Magnetoresistance in Mn pyrochlore: electrical transport in a low carrier density ferromagnet

TL;DR

This paper investigates magnetoresistance in low carrier density ferromagnetic Mn pyrochlore, highlighting the role of spin polarons and proposing a model for colossal magnetoresistance distinct from perovskite manganites.

Contribution

It introduces a new understanding of colossal magnetoresistance in Mn pyrochlore through spin polaron formation at low electron densities.

Findings

Large magnetoresistance observed near and above $T_c$

Spin polarons form in an intermediate temperature regime

Proposes a different mechanism from perovskite manganites

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$_3$.