Disorder driven maximum in the magnetoresistance of spin polaron systems

TL;DR

This paper investigates how structural disorder influences the magnetoresistance in spin polaron systems, revealing an optimal disorder level that maximizes magnetoresistance, with implications for magnetic semiconductor applications.

Contribution

It demonstrates that disorder enhances magnetoresistance up to an optimal point, beyond which it causes localization, and provides a comprehensive map of magnetoresistance dependence on disorder and coupling.

Findings

Disorder increases resistivity peak at T_c and magnetoresistance.

Maximum magnetoresistance (~80%) occurs at an optimal disorder level.

Too much disorder leads to Anderson insulator behavior.

Abstract

Ferromagnetic polarons are self trapped states of an electron in a locally spin polarised environment. They occur close to the magnetic $T_c$ in low carrier density local moment magnets when the electron-spin coupling is comparable to the hopping scale. In non disordered systems the primary signatures are a modest non-monotonicity in the temperature dependent resistivity $\rho(T)$, and a magnetoresistance that can be $\sim 20-30 \%$ at $T_c$, at fields that, in energy units, are $\sim 0.01 k_BT_c$. We find that structural disorder, in the form of pinning centers, promotes polaron formation, hugely increases the resistivity peak at $T_c$, and can enhance the magnetoresistance to $\sim 80\%$. The change in magnetoresistance with disorder is, however, non-monotonic. Too much disorder just creates an Anderson insulator - with the resistivity unresponsive to the magnetisation. This paper establishes the optimum disorder for maximising the magnetoresistance, suggests the physical process behind the unusual disorder dependence, and provides a magnetoresistance map - in terms of coupling and disorder - that locates some of the existing magnetic semiconductors within this framework.