  Heteroepitaxial spinel ferrites NiFe2O4 and CoFe2O4 films have been prepared
by pulsed laser deposition (PLD) at various temperatures (175 - 690 {\deg}C)
under ozone/oxygen pressure of 10 mTorr. Due to enhanced kinetic energy of
ablated species at low pressure and enhanced oxidation power of ozone, epitaxy
has been achieved at significantly lower temperatures than previously reported.
Films grown at temperature below 550 {\deg}C show a novel growth mode, which we
term "vertical step-flow" growth mode. Epitaxial spinel ferrite films with
atomically flat surface over large areas and enhanced magnetic moment can be
routinely obtained. Interestingly, the growth mode is independent of the nature
of substrates (spinel MgAl2O4, perovskite SrTiO3, and rock salt MgO) and film
thicknesses. The underlying growth mechanism is discussed.
