  Particle-in-cell simulations are used to investigate the formation of
magnetic fields, B, in plasmas with perpendicular electron density and
temperature gradients. For system sizes, L, comparable to the ion skin depth,
d_i, it is shown that B ~ d_i/L, consistent with the Biermann battery effect.
However, for large L/d_i, it is found that the Weibel instability (due to
electron temperature anisotropy) supersedes the Biermann battery as the main
producer of B. The Weibel-produced fields saturate at a finite amplitude
(plasma \beta \approx 100), independent of L. The magnetic energy spectra below
the electron Larmor radius scale is well fitted by power law with slope -16/3,
as predicted in Schekochihin et al., Astrophys. J. Suppl. Ser 182, 310 (2009).
