NEW SPIN DYNAMICS IN GMR CA DOPED MANGANITES

Spin dynamics have been studied on single crystals of weakly doped La1 -xCaxMnO3. At 17 K a spin-wave branch is found due to superexchange co upling (ferromagnetic in the a, b plane, increasing with x, antiferrom agnetic along c decreasing with x). An additional isotropic and weakly dispersed spin-wave branch is observed around ferromagnetic zone cent ers at lower energy. This branch shows a gap decreasing with ?c and a stiffness constant increasing with x. The evolution of this branch wit h temperature is presented. In the paramagnetic phase the excitations become diffusive and the q-dependence of the width of the quasi-elasti c spectra: Gamma(q)= Gamma(0) + Dq(2) reflects the dispersion law foun d in the ordered phase. These further excitations are attributed to th e coupling induced by the mobile charge carriers with the localized sp in lattice. (C) 1998 Elsevier Science B.V. All rights reserved.