MAGNETIC COUPLING INDUCED BY HOLE DOPING IN PEROVSKITES LA1-XCAXMNO3 : A NEUTRON SCATTERING STUDY

Elastic and inelastic neutron scattering has been used to study static and dynamic magnetic properties in Ca-doped ${\mathrm{LaMnO}}_{3}$ perovskites in the region of low doping (5% and 8%) and to determine the phase diagram. In addition to the spin-wave branch related to the superexchange coupling between Mn ions (ferromagnetic in the basal planes and antiferromagnetic along the direction perpendicular to them), a new type of ferromagnetic spin excitation branch is found induced by hole doping. This is revealed by well-defined propagative spin wave modes with a gap and a nearly isotropic dispersion law at small q values: $\ensuremath{\omega}={\ensuremath{\omega}}_{0}{+Dq}^{2}.$ At ${T}_{N},$ these modes change from a collective to a diffusive character. The width in energy associated to the quasielastic scattering exhibits a similar dispersion law: $\ensuremath{\Gamma}={\ensuremath{\Gamma}}_{0}{+dq}^{2}.$ All these results are compared with other experimental results on similar compounds and discussed in the frame of existing theories.