapse effectively eliminates one of the magnetic components, confining the
field in the pancake plane. Second, in agreement with recent numerical
simulations, we find that the shear anisotropy can amplify the magnetic field
well beyond the isotropic case. Our results suggest that the magnetic strengths
observed in spiral and disk galaxies today might have originated from seeds
considerably weaker than previous estimates.
