  We give detailed analysis of the effect of depolarizing field in
nanometer-size ferroelectric capacitors studied by Kim et al. [Phys. Rev. Lett.
95, 237602 (2005)]. We calculate a critical thickness of the homogeneous state
and its stability with respect to domain formation for strained thin films of
BaTiO3 on SrRuO3/SrTiO3 substrate within the Landau theory. While the former
(2.5nm) is the same as given by ab-initio calculations, the actual critical
thickness is set by the domains at 1.6nm. There is a large Merz's activation
field for polarization relaxation. Remarkably, the results show a_negative_
slope of the "actual" hysteresis loops, a hallmark of the domain structures in
ideal thin films with imperfect screening.
