Magnetic Anisotropy And Domain Structure Of The Layered Manganite La1.36sr1.64mn2o7

The magnetic anisotropy and domain structure of La1.36Sr1.64Mn2O7 were investigated 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 cancellation 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 developed maze shaped domain pattern with 10 mu m domain width is observed which shows signatures of domain branching and flux closure typical for a moderately anisotropic uniaxial magnet.