Electronic Origin of Half-metal to Semiconductor Transition and Colossal Magnetoresistance in Spinel HgCr2Se4

TL;DR

This study reveals the electronic mechanisms behind the half-metal to semiconductor transition and colossal magnetoresistance in HgCr2Se4, highlighting a Lifshitz transition driven by exchange interactions and chemical nonstoichiometry.

Contribution

It provides spectroscopic evidence linking electronic structure changes to phase transitions and magnetoresistance in HgCr2Se4, a unique half-metal.

Findings

Fermi surface consists of a single electron pocket in n-HgCr2Se4

Lifshitz transition occurs across the FM-PM transition

Exchange interaction causes giant band splitting

Abstract

Half-metals are ferromagnets hosting spin-polarized conducting carriers and crucial for spintronics applications. The chromium spinel HgCr2Se4 represents a unique type of half-metal, which features a half-metal to semiconductor transition (HMST) and exhibits colossal magnetoresistance (CMR) across the ferromagnetic-paramagnetic (FM-PM) transition. Using angle-resolved photoemission spectroscopy (ARPES), we find that the Fermi surface of n-type HgCr2Se4 (n-HgCr2Se4) consists of a single electron pocket which moves above the Fermi level (EF) upon the FM-PM transition, leading to the HMST. Such a Lifshitz transition manifests a giant band splitting which originates from the exchange interaction unveiled with a specific chemical nonstoichiometry. The exchange band splitting and the chemical nonstoichiometry are two key ingredients to the HMST and CMR, consistent with our ab-initio calculation. Our findings provide spectroscopic evidences of the electronic origin of the anomalous properties of HgCr2Se4, which address the unique phase transition in half-metals.