Effect of layering and doping on the magnetic anisotropy of the layered manganites La2-2xSr1+2xMn2O7 (x=0.3-0.4)

Magnetization experiments were performed on a series of single crystals of the bilayer manganites La2-2xSr1+2xMn2O7. The magnetic anisotropy constants were determined by fitting expressions based on the tetragonal anisotropy energy to the magnetization curves. It is shown that there is a significant dipolar contribution to the first order anisotropy constant arising from t he layered crystal structure. This contribution determines the magnetizatio n direction in the doping range x = 0.32-0.33 where the anisotropy due to t he electronic structure is small. With increasing doping the magnetic aniso tropy changes from strong uniaxial anong the c axis (K approximate to 2.5 x 10(6) erg/cm(3)) at x = 0.3 to strong easy plane (K approximate to -3.7 x 10(6) erg/cm(3)) at x = 0.4 in an almost linear fashion. This evolution is explained through the change of the orbital nature of the e(g) electrons fr om predominantly d(3z2-r2) to predominantly d(x2-y2). On the samples displa ying easy-plane anisotropy a small anisotropy in the basal plane of about - 7 x 10(4) erg/cm(3) was determined with (110) being the easy axis. An estim ate of the spin-wave gap based on these results is in reasonable agreement with experimental determinations. (C) 2001 American Institute of Physics.