MAGNETORESISTANCE IN MN PYROCHLORE - ELECTRICAL-TRANSPORT IN A LOW CARRIER DENSITY FERROMAGNET

We discuss magnetotransport in a low density electron gas coupled to s pin fluctuations near and above a ferromagnetic transition. Provided t he density is low enough [n less than or similar to 1/xi(3)(T), with x i(T) the ferromagnetic correlation length], spin polarons form in an i ntermediate temperature regime above T-c. Both in the spin polaron reg ime and in the itinerant regime nearer T-c, the magnetoresistance is l arge. We propose that this provides a good model for ''colossal'' magn etoresistance in the pyrochlore Tl2-xScxMn2O7, fundamentally different from the mechanism in the perovskite manganites such as La1-xSrxMnO3.