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

Abstract

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 [ $n\lesssim 1/{\xi }^3\left(T\right)$, with $\xi \left(T\right)$ the ferromagnetic correlation length], spin polarons form in an intermediate temperature regime above $T_c$. Both in the spin polaron regime and in the itinerant regime nearer $T_c$, the magnetoresistance is large. We propose that this provides a good model for “colossal” magnetoresistance in the pyrochlore ${\mathrm{Tl}}_{2-x}{\mathrm{Sc}}_x{\mathrm{Mn}}_2{\mathrm{O}}_7$, fundamentally different from the mechanism in the perovskite manganites such as ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{MnO}}_3$.