Zone Boundary Softening of the Spin-Wave Dispersion in Doped Ferromagnetic Manganites

We argue that the new distinct features observed in doped ferromagnetic manganites R_1-xDₓMnO₃ ( RD0ex0ex=0ex0exLaSr, PrSr, NdSr, and LaCa), the softening of the spin-wave dispersion at the zone boundary and the increase of the spin-wave stiffness constant with x, have purely magnetic spin origin. They indicate the breakdown of the canonical double-exchange limit, and reflect otherwise natural consequence of the eg-band filling in the half-metallic regime. Details of the realistic electronic structure are important and significantly modify the analysis based on the minimal tight-binding Hamiltonian for eg electrons.