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 ($n\lesssim 1/{\xi }^3(T)$, with $\xi (T)$ 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 Tl${}_{2-x}$Sc${}_x$Mn${}_2$O${}_7$, fundamentally different from the mechanism in the perovskite manganites such as La${}_{1-x}$Sr${}_x$MnO${}_3$.

4 pages, LaTex, + 3 figures

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