The magnetic anisotropy and domain structure of La1.36Sr1.64Mn2O7 were inve
stigated using magnetooptical imaging and magnetization measurements. First
and second order anisotropy constants of K-1=2.5 X 10(5) erg/cm(3) and K-2
=2.8X10(5) erg/cm(3) at 20 K are obtained. With increasing temperature K-1
and K-2 drop rapidly until K1 changes sign near 80 K. A model of near cance
llation of a strong single-ion anisotropy (k(u) similar or equal to 0.7X10(
6)erg/cm(3) at 20 K) and large dipolar effects originating from the layered
structure can account for the observed anisotropy. Below 75 K a well devel
oped maze shaped domain pattern with 10 mu m domain width is observed which
shows signatures of domain branching and flux closure typical for a modera
tely anisotropic uniaxial magnet.