A quantum theory of double exchange, 3

Metallic double-exchange ferromagnets such as doped LaMnO 3 are investigatd on the basis of an efffective Hamiltonian derived from the s - d model Hamiltonian. Resistance at low temperatures is calculated in the spin wave approximation and it is found to be proportional to T 9/2 . Magnetic behaviors of the system at finite temperatures are examined by the use of a simple molecular field theory. A magnetization-temperature curve which firs well the experimental data of Leung et al. is obtained. Resistance and magneto-resistance are also studied.