SPIN-WAVES OF LIGHTLY DOPED LA1-XSRXMNO3

Neutron scattering has been used to study the spin dynamics of the lig htly doped La1-xSrxMnO3. As the dopant concentration increases from 0 to 0.09, the magnetic long-range order changes from a slightly to a st rongly canted antiferromagnetic order, while the corresponding transit ion temperature is reduced from 140 to about 120 K. The spin waves sho w an anisotropic dispersion relation which can be described by a stron g in-plane ferromagnetic coupling J(ab) and a weak inter-plane antifer romagnetic coupling J(c). The spin-wave dispersion exhibits a finite e nergy gap which decreases with the dopant concentration from about 2.5 meV for x = 0 to about 1.5 meV for x = 0.09. Furthermore, while the f erromagnetic in-plane coupling is slightly enhanced upon doping, the i nter-plane antiferromagnetic coupling is strongly reduced, leading to a two-dimensional ferromagnetic system near the concentration x = 0.09 .