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 e(g) electrons favors in the ferromagnetic metallic phase an orbital liquid (i.e., orbital-disordered) state. Therefore the spinwave stiffness is isotropic, and increases with hole doping x, providing a direct measure of the kinetic energy proportional to(x) of strongly correlated e(g) 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.