  The weak ferromagnetism of La_{1.99}Sr_{0.01}CuO_4 epitaxial thin films is
investigated using magnetoresistance measurement. While a steplike negative
magnetoresistance associated with the weak ferromagnetic transition is clearly
observed in the films grown on YAlO_3(001), it is notably suppressed in the
films grown on SrTiO_3(100) and
(LaAlO_3)_{0.3}(SrAl_{0.5}Ta_{0.5}O_3)_{0.7}(100), and almost disappears in
films grown on LaSrAlO_4(001). The strong suppression of the steplike
magnetoresistance provides evidence that the CuO_2 planes are much less
corrugated in thin films grown on tetragonal substrates, particularly on
LaSrAlO_4(001), than in bulk crystals.
