Abstract
Spin dynamics have been studied on single crystals of weakly doped La 1− x Ca x MnO 3. At 17 K a spin-wave branch is found due to superexchange coupling (ferromagnetic in the a, b plane, increasing with x, antiferromagnetic along c decreasing with x). An additional isotropic and weakly dispersed spin-wave branch is observed around ferromagnetic zone centers at lower energy. This branch shows a gap decreasing with x and a stiffness constant increasing with x. The evolution of this branch with temperature is presented. In the paramagnetic phase the excitations become diffusive and the q-dependence of the width of the quasi-elastic spectra: Γ( q) = Γ 0 + Dq 2 reflects the dispersion law found in the ordered phase. These further excitations are attributed to the coupling induced by the mobile charge carriers with the localized spin lattice.
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