Energy-gap driven low-temperature magnetic and transport properties in Cr$_{1/3}M$S$_2$ ($M$ = Nb or Ta)

TL;DR

This study reveals that the low-temperature magnetic and transport properties of Cr$_{1/3}M$S$_2$ are driven by an electronic energy gap, with experimental and theoretical evidence showing half-metallicity and dynamic spin fluctuations.

Contribution

The paper provides a unifying explanation for the unusual low-temperature properties of Cr$_{1/3}M$S$_2$ by linking electronic structure, gap features, and magnetic behavior, including the influence of chiral soliton lattice effects.

Findings

Materials are half-metals with a 40-100 meV gap in one spin channel.

Magnetometry confirms the existence of the electronic gap.

Spin fluctuations across the gap are observed over a wide frequency range.

Abstract

The helimagnets Cr$_{1/3}M$S$_2$ ($M$ = Nb or Ta) have attracted renewed attention due to the discovery of a chiral soliton lattice (CSL) stabilized in an applied magnetic field, but reports of unusual low-temperature transport and magnetic properties in this system lack a unifying explanation. Here we present electronic structure calculations that demonstrate the materials are half-metals. There is also a gap-like feature (width in range 40-100 meV) in the density of states of one spin channel. This electronic structure explains the low-temperature electronic and magnetic properties of Cr$_{1/3}M$S$_2$ ($M$ = Nb or Ta), with the gap-like feature particularly important for explaining the magnetic behavior. Our magnetometry measurements confirm the existence of this gap. Dynamic spin fluctuations driven by excitations across this gap are seen over a wide range of frequencies (0.1 Hz to MHz) with AC susceptibility and muon-spin relaxation ($\mu^+$SR) measurements. We show further how effects due to the CSL in Cr$_{1/3}$NbS$_2$, as detected with $\mu^+$SR, dominate over the gap-driven magnetism when the CSL is stabilized as the majority phase.