  We determine the critical Zeeman energy splitting for Fermi superfluidity at
infinite s-wave scattering length according to the Monte Carlo and experimental
results of the equations of state. Based on the universality hypothesis, we
show that there exist two critical fields $H_{c1}$ and $H_{c2}$, between which
a superfluid-normal mixed phase is energetically favored, and model-independent
formulae for $H_{c1}$, $H_{c2}$ and the critical population imbalance $P_c$ are
derived. Using recent Monte Carlo and experimental results of $P_c$, $H_{c1}$
and $H_{c2}$ are determined. It is found $H_{c1}=0.41\epsilon_{\text F}$ and
$H_{c2}=0.50\epsilon_{\text F}$, with $\epsilon_{\text F}$ being the Fermi
energy of non-interacting gas.
