Spin correlations and colossal magnetoresistance in HgCr$_2$Se$_4$

TL;DR

This paper investigates the mechanism behind colossal magnetoresistance in HgCr₂Se₄, highlighting the role of spin correlations and a percolation transition from magnetic polarons to a conductive network.

Contribution

It reveals the significance of spin correlations in CMR and identifies a percolation transition in transport behavior across a wide temperature range.

Findings

CMR effect occurs below T* ≈ 2.1 T_C

Transport transitions from semiconducting to metallic

Spin correlations influence conduction in the paramagnetic phase

Abstract

This study aims to unravel the mechanism of colossal magnetoresistance (CMR) observed in n-type HgCr$_2$Se$_4$, in which low-density conduction electrons are exchange-coupled to a three-dimensional Heisenberg ferromagnet with a Curie temperature $T_C\approx$ 105 K. Near room temperature the electron transport exhibits an ordinary semiconducting behavior. As temperature drops below $T^*\simeq2.1T_C$, the magnetic susceptibility deviates from the Curie-Weiss law, and concomitantly the transport enters an intermediate regime exhibiting a pronounced CMR effect before a transition to metallic conduction occurs at $T<T_C$. Our results suggest an important role of spin correlations not only near the critical point, but also for a wide range of temperatures ($T_C<T<T^*$) in the paramagnetic phase. In this intermediate temperature regime the transport undergoes a percolation type of transition from isolated magnetic polarons to a continuous network when temperature is lowered or magnetic field becomes stronger.