Percolation and Colossal Magnetoresistance in Eu-based Hexaborides

TL;DR

This paper investigates how substituting calcium for europium in EuB6 affects its magnetic and electronic properties, revealing percolation phenomena and colossal magnetoresistance effects similar to manganese oxides.

Contribution

It demonstrates the impact of Ca substitution on magnetic order, electronic phase separation, and magnetoresistance in EuB6-based materials, highlighting percolation effects.

Findings

Curie temperature decreases with Ca substitution and vanishes at x ≤ 0.3.

Phase separation into Eu- and Ca-rich clusters occurs at x ≈ 0.27.

Colossal negative magnetoresistance observed at low temperatures.

Abstract

Upon substituting Ca for Eu in the local-moment ferromagnet EuB$_6$, the Curie temperature $T_C$ decreases substantially with increasing dilution of the magnetic sublattice and is completely suppressed for $x$ $\leq$ 0.3. The Ca substitution leads to significant changes of the electronic properties across the Eu$_x$Ca$_{1-x}$B$_6$ series. Electron microscopy data for $x$ $\approx$ 0.27 indicate a phase separation into Eu- and Ca-rich clusters of 5 to 10 nm diameter, leading to percolation-type phenomena in the electrical transport properties. The related critical concentration $x_p$ is approximately 0.3. For $x$ $\approx$ 0.27, we observe colossal negative magnetoresistance effects at low temperatures, similar in magnitude as those reported for manganese oxides.