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

Abstract

We argue that the new distinct features observed in doped ferromagnetic manganites ${R}_{1\ensuremath{-}x}{D}_{x}{\mathrm{MnO}}_{3}$ ( $RD\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\mathrm{LaSr}$, 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 ${e}_{g}$-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 ${e}_{g}$ electrons.