Magnetoresistance in Mn Pyrochlore: Electrical Transport in a Low Carrier Density Ferromagnet

We discuss magnetotransport in a low density electron gas coupled to spin fluctuations near and above a ferromagnetic transition. Provided the density is low enough [ n1/ξ³(T), with ξ(T) the ferromagnetic correlation length], spin polarons form in an intermediate temperature regime above Tc. Both in the spin polaron regime and in the itinerant regime nearer Tc, the magnetoresistance is large. We propose that this provides a good model for ``colossal'' magnetoresistance in the pyrochlore Tl_2-xScₓMn₂O₇, fundamentally different from the mechanism in the perovskite manganites such as La_1-xSrₓMnO₃.