Weak ferromagnetism of La_{1.99}Sr_{0.01}CuO₄ thin films: evidence for removal of corrugation in CuO₂ plane by epitaxial strain

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.