Anomalous magnetoresistance of EuB$_{5.99}$C$_{0.01}$: Enhancement of magnetoresistance in systems with magnetic polarons

TL;DR

This study investigates the large negative magnetoresistance in EuB$_{5.99}$C$_{0.01}$, attributing it to magnetic polaron behavior and carbon concentration fluctuations, revealing how these factors influence magnetic and electrical properties.

Contribution

It introduces the role of carbon-rich regions as spacers affecting magnetic polaron percolation and magnetoresistance in EuB$_{5.99}$C$_{0.01}$, a novel insight into magnetoresistance mechanisms.

Findings

Negative magnetoresistance linked to carbon fluctuations.

Carbon-rich regions create helimagnetic domains affecting resistivity.

Spacers hinder magnetic polaron percolation, increasing resistivity.

Abstract

We present results of measurements of electrical, magnetic and thermal properties of EuB$_{5.99}$C$_{0.01}$. The observed anomalously large negative magnetoresistance as above, so below the Curie temperature of ferromagnetic ordering $T_C$ is attributed to fluctuations in carbon concentration. Below $T_C$ the carbon richer regions give rise to helimagnetic domains, which are responsible for an additional scattering term in the resistivity, which can be suppressed by a magnetic field. Above $T_C$ these regions prevent the process of percolation of magnetic polarons (MPs), acting as "spacers" between MPs. We propose that such "spacers", being in fact volumes incompatible with existence of MPs, may be responsible for the decrease of the percolation temperature and for the additional (magneto)resistivity increase in systems with MPs.