PHASE-TRANSITION IN THE DOUBLE-EXCHANGE MODEL - A SCHWINGER BOSON APPROACH

Local alignment of a conduction electron's spin with core spins caused by strong ferromagnetic coupling imposes a severe restriction on the Hilbert space. This is incorporated in a representation in which the e lectron is a composite object. Within a mean-field approximation we fi nd a transition from a ferromagnetic metal to a paramagnetic state at a temperature T-c much less than T-F, the Fermi temperature, i.e., the re is a separation of energy scales. The electron Green function exhib its a two-fluid character: a Fermi liquid component associated with th e ordered spins which disappears above T-c, and an incoherent componen t with disordered spins which breaks particle-hole symmetry. Implicati ons for manganites, which exhibit very large magnetoresistance, are di scussed.