Double-exchange is not the cause of ferromagnetism in doped manganites
  The coexistence of ferromagnetism and metallic conduction in doped manganites has long been explained by a double-exchange model in which the ferromagnetic exchange arises from the carrier hopping. We evaluate the zero-temperature spin stiffness D(0) and the Curie temperature T_{C} on the basis of the double-exchange model using the measured values of the bare bandwidth W and the Hund's rule coupling J_{H}. The calculated D(0) and T_{C} values are too small compared with the observed ones even in the absence of interactions. A realistic onsite interorbital Coulomb repulsion can reduce D(0) substantially in the case of a 2-orbital model. Furthermore, experiment shows that D(0) is simply proportional to x in La_{1-x}Sr_{x}MnO_{3} system, independent of whether the ground state is a ferromagnetic insulator or metal. These results strongly suggest that the ferromagnetism in manganites does not originate from the double-exchange interaction. On the other hand, an alternative model based on the d-p exchange can semi-quantitatively explain the ferromagnetism of doped manganites at low temperatures. 
G. M. Zhao (University of Zurich)