Why spin excitations in metallic ferromagnetic manganites are isotropic

We consider the t-J model for doped manganites, and show that double exchange for correlated eg electrons favors in the ferromagnetic metallic phase an orbital liquid (i.e., orbital-disordered) state. Therefore the spin-wave stiffness is isotropic, and increases with hole doping x, providing a direct measure of the kinetic energy ∝x of strongly correlated eg electrons. The superexchange interactions, which stabilize orbital order at low doping, are frustrated in the orbital liquid and only reduce the stiffness constant, leading to a quantitative explanation of the magnon dispersion.