Highly Degenerate Canted Spin Structure in Bilayer Manganite La${}_{1.1}$Sr${}_{1.9}$Mn${}_2$O${}_7$

We have examined magnetic ordering of the two-dimensional (2D) bilayer manganite La${}_{1.1}$Sr${}_{1.9}$Mn${}_2$O${}_7$ with neutron diffraction technique. Due to the two-dimensionality, there appears intense 2D ridge scattering along the tetragonal $(00l)$ direction, and the analyses of such scattering leads to a conclusion that the low temperature spin structure is a canted antiferromagnetic ordering.